MAX1778/MAX1880–MAX1885
General Description
The MAX1778/MAX1880–MAX1885 multiple-output
DC-DC converters provide the regulated voltages required
by active matrix thin-film transistor (TFT) liquid crystal
displays (LCD) in a low-profile TSSOP package. One
high-power step-up converter and two low-power charge
pumps convert the 2.7V to 5.5V input voltage into three
independent output voltages. A built-in linear regulator and
VCOM buffer complete the power-supply requirements.
The main step-up converter accurately generates an
externally set output voltage up to 13V that can supply the
display’s row/column drivers. The converter’s high switching frequency and current-mode PWM architecture provide
fast transient response and allow the use of small lowprofile inductors and ceramic capacitors. The low-power
BiCMOS control circuitry and internal 14V switch (0.35Ω
N-channel MOSFET) enable efficiencies up to 91%.
The dual low-power charge pumps (MAX1778/MAX1880/
MAX1881/MAX1882 only) independently regulate one
positive output (VPOS) and one negative output (VNEG).
These low-power outputs use external diode and capacitor stages (as many stages as required) to regulate output
voltages up to +40V and -40V. A unique control scheme
minimizes output ripple as well as capacitor sizes for both
charge pumps.
A resistor-programmable, 40mA, low-dropout linear
regulator (MAX1778/MAX1881/MAX1883/MAX1884 only)
provides preregulation or postregulation for any of the
supplies. For higher current applications, an external
transistor can be added. Additionally, the VCOM buffer
provides a high current output that is ideal for driving the
capacitive backplane of TFT LCD panels. The VCOM
buffer’s output voltage is preset with an internal 50%
resistive-divider or can be externally adjusted for other
voltages.
Quad-Output TFT LCD DC/DC
Converters with Buffer
Features
●● 500kHz/1MHz Current-Mode PWM Step-Up
Regulator
• Up to +13V Main High-Power Output
±1% Accurate
• High Efficiency (91%)
●● Dual Regulated Charge-Pump Outputs (MAX1778/
MAX1880–MAX1882 only)
• Up to +40V Positive Charge-Pump Output
• Up to -40V Negative Charge-Pump Output
●● Low-Dropout 40mA Linear Regulator (MAX1778/
MAX1881/MAX1883/MAX1884 only)
• Up to +15V LDO Input
●● Optional Higher Current with External Transistor
●● 2.7V to 5.5V Input Supply
●● Internal Supply Sequencing and Soft-Start
●● Power-Ready Output
●● Adjustable Fault-Detection Latch
●● Thermal Protection (+160°C)
●● 0.1μA Shutdown Current
●● 0.7mA IN Quiescent Current
●● Ultra-Small External Components
●● Thin TSSOP Package (1.1mm max height)
Ordering Information
TEMP RANGE
PIN-PACKAGE
MAX1778EUG
PART
-40°C to +85°C
24 TSSOP
MAX1778EUG+
-40°C to +85°C
24 TSSOP
MAX1880EUG
-40°C to +85°C
24 TSSOP
MAX1881EUG
-40°C to +85°C
24 TSSOP
MAX1882EUG
-40°C to +85°C
24 TSSOP
MAX1883EUP
-40°C to +85°C
20 TSSOP
MAX1884EUP
-40°C to +85°C
20 TSSOP
MAX1885EUP
-40°C to +85°C
20 TSSOP
The MAX1778/MAX1880–MAX1885 are protected against
output undervoltage and thermal overload conditions by a
latched fault detection circuit that shuts down the device.
All devices are available in the ultrathin TSSOP package
(1.1mm max height).
+Denotes lead(Pb)-free/RoHS-compliant package.
Applications
Typical operating Circuit appears at end of data sheet.
●● TFT LCD Notebook Displays
●● TFT LCD Desktop Monitor Panels
19-1979; Rev 3; 4/15
Pin Configurations and Selector Guide appear at end of
data sheet.
MAX1778/MAX1880–MAX1885
Quad-Output TFT LCD DC/DC
Converters with Buffer
Absolute Maximum Ratings
IN, SHDN, TGND, FLTSET to GND...........................-0.3V to +6V
DRVN to GND........................................-0.3V to (VSUPN + 0.3V)
DRVP to GND........................................-0.3V to (VSUPP + 0.3V)
PGND to GND.....................................................................±0.3V
RDY, SUPB to GND................................................-0.3V to +14V
LX, SUPP, SUPN to PGND .....................................-0.3V to +14V
SUPL to GND..........................................................-0.3V to +18V
LDOOUT to GND....................................-0.3V to (VSUPL + 0.3V)
INTG, REF, FB, FBN, FBP to GND...............-0.3V to (VIN + 0.3V)
FBL to GND.............-0.3V to the lower of (VSUPL + 0.3V) or +6V
BUFOUT, BUF+, BUF- to GND..............-0.3V to (VSUPB + 0.3V)
Continuous Power Dissipation (TA = +70°C)
20-Pin TSSOP (derate 10.9mW/°C above +70°C)......879mW
24-Pin TSSOP (derate 12.2mW/°C above +70°C)......975mW
Operating Temperature Range
MAX1778EUG, MAX1883EUP.........................-40°C to +85°C
Junction Temperature.......................................................+150°C
Storage Temperature Range..............................-65°C to +150°C
Lead Temperature (soldering, 10s)...................................+300°C
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these
or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect
device reliability.
Electrical Characteristics
(VIN = +3.0V, SHDN = IN, VSUPP = VSUPN = VSUPB = VSUPL = 10V, LDOOUT = FBL, BUF- = BUFOUT, BUF+ = FLTSET = TGND =
PGND = GND, CREF = 0.22μF, CBUF = 1μF, TA = 0°C to +85°C. Typical values are at TA = +25°C, unless otherwise noted.)
PARAMETER
Input Supply Range
Input Undervoltage Threshold
IN Quiescent Supply Current
SUPP Quiescent Current
SUPN Quiescent Current
SYMBOL
CONDITIONS
VIN
VUVLO
IIN
ISUPP
ISUPN
MIN
TYP
2.7
VIN rising, 40mV hysteresis (typ)
VFB = VFBP =
1.5V, VFBN =
-0.2V
VFBP = 1.5V
VFBN = -0.2V
MAX1778/MAX1880/
MAX1883 (fOSC = 1MHz)
2.2
MAX
UNITS
5.5
V
2.4
2.6
V
0.7
1
mA
MAX1881/MAX1882/
MAX1884/MAX1885
(fOSC = 500kHz)
0.6
1
MAX1778/MAX1880
(fOSC = 1MHz)
0.4
0.7
MAX1881/MAX1882
(fOSC = 500kHz)
0.3
0.5
MAX1778/MAX1880
(fOSC = 1MHz)
0.4
0.7
MAX1881/MAX1882
(fOSC = 500kHz)
0.3
0.5
mA
mA
IN Shutdown Current
VSHDN = 0, VIN = 5V
0.1
10
µA
SUPP Shutdown Current
VSHDN = 0, VSUPP = 13V,
MAX1778/MAX1880/MAX1881/MAX1882
0.1
10
µA
SUPN Shutdown Current
VSHDN = 0, VSUPN = 13V,
MAX1778/MAX1880/MAX1881/MAX1882
0.1
10
µA
SUPL Shutdown Current
VSHDN = 0, VSUPL = 13V
MAX1778/MAX1881/MAX1883/MAX1884
0.1
10
µA
SUPB Shutdown Current
VSHDN = 0, VSUPB = 13V
6
13
µA
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Maxim Integrated │ 2
MAX1778/MAX1880–MAX1885
Quad-Output TFT LCD DC/DC
Converters with Buffer
Electrical Characteristics (continued)
(VIN = +3.0V, SHDN = IN, VSUPP = VSUPN = VSUPB = VSUPL = 10V, LDOOUT = FBL, BUF- = BUFOUT, BUF+ = FLTSET = TGND =
PGND = GND, CREF = 0.22μF, CBUF = 1μF, TA = 0°C to +85°C. Typical values are at TA = +25°C, unless otherwise noted.)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
13
V
1.247
1.260
MAIN STEP-UP CONVERTER
Main Output Voltage Range
VMAIN
FB Regulation Voltage
VFB
FB Input Bias Current
IFB
Operating Frequency
fOSC
VIN
Integrator enabled, CINTG = 1000pF
1.234
Integrator disabled (INTG = REF)
1.220
-50
+50
nA
MAX1778/MAX1880/MAX1883
0.85
1
1.15
MHz
MAX1881/MAX1882/MAX1884/MAX1885
425
500
575
kHz
80
85
91
%
ILX = 0 to 200mA,
VMAIN = 10V
Integrator enabled,
CINTG = 1000pF
0.01
Integrator disabled
(INTG = REF)
0.2
%
Line Regulation
0.1
Integrator Transconductance
LX Switch On-Resistance
LX Leakage Current
ILX
ILX = 100mA
ILIM
0.275
tSS
FB Fault Trip Level
Ω
0.01
20
µA
0.38
0.5
0.75
Phase III = soft-start (1024/fOSC)
1.12
1.15
Maximum RMS LX Current
Soft-Start Period
0.7
Phase II = soft-start (1024/fOSC)
Phase IV = fully on (after 3072/fOSC)
µS
0.35
VLX = 13V
Phase I = soft-start (1024/fOSC)
LX Current Limit
%/V
317
RLX(ON)
V
VFB = 1.25V, INTG = GND
Oscillator Maximum Duty Cycle
Load Regulation
1.280
Power-up to the end of Phase III
A
1.5
1.85
1
A
3072 / fOSC
s
Falling edge, FLTSET = GND
1.07
1.1
1.14
Falling edge, FLTSET = 1V
0.955
0.99
1.025
V
POSITIVE CHARGE PUMP (MAX1778/MAX1880/MAX1881/MAX1882 only)
SUPP Input Supply Range
VSUPP
Operating Frequency
fCHP
FBP Regulation Voltage
VFBP
FBP Input Bias Current
IFBP
DRVP PCH On-Resistance
RPCH(ON)
DRVP NCH On-Resistance
RNCH(ON)
2.7
1.2
VFBP = 1.5V
VFBP = 1.2V
VFBP = 1.3V
FBP Fault Trip Level
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1.25
-50
V
Hz
1.3
V
+50
nA
5
10
Ω
2
4
20
Maximum RMS DRVP Current
FBP Power-Ready Trip Level
13
0.5 x fOSC
Ω
kΩ
0.1
A
Rising edge
1.09
1.125
1.16
Falling edge, FLTSET = GND
1.08
1.11
1.16
Falling edge, FLTSET = 1V
0.955
0.99
1.025
V
V
Maxim Integrated │ 3
MAX1778/MAX1880–MAX1885
Quad-Output TFT LCD DC/DC
Converters with Buffer
Electrical Characteristics (continued)
(VIN = +3.0V, SHDN = IN, VSUPP = VSUPN = VSUPB = VSUPL = 10V, LDOOUT = FBL, BUF- = BUFOUT, BUF+ = FLTSET = TGND =
PGND = GND, CREF = 0.22μF, CBUF = 1μF, TA = 0°C to +85°C. Typical values are at TA = +25°C, unless otherwise noted.)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
NEGATIVE CHARGE PUMP (MAX1778/MAX1880/MAX1881/MAX1882 only)
SUPN Input Supply Range
VSUPN
Operating Frequency
fCHP
FBN Regulation Voltage
VFBN
FBN Input Bias Current
IFBN
DRVN PCH On-Resistance
RPCH(ON)
DRVN NCH On-Resistance
RNCH(ON)
2.7
13
0.5 x fOSC
-50
VFBN = 0
0
-50
VFBN = +50mV
VFBN = -50mV
+50
mV
+50
nA
5
10
Ω
2
4
20
Maximum RMS DRVN Current
V
Hz
Ω
kΩ
0.1
A
FBN Power-Ready Trip Level
Falling edge
80
125
165
mV
FBN Fault Trip Level
Rising edge
80
140
190
mV
15
V
4
4.3
V
µA
LOW-DROPOUT LINEAR REGULATOR (MAX1778/MAX1881/MAX1883/MAX1884 only)
SUPL Input Supply Range
VSUPL
SUPL Undervoltage Lockout
SUPL Quiescent Current
4.5
Rising edge, 50mV hysteresis (typ)
ISUPL
Dropout Voltage (Note 1)
VDROP
FBL Regulation Voltage
VFBL
3.8
ILDO = 100µA
LDO is set to
regulate at 9V
120
220
ILDO = 40mA
130
300
ILDO = 5mA
70
VSUPL = 10V, LDO regulating at 9V,
ILDO = 15mA
1.235
1.25
mV
1.265
V
LDO Load Regulation
VSUPL = 10V, LDO regulating at 9V,
ILDO = 100µA to 40mA
1.2
%
LDO Line Regulation
VSUPL = 4.5V to 15V, FBL = LDOOUT,
ILDO = 15mA
0.02
%/V
FBL Input Bias Current
LDO Current Limit
IFBL
ILDOLIM
VFBL = 1.25V
VSUPL = 10V, VLDOOUT = 9V, VFBL = 1.2V
-0.8
40
+0.8
µA
130
220
mA
13
V
420
850
µA
VCOM BUFFER
SUPB Input Supply Range
VSUPB
SUPB Quiescent Current
ISUPB
VSUPB = 13V
4.5
Power-Supply Rejection Ratio
PSRR
VSUPB = 4.5V to 13V, VCM = 2.25V
85
Input Common-Mode Voltage
Range
VCM
|VOS| < 10mV
1.2
VCM = 1.2V to 8.8V
75
BUFOUT Leakage Current
Common-Mode Rejection Ratio
Input Bias Current
Input Offset Current
Gain Bandwidth Product
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-10
CMRR
IBIAS
VCM = 5V
-100
IOS
VCM = 5V
-100
GBW
CBUF = 1µF
+10
98
µA
dB
8.8
V
dB
-10
+100
+100
13
nA
nA
kHz
Maxim Integrated │ 4
MAX1778/MAX1880–MAX1885
Quad-Output TFT LCD DC/DC
Converters with Buffer
Electrical Characteristics (continued)
(VIN = +3.0V, SHDN = IN, VSUPP = VSUPN = VSUPB = VSUPL = 10V, LDOOUT = FBL, BUF- = BUFOUT, BUF+ = FLTSET = TGND =
PGND = GND, CREF = 0.22μF, CBUF = 1μF, TA = 0°C to +85°C. Typical values are at TA = +25°C, unless otherwise noted.)
PARAMETER
Output Voltage
SYMBOL
VBUFOUT
CONDITIONS
BUF+ = GND
MIN
TYP
MAX
IBUFOUT = 0
4.99
5.01
IBUFOUT = ±5mA
4.97
5.03
IBUFOUT = ±45mA
4.93
5.07
IBUFOUT = ±5mA
-30
+30
IBUFOUT = ±45mA
-70
+70
Input Offset Voltage
VOS
VSUPB = 4.5V to
13V, VCM = 1.2V to
(VSUPB - 1.2V)
Output Voltage Swing High
VOH
IBUFOUT = -45mA, ∆VOS = 1V
Output Voltage Swing Low
VOL
IBUFOUT = +45mA, ∆VOS = 1V
9
9.6
0.4
Peak Buffer Output Current
Falling edge, 20mV hysteresis (typ)
V
mV
V
1
±150
BUF+ Dual Mode™ Threshold
Voltage
UNITS
V
mA
80
125
170
mV
1.231
1.25
1.269
V
0.9
1.05
1.2
V
0.9
V
REFERENCE
Reference Voltage
VREF
-2µA < IREF < 50µA
Reference Undervoltage
Threshold
LOGIC SIGNALS
SHDN Input Low Voltage
SHDN Input High Voltage
SHDN Input Current
2.1
ISHDN
V
0.01
0.67 x
VREF
FLTSET Input Voltage Range
80
1
µA
0.85 x
VREF
V
FLTSET Threshold Voltage
Rising edge, 25mV hysteresis (typ)
125
170
mV
FLTSET Input Current
VFLTSET = 1V
0.1
50
nA
RDY Output Low Voltage
ISINK = 2mA
0.25
0.5
V
RDY Output High Leakage
VRDY = 13V
0.01
1
µA
Thermal Shutdown
Rising temperature
160
°C
Dual Mode is a trademark of Maxim Integrated Products, Inc.
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Maxim Integrated │ 5
MAX1778/MAX1880–MAX1885
Quad-Output TFT LCD DC/DC
Converters with Buffer
Electrical Characteristics
(VIN = +3.0V, SHDN = IN, VSUPP = VSUPN = VSUPB = VSUPL = 10V, LDOOUT = FBL, BUF- = BUFOUT, BUF+ = FLTSET = TGND =
PGND = GND, CREF = 0.22μF, CBUF = 1μF, TA = -40°C to +85°C, unless otherwise noted.) (Note 2)
PARAMETER
Input Supply Range
Input Undervoltage Threshold
IN Quiescent Supply Current
SUPP Quiescent Current
SUPN Quiescent Current
SYMBOL
CONDITIONS
VIN
VUVLO
IIN
ISUPP
ISUPN
VIN Rising, 40mV hysteresis (typ)
VFB =
VFBP = 1.5V,
VFBN = -0.2V
VFBP = 1.5V
VFBN = -0.2V
MIN
MAX
UNITS
2.7
5.5
V
2.2
2.6
V
MAX1778/MAX1880/
MAX1883 (fOSC = 1MHz)
1
MAX1881/MAX1882/MAX1884/
MAX1885 (fOSC = 500kHz)
1
mA
MAX1778/MAX1880
(fOSC = 1MHz)
0.7
MAX1881/MAX1882
(fOSC = 500kHz)
0.5
MAX1778/MAX1880
(fOSC = 1MHz)
0.7
MAX1881/MAX1882
(fOSC = 500kHz)
0.5
mA
mA
IN Shutdown Current
VSHDN = 0, VIN = 5V
10
µA
SUPP Shutdown Current
VSHDN = 0, VSUPP = 13V,
MAX1778/MAX1880/MAX1881/MAX1882
10
µA
SUPN Shutdown Current
VSHDN = 0, VSUPN = 13V,
MAX1778/MAX1880/MAX1881/MAX1882
10
µA
SUPL Shutdown Current
VSHDN = 0, VSUPL = 13V,
MAX1778/MAX1881/MAX1883/MAX1884
10
µA
SUPB Shutdown Current
VSHDN = 0, VSUPB = 13V
13
µA
VIN
13
V
Integrator enabled, CINTG = 1000pF
1.223
1.269
Integrator disabled (INTG = REF)
1.21
1.29
VFB = 1.25V, INTG = GND
-50
+50
nA
MAX1778/MAX1880/MAX1883
0.75
1.25
MHz
MAX1881/MAX1882/MAX1884/MAX1885
375
625
kHz
79
91
%
MAIN STEP-UP CONVERTER
Main Output Voltage Range
VMAIN
FB Regulation Voltage
VFB
FB Input Bias Current
IFB
Operating Frequency
FOSC
Oscillator Maximum Duty
Cycle
LX Switch On-Resistance
RLX(ON)
LX Leakage Current
ILX
LX Current Limit
ILIM
FB Fault Trip Level
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V
ILX = 100mA
0.7
Ω
VLX = 13V
20
µA
Phase I = soft-start (1024/fOSC)
0.275
0.525
Phase IV = fully on (after 3072/fOSC)
1.1
2.05
Falling edge, FLTSET = GND
1.07
1.14
A
V
Maxim Integrated │ 6
MAX1778/MAX1880–MAX1885
Quad-Output TFT LCD DC/DC
Converters with Buffer
Electrical Characteristics (continued)
(VIN = +3.0V, SHDN = IN, VSUPP = VSUPN = VSUPB = VSUPL = 10V, LDOOUT = FBL, BUF- = BUFOUT, BUF+ = FLTSET = TGND =
PGND = GND, CREF = 0.22μF, CBUF = 1μF, TA = -40°C to +85°C, unless otherwise noted.) (Note 2)
PARAMETER
SYMBOL
CONDITIONS
MIN
MAX
UNITS
POSITIVE CHARGE PUMP (MAX1778/MAX1880/MAX1881/MAX1882 only)
SUPP Input Supply Range
VSUPP
2.7
13
V
FBP Regulation Voltage
VFBP
1.2
1.3
V
FBP Input Bias Current
IFBP
-50
+50
nA
10
Ω
4
Ω
DRVP PCH On-Resistance
DRVP NCH On-Resistance
VFBP = 1.5V
RPCH(ON)
RNCH(ON)
VFBP = 1.2V
VFBP = 1.3V
20
Rising edge
1.09
1.16
V
VSUPN
2.7
13
V
VFBN
-50
+50
mV
-50
+50
nA
10
Ω
FBP Power-Ready Trip Level
kΩ
NEGATIVE CHARGE PUMP (MAX1778/MAX1880/MAX1881/MAX1882 only)
SUPN Input Supply Range
FBN Regulation Voltage
FBN Input Bias Current
IFBN
DRVN PCH On-Resistance
RPCH(ON)
DRVN NCH On-Resistance
RNCH(ON)
FBN Power-Ready Trip Level
VFBN = 0
VFBN = +50mV
4
Ω
VFBN = -50mV
20
kΩ
Falling edge
80
165
mV
4.5
15
V
LOW DROPOUT LINEAR REGULATOR (MAX1778/MAX1881/MAX1883/MAX1884 only)
SUPL Input Supply Range
VSUPL
SUPL Undervoltage Lockout
Rising edge, 50mV hysteresis (typ)
4.3
V
ILDO = 100µA
240
µA
VDROP
LDO regulating to 9V, ILDO = 40mA
330
mV
VFBL
VSUPL = 10V, LDO regulating to 9V,
ILDO = 15mA
1.265
V
SUPL Quiescent Current
ISUPL
Dropout Voltage (Note 1)
FBL Regulation Voltage
3.8
1.222
LDO Load Regulation
VSUPL = 10V, LDO regulating to 9V,
ILDO = 100µA to 40mA
1.2
%
LDO Line Regulation
VSUPL = 4.5V to 15V, FBL = LDOOUT,
ILDO = 15mA
0.02
%/V
-1.2
+1.2
µA
40
260
mA
FBL Input Bias Current
LDO Current Limit
IFBL
ILDOLIM
VFBL = 1.25V
VSUPL = 10V, VLDOOUT = 9V, VFBL = 1.2V
VCOM BUFFER
SUPB Input Supply Range
VSUPB
SUPB Quiescent Current
ISUPB
4.5
BUFOUT Leakage Current
Input Common-Mode Voltage
Range
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VCM
13
V
850
µA
-10
+10
µA
1.2
8.8
V
VSUPB = 13V
|VOS| < 10mV
Maxim Integrated │ 7
MAX1778/MAX1880–MAX1885
Quad-Output TFT LCD DC/DC
Converters with Buffer
Electrical Characteristics (continued)
(VIN = +3.0V, SHDN = IN, VSUPP = VSUPN = VSUPB = VSUPL = 10V, LDOOUT = FBL, BUF- = BUFOUT, BUF+ = FLTSET = TGND =
PGND = GND, CREF = 0.22μF, CBUF = 1μF, TA = -40°C to +85°C, unless otherwise noted.) (Note 2)
PARAMETER
Input Bias Current
Input Offset Current
Output Voltage
SYMBOL
CONDITIONS
IBIAS
VCM = 5V
IOS
VCM = 5V
VBUFOUT
BUF+ = GND
MAX
UNITS
+500
nA
nA
-500
+500
IBUFOUT = 0
4.988
5.012
IBUFOUT = ±5mA
4.97
5.03
IBUFOUT = ±45mA
4.93
5.07
IBUFOUT = ±5mA
-30
+30
IBUFOUT = ±45mA
-70
+70
Input Offset Voltage
VOS
VSUPB = 4.5V to 13V
VCM = 1.2V to
(VSUPB - 1.2V)
Output Voltage Swing High
VOH
IBUFOUT = -45mA, ∆VOS = 1V
Output Voltage Swing Low
VOL
IBUFOUT = +45mA, ∆VOS = 1V
BUF+ Dual-Mode
Threshold Voltage
MIN
-500
Falling edge, 20mV hysteresis (typ)
9
V
mV
V
1
V
80
170
mV
1.223
1.269
V
0.9
1.2
V
0.9
V
1
µA
0.74 x
VREF
0.85 x
VREF
V
80
170
mV
nA
REFERENCE
Reference Voltage
VREF
-2µA < IREF < 50µA
Reference Undervoltage
Threshold
LOGIC SIGNALS
SHDN Input Low Voltage
2.1
SHDN Input High Voltage
SHDN Input Current
ISHDN
FLTSET Input Voltage
Range
V
FLTSET Threshold Voltage
Rising edge, 25mV hysteresis (typ)
FLTSET Input Current
VFLTSET = 1V
50
RDY Output Low Voltage
ISINK = 2mA
0.5
V
RDY Output High Leakage
VRDY = 13V
1
µA
Note 1: Dropout voltage is defined as the VSUPL - VLDOOUT, when VSUPL is 100mV below the set value of VLDOOUT.
Note 2: Specifications to -40°C are guaranteed by design, not production tested.
www.maximintegrated.com
Maxim Integrated │ 8
MAX1778/MAX1880–MAX1885
Quad-Output TFT LCD DC/DC
Converters with Buffer
Typical Operating Characteristics
(Circuit of Figure 1, VIN = +3.3V, SHDN = IN, VMAIN = VSUPP = VSUPN = VSUPB = VSUPL = 8V, BUF- = BUFOUT, BUF+ = FLTSET =
TGND = PGND = GND, TA = +25°C.)
MAIN 8V OUTPUT EFFICIENCY
vs. LOAD CURRENT
7.96
CINTG = 470pF
RCOMP = 24kΩ
CCOMP = 470pF
7.88
0
200
400
600
70
60
40
0
200
400
70
40
FIGURE 8
VOUT = 12V
CINTG = 470pF
0
100
200
300
400
500
1.05
1.00
0.95
0.90
0.80
3.0
3.5
50
VPOS = 20V
5
10
INEG (mA)
www.maximintegrated.com
MAX1778 toc03
19.8
VSUPP = 7.5V
VSUPP = 7V
0
5
15
20
-4.90
-4.92
IPOS = 10mA
-5.02
6
8
VSUPP (V)
10
12
14
VSUPN = 7V
-4.98
10
4
20
-4.96
-5.00
2
VSUPN = 6V
-4.94
15
5
15
NEGATIVE CHARGE-PUMP OUTPUT VOLTAGE
vs. LOAD CURRENT
IPOS = 1mA
20
10
IPOS (mA)
35
25
600
VSUPP = 8V
19.6
19.2
5.5
MAX1778 toc08
MAX1778 toc07
VSUPP = 10V
0
5.0
40
30
70
30
4.5
MAXIMUM POSITIVE CHARGE-PUMP
OUTPUT VOLTAGE vs. SUPPLY VOLTAGE
VSUPP = 8V
40
4.0
500
VSUPP = 10V
19.4
2.5
400
20.0
0.85
600
300
20.2
1.10
VPOS (V)
EFFICIENCY (%)
VSUPP = 7.5V
60
200
VIN (V)
100
80
100
POSITIVE CHARGE-PUMP OUTPUT VOLTAGE
vs. LOAD CURRENT
MAX1778 toc05
1.15
POSITIVE CHARGE-PUMP EFFICIENCY
vs. LOAD CURRENT
VSUPP = 7V
0
IOUT (mA)
MAX1778
IOUT (mA)
90
FIGURE 8
CINTG = 470pF
11.76
800
VPOS (V)
EFFICIENCY (%)
VIN = 3.3V
1.20
SWITCHING FREQUENCY (MHz)
VIN = 5V
50
600
STEP UP CONVERTERS
SWITCHING FREQUENCY vs. INPUT VOLTAGE
MAX1778 toc04
100
60
11.84
IOUT (mA)
MAIN 12V OUTPUT EFFICIENCY
vs. LOAD CURRENT
80
VIN = 5V
11.92
VOUT = 8V
RCOMP = 24kΩ
CCOMP = 470pF
CINTG = 470pF
IOUT (mA)
90
VIN = 3.3V
12.00
50
800
12.08
VIN = 3.3V
VNEG (V)
7.92
80
12.16
MAX1778 toc06
VIN = 5V
8.00
90
EFFICIENCY (%)
VOUT (V)
8.04
VIN = 5V
VOUT (V)
VIN = 3.3V
12.24
MAX1778 toc02
8.08
100
MAX1778 toc01
8.12
MAIN 12V OUTPUT VOLTAGE
vs. LOAD CURRENT
MAX1778 toc09
MAIN 8V OUTPUT VOLTAGE
vs. LOAD CURRENT
-5.04
VSUPN = 8V
0
10
20
INEG (mA)
30
40
Maxim Integrated │ 9
MAX1778/MAX1880–MAX1885
Quad-Output TFT LCD DC/DC
Converters with Buffer
Typical Operating Characteristics (continued)
(Circuit of Figure 1, VIN = +3.3V, SHDN = IN, VMAIN = VSUPP = VSUPN = VSUPB = VSUPL = 8V, BUF- = BUFOUT, BUF+ = FLTSET =
TGND = PGND = GND, TA = +25°C.)
-6
VSUPN = 7V
70
60
VSUPN = 8V
-8
MAX1778 toc12
1.26
1.25
INEG = 1mA
-10
50
1.24
-12
40
30
INEG = 10mA
1.27
VREF (V)
80
-4
VNEG (V)
EFFICIENCY (%)
VSUPN = 6V
MAX1778 toc11
VNEG = -5V
90
-2
MAX1778 toc10
100
REFERENCE VOLTAGE
vs. REFERENCE LOAD CURRENT
MAXIMUM NEGATIVE CHARGE-PUMP
OUTPUT VOLTAGE vs. SUPPLY VOLTAGE
NEGATIVE CHARGE-PUMP EFFICIENCY
vs. LOAD CURRENT
0
10
20
INEG (mA)
30
40
-14
STEP-UP CONVERTER LOAD-TRANSIENT
RESPONSE
MAX1778 toc13
200mA
4
6
8
10
12
1.23
14
8.1V
B
7.9V
C
0
40µs/div
A. IMAIN = 20mA to 200mA, 200mA/div
B. VMAIN = 8V, 100mV/div
C. INDUCTOR CURRENT, 1A/div
CINTG = 1000pF
www.maximintegrated.com
40
60
80
100
STEP-UP CONVERTER LOAD-TRANSIENT
RESPONSE WITHOUT INTEGRATOR
STEP-UP CONVERTER LOAD-TRANSIENT
RESPONSE (1µs PULSES)
200mA
MAX1778 toc15
A
0
0.5A
A
0
8.0V
B
8.0V
7.9V
1A
20
IREF (µA)
8.1V
8.0V
0
VSUPN (V)
MAX1778 toc14
A
0
2
B
7.9V
1A
1A
C
0
40µs/div
A. IMAIN = 20mA to 200mA, 200mA/div
B. VMAIN = 8V, 100mV/div
C. INDUCTOR CURRENT, 1A/div
INTG = REF
0.5A
C
0
4µs/div
A. IMAIN = 0 to 500mA, 500mA/div
B. VMAIN = 8V, 100mV/div
C. INDUCTOR CURRENT, 500mA/div
Maxim Integrated │ 10
MAX1778/MAX1880–MAX1885
Quad-Output TFT LCD DC/DC
Converters with Buffer
Typical Operating Characteristics (continued)
(Circuit of Figure 1, VIN = +3.3V, SHDN = IN, VMAIN = VSUPP = VSUPN = VSUPB = VSUPL = 8V, BUF- = BUFOUT, BUF+ = FLTSET =
TGND = PGND = GND, TA = +25°C.)
MAX1778 toc18
MAX1778 toc17
MAX1778 toc16
2V
A
8V
STEP-UP CONVERTER
SOFT-START (HEAVY LOAD)
STEP-UP CONVERTER
SOFT-START (LIGHT LOAD)
RIPPLE VOLTAGE WAVEFORMS
A
0
B
B
6V
C
0.5A
C
0
1.0A
0
POWER-UP SEQUENCE
(CIRCUIT OF FIGURE 10)
POWER-UP SEQUENCE
2V
A
0
MAX1778 toc21
A
2V
0
4V
B
0
B
10V
C
5V
20V
C
0
10V
D
0
0
-10V
E
www.maximintegrated.com
B
0
0
D
-5V
2ms/div
A. VSHDN = O TO 2V, 2V/div
B. RDY, 5V/div
C. POSITIVE CHARGE PUMP = VPOS = 20V, RLOAD = 4kΩ, 10V/div
D. STEP-UP CONVERTER: VMAIN = 8V, RLOAD = 40Ω, 10V/div
E. NEGATIVE CHARGE PUMP: VNEG = -5V, RLOAD = 500Ω, 10V/div
A
2V
20V
5V
1ms/div
A. VSHDN = O TO 2V, 2V/div
B. VMAIN = 8V, 2V/div
C. INDUCTOR CURRENT, 500mA/div
RLOAD = 20Ω
POWER-UP INTO SHORT-CIRCUIT
(CIRCUIT OF FIGURE 10)
MAX1778 toc20
4V
MAX1778 toc19
C
0.5A
1ms/div
A. VSHDN = O TO 2V, 2V/div
B. VMAIN = 8V, 2V/div
C. INDUCTOR CURRENT, 500mA/div
RLOAD = 400Ω
1µs/div
A. VMAIN = 8V, IMAIN = 200mA, 10mV/div
B. VNEG = -5V, INEG = 10mA, 20mV/div
C. VPOS = 20V, IPOS = 5mA, 20mV/div
B
6V
4V
4V
20V
A
0
8V
8V
-5V
2V
1ms/div
A. RDY, 2V/div
B. POSITIVE CHARGE PUMP, VPOS(SYS) = 20V, 10V/div
C. STEP-UP CONVERTER: VMAIN(SYS) = 8V, 10V/div
D. NEGATIVE CHARGE PUMP, VNEG = -5V, -5V/div
10V
C
5V
0
100µs/div
A. RDY, 2V/div
B. GATE OF N-CH MOSFET, 5V/div
C. STEP-UP CONVERTER, VMAIN(START) = 8V, 5V/div
VMAIN(SYS) = GND
Maxim Integrated │ 11
MAX1778/MAX1880–MAX1885
Quad-Output TFT LCD DC/DC
Converters with Buffer
Typical Operating Characteristics (continued)
(Circuit of Figure 1, VIN = +3.3V, SHDN = IN, VMAIN = VSUPP = VSUPN = VSUPB = VSUPL = 8V, BUF- = BUFOUT, BUF+ = FLTSET =
TGND = PGND = GND, TA = +25°C.)
5.02
ILDOOUT = 40mA
4.90
4.85
4.98
5.02
4.96
4.92
4.75
4.90
10
12
4.92
0.01
0.1
1
10
100
4.90
-40
-15
ILDOOUT (mA)
10
35
60
85
TEMPERATURE (°C)
LDO SUPPLY CURRENT
vs. LDO OUTPUT CURRENT
(INTERNAL LINEAR REGULATOR)
4.0
MAX1778 toc25
VLDOOUT = 5V
120
80
VLDOOUT = 5V
3.5
ISUPL - ILDOOUT (mA)
160
VSUPL - VLDOOUT (mV)
ILDOOUT = 40mA
4.98
4.94
DROPOUT VOLTAGE
vs. LDO LOAD CURRENT
(INTERNAL LINEAR REGULATOR)
3.0
2.5
2.0
1.5
1.0
40
0
5.00
4.96
VSUPL (V)
200
ILDOOUT = 0
MAX1778 toc26
8
5.06
5.04
4.94
6
5.08
5.00
4.80
4
5.10
VLDO (V)
VLDOOUT (V)
VLDOOUT (V)
4.95
LDO OUTPUT VOLTAGE vs. TEMPERATURE
(INTERNAL LINEAR REGULATOR)
MAX1778 toc23
ILDOOUT = 0
5.00
5.04
MAX1778 toc22
5.05
LDO OUTPUT VOLTAGE
vs. LDO OUTPUT CURRENT
(INTERNAL LINEAR REGULATOR)
MAX1778 toc24
LDO OUTPUT VOLTAGE
vs. LDO INPUT VOLTAGE
(INTERNAL LINEAR REGULATOR)
0.5
0
10
20
ILDOOUT (mA)
www.maximintegrated.com
30
40
0
0
10
20
30
40
ILDOOUT (mA)
Maxim Integrated │ 12
MAX1778/MAX1880–MAX1885
Quad-Output TFT LCD DC/DC
Converters with Buffer
Typical Operating Characteristics (continued)
(Circuit of Figure 1, VIN = +3.3V, SHDN = IN, VMAIN = VSUPP = VSUPN = VSUPB = VSUPL = 8V, BUF- = BUFOUT, BUF+ = FLTSET =
TGND = PGND = GND, TA = +25°C.)
60
40
20
1
10
40mA
0
A
5.00V
B
1
STABLE REGION
0.1
CLDOOUT = 4.7µF
ILDOOUT = 40mA
MAX1778 toc29
CLDOOUT = 1µF
10
CLDOOUT ESR (Ω)
PSRR (dB)
80
0
100
MAX1778 toc27
100
LOAD-TRANSIENT RESPONSE
(INTERNAL LINEAR REGULATOR)
REGION OF STABLE CLDOOUT ESR
vs. LOAD CURRENT
MAX1778 toc28
POWER-SUPPLY REJECTION RATIO
vs. FREQUENCY
4.96V
100
1000
0.01
FREQUENCY (kHz)
LOAD-TRANSIENT RESPONSE NEAR
DROPOUT (INTERNAL LINEAR REGULATOR)
A
0
10
20
ILDOOUT (mA)
40
30
INTERNAL LINEAR-REGULATOR
STARTUP
MAX1778 toc31
5.0V
MAX1778 toc32
A
8.0V
5.00V
100µs/div
A. ILDO = 100µA TO 40mA, 40mA/div
B. VLDO = 5V, 20mV/div
VSUPL = VLDO + 500mV
INTERNAL LINEAR-REGULATOR
RIPPLE REJECTION
MAX1778 toc30
40mA
0
B
A
2V
0
B
4V
2V
B
1.0A
C
0
0.5A
C
0
4.94V
100µs/div
A. ILDO = 100µA TO 40mA, 40mA/div
B. VLDO = 5V, 20mV/div
VIN = VLDO + 100mV
www.maximintegrated.com
10µs/div
A. VLDOOUT = 5V, ILDOOUT = 40mA, 10mV/div
B. VMAIN = VSUPL = 8V, 200mV/div
C. IMAIN = 0 TO 750mA, 500mA/div
4V
2V
400µs/div
A. VSHDN = 0 TO 2V, 2V/div
B. VLDOOUT = 5V, RLDOOUT = 125Ω, 2V/div
C. VMAIN = 8V, RMAIN = 40Ω, 2V/div
Maxim Integrated │ 13
MAX1778/MAX1880–MAX1885
Quad-Output TFT LCD DC/DC
Converters with Buffer
Typical Operating Characteristics (continued)
(Circuit of Figure 1, VIN = +3.3V, SHDN = IN, VMAIN = VSUPP = VSUPN = VSUPB = VSUPL = 8V, BUF- = BUFOUT, BUF+ = FLTSET =
TGND = PGND = GND, TA = +25°C.)
ILDO = 750mA
2.49
2.53
VLDO (V)
ILDO = 0
2.51
FIGURE 7
2.5
3.0
3.5
4.0
4.5
5.0
50mA
0.1
1
10
100
1000
100µs/div
A. ILDO = 50mA TO 250mA, 200mA/div
B. VLDO = 2.5V, 50mV/div
FIGURE 7
INPUT OFFSET VOLTAGE DEVIATION
vs. COMMON-MODE VOLTAGE
EXTERNAL LINEAR-REGULATOR
RIPPLE REJECTION
1.5
1A
0.5A
C
0
10µs/div
A. VLDO = 2.5V, ILDO = 200mA, 10mV/div
B. VMAIN = VSUPL = 8V, 200mV/div
C. IMAIN = 0 TO 750mA, 500mA/div
FIGURE 7
VSUPB = 13V
0.5
-0.5
0.2
-0.2
-0.6
-1.5
-2.5
VCM = VSUPB / 2
0.6
∆VOS (mV)
7.8V
VSUPB = 4.5V
∆VOS (mV)
B
8.0V
INPUT OFFSET VOLTAGE DEVIATION
vs. BUFFER SUPPLY VOLTAGE
1.0
MAX1778 toc37
2.5
A
2.5V
B
2.45V
ILDO (mA)
MAX1778 toc36
A
2.50V
VIN (V)
www.maximintegrated.com
250mA
2.55V
2.49
2.45
5.5
MAX1778 toc35
2.51
2.47
2.47
2.45
FIGURE 7
EXTERNAL LINEAR-REGULATOR
LOAD-TRANSIENT RESPONSE
MAX1778 toc38
2.53
VLDO (V)
2.55
MAX1778 toc33
2.55
LINEAR-REGULATOR OUTPUT VOLTAGE
vs. LOAD CURRENT
(EXTERNAL LINEAR REGULATOR)
MAX1778 toc34
LINEAR-REGULATOR OUTPUT VOLTAGE
vs. INPUT VOLTAGE
(EXTERNAL LINEAR REGULATOR)
0
2
4
6
8
VCM (V)
10
12
14
-1.0
4
6
8
10
12
14
VSUPB (V)
Maxim Integrated │ 14
MAX1778/MAX1880–MAX1885
Quad-Output TFT LCD DC/DC
Converters with Buffer
Typical Operating Characteristics (continued)
(Circuit of Figure 1, VIN = +3.3V, SHDN = IN, VMAIN = VSUPP = VSUPN = VSUPB = VSUPL = 8V, BUF- = BUFOUT, BUF+ = FLTSET =
TGND = PGND = GND, TA = +25°C.)
3.5
IBIAS (nA)
-0.2
2.5
2.0
1.5
TA = +85°C
1.0
-0.6
0.5
-15
10
35
60
85
-30
-15
0
15
30
0
45
8
10
12
BUFFER INPUT BIAS CURRENT
vs. TEMPERATURE
BUFFER SUPPLY CURRENT
vs. COMMON-MODE VOLTAGE
MAX1778 toc42
12
VCM = VSUPB/2
11
8
7
6
8
10
12
4
14
VSUPB = 13V
0.46
0.42
VSUPB = 4.5V
0.38
0.34
5
VCM = VSUPB / 2
-40
-15
10
35
60
85
0.30
0
2
4
6
8
10
12
VSUPB (V)
TEMPERATURE (°C)
VCM (V)
BUFFER SUPPLY CURRENT
vs. BUFFER SUPPLY VOLTAGE
NO-LOAD BUFFER SUPPLY CURRENT
vs. TEMPERATURE
VCOM BUFFER
SMALL-SIGNAL RESPONSE
0.46
VSUPB = 13V
VCM = VSUPB/2
0.9
0.8
ISUPB (mA)
0.38
0.5
0.4
4.05V
6
3.95V
0.1
8
10
VSUPB (V)
www.maximintegrated.com
12
14
0
B
4.00V
0.2
VCM = VSUPB/2
A
3.95V
0.6
0.3
0.34
4.05V
4.00V
0.7
0.42
14
MAX1778 toc47
MAX1778 toc46
1.0
MAX1778 toc45
0.50
14
0.50
ISUPB (mA)
IBIAS (nA)
IBIAS (nA)
9
6
ISUPB (mA)
6
BUFFER INPUT BIAS CURRENT
vs. BUFFER SUPPLY VOLTAGE
6
4
4
VCM (V)
10
0.30
2
IBUFOUT (mA)
8
4
0
TEMPERATURE (°C)
10
4
VSUPB = 4.5V
4
2
VCM = VSUPB/2
-45
6
MAX1778 toc44
-40
0.0
TA = +85°C
MAX1778 toc43
0
VSUPB = 13V
8
TA = +25°C
3.0
0.2
gm (S)
∆VOS (mV)
TA = -40°C
4.0
10
MAX1778 toc40
VSUPB = 13V
VCM = VSUPB/2
0.6
4.5
MAX1778 toc39
1.0
BUFFER INPUT BIAS CURRENT
vs. COMMON-MODE VOLTAGE
BUFFER TRANSCONDUCTANCE
vs. BUFFER OUTPUT CURRENT
MAX1778 toc41
INPUT OFFSET VOLTAGE DEVIATION
vs. TEMPERATURE
-40
-15
10
35
TEMPERATURE (°C)
60
85
4µs/div
A. VBUF+ = 3.95V TO 4.05V, 50mV/div
B. BUFOUT = BUF-, 50mV/div
CBUF = 1µF, VSUPB = 8V
Maxim Integrated │ 15
MAX1778/MAX1880–MAX1885
Quad-Output TFT LCD DC/DC
Converters with Buffer
Typical Operating Characteristics (continued)
(Circuit of Figure 1, VIN = +3.3V, SHDN = IN, VMAIN = VSUPP = VSUPN = VSUPB = VSUPL = 8V, BUF- = BUFOUT, BUF+ = FLTSET =
TGND = PGND = GND, TA = +25°C.)
VCOM BUFFER
LARGE-SIGNAL RESPONSE
VCOM BUFFER
LOAD-TRANSIENT RESPONSE
MAX1778 toc48
VCOM BUFFER
LOAD-TRANSIENT RESPONSE
MAX1778 toc50
MAX1778 toc49
4.50V
200mA
A
4.00V
500mA
A
0
3.50V
4.50V
-200mA
-500mA
4.2V
4.5V
4.0V
3.8V
B
4.00V
B
4.0V
B
3.5V
8.0V
3.50V
A
0
C
10µs/div
8.0V
C
4µs/div
A. VBUF+ = 3.50V TO 4.50V, 0.5V/div
B. BUFOUT = BUF-, 0.5V/div
CBUF = 1µF, VSUPB = 8V
4µs/div
A. IBUFOUT = 200mA PULSES, 200mA/div
B. BUFOUT = BUF-, 200mV/div
C. VMAIN = 8V, 50mV/div
VSUPB = VMAIN, BUF+ = GND, CBUF = 1µF
A. IBUFOUT = 400mA PULSES, 500mA/div
B. BUFOUT = BUF-, 0.5V/div
C. VMAIN = 8V, 100mV/div
VSUPB = VMAIN, BUF+ = GND, CBUF = 1µF
VCOM BUFFER STARTUP
MAX1778 toc51
4V
2V
4V
A
0
4V
4V
B
MAX1778 toc52
2V
0
2V
VCOM BUFFER STARTUP
(PRECHARGED BUFOUT)
A
B
2V
0
0
8.1V
C
7.8V
100µs/div
A. RDY, 2V/div
B. BUFOUT = BUF-, CBUF = 1µF, 2V/div
C. VSUPB = VMAIN = 8V, IMAIN = 20mA, 200mV/div
BUF+ = GND
www.maximintegrated.com
8.2V
C
8.0V
7.9V
1µs/div
A. RDY, 2V/div
B. BUFOUT = BUF-, CBUF = 1µF, 2V/div
C. VSUPB = VMAIN = 8V, IMAIN = 20mA, 200mV/div
FIGURE 11
Maxim Integrated │ 16
MAX1778/MAX1880–MAX1885
Quad-Output TFT LCD DC/DC
Converters with Buffer
Pin Description
PIN
NAME
FUNCTION
1
FB
Main Step-Up Regulator Feedback Input. Regulates to 1.25V
nominal. Connect a resistive divider from the output (VMAIN) to FB
to analog ground (GND).
2
INTG
Main Step-Up Integrator Output. When using the integrator,
connect 1000pF to analog ground (GND). To disable the integrator,
connect INTG to REF.
MAX1778
MAX1881
MAX1880
MAX1882
MAX1883
MAX1884
MAX1885
1
1
1
2
2
2
3
3
3
3
IN
Main Supply Voltage. The supply voltage powers the control
circuitry for all the regulators and can range from 2.7V to 5.5V.
Bypass with a 0.1µF capacitor between IN and GND, as close to
the pins as possible.
4
4
4
4
BUF+
VCOM Buffer (Operational Transconductance Amplifier) Positive
Feedback Input. Connect to GND to select the internal resistive
divider that sets the positive input to half the amplifier’s supply
voltage (VBUF+ = VSUPB /2).
5
5
5
5
BUF-
VCOM Buffer (Operational Transconductance Amplifier) Negative
Feedback Input
6
6
6
6
SUPB
VCOM Buffer (Operational Transconductance Amplifier) Supply
Voltage
7
7
7
7
BUFOUT
VCOM Buffer (Operational Transconductance Amplifier) Output
8
8
8
8
GND
Analog Ground. Connect to power ground (PGND) underneath the
IC.
9
9
9
9
REF
Internal Reference Bypass Terminal. Connect a 0.22µF ceramic
capacitor from REF to analog ground (GND). External load
capability up to 50µA.
10
10
—
—
FBP
Positive Charge-Pump Regulator Feedback Input. Regulates
to 1.25V nominal. Connect a resistive divider from the positive
charge-pump output (VPOS) to FBP to analog ground (GND).
11
11
—
—
FBN
Negative Charge-Pump Regulator Feedback Input. Regulates to
0V nominal. Connect a resistive divider from the negative chargepump output (VNEG) to FBN to the reference (REF).
12
12
10
10
SHDN
Active-Low Shutdown Control Input. Pull SHDN low to force the
controller into shutdown. If unused, connect SHDN to IN for normal
operation. A rising edge on SHDN clears the fault latch.
SUPL
Low-Dropout Linear Regulator Input Voltage. Can range from
4.5V to 15V. Bypass with a 1µF capacitor to GND (see Capacitor
Selection and Regulator Stability). Connect both input pins together
externally.
13
—
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11
—
Maxim Integrated │ 17
MAX1778/MAX1880–MAX1885
Quad-Output TFT LCD DC/DC
Converters with Buffer
Pin Description (continued)
PIN
MAX1778
MAX1881
MAX1880
MAX1882
MAX1883
MAX1884
MAX1885
NAME
FUNCTION
Linear Regulator Output. Sources up to 40mA. Bypass to GND with
a ceramic capacitor determined by:
14
—
12
—
LDOOUT
15
—
13
—
FBL
16
16
14
14
FLTSET
17
17
—
—
SUPN
Negative Charge-Pump Driver Supply Voltage. Bypass to power
ground (PGND) with a 0.1µF capacitor.
18
18
—
—
DRVN
Negative Charge-Pump Driver Output. Output high level is VSUPN
and low level is PGND.
19
19
—
—
SUPP
Positive Charge-Pump Driver Supply Voltage. Bypass to power
ground (PGND) with a 0.1µF capacitor.
20
20
—
—
DRVP
Positive Charge-Pump Driver Output. Output high level is VSUPP
and low level is PGND
21
21
17
17
PGND
Power Ground. Connect to analog ground (GND) underneath the
IC.
22
22
18
18
LX
Main Step-Up Regulator Power MOSFET N-Channel Drain. Place
output diode and output capacitor as close as possible to PGND.
23
23
19
19
TGND
24
24
20
20
RDY
Active-Low, Open-Drain Output. Indicates all outputs are ready.
On-resistance is 125Ω (typ).
—
13–15
15, 16
11–13,
15, 16
N.C.
No Connection. Not internally connected.
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ILDOOUT(MAX)
C LDOOUT ≥ 0.5ms X
VLDOOUT
Voltage Setting Input. Connect a resistive divider from the linear
regulator output (VLDOOUT) to FBL to analog ground (GND).
Fault Trip-Level Set Input. Connect to a resistive divider between
REF and GND to set the main step-up converter’s and positive
charge pump’s fault thresholds between 0.67 x VREF and 0.85 x
VREF. Connect to GND for the preset fault threshold (0.9 x VREF).
Must be connected to ground.
Maxim Integrated │ 18
MAX1778/MAX1880–MAX1885
INPUT
VIN = 3.3V
Quad-Output TFT LCD DC/DC
Converters with Buffer
L1
6.8µH
CIN
4.7µF
RRDY
100kΩ
TO LOGIC
LDO
VLDOOUT = 5V CLDO
4.7µF
C1
0.22µF
LX
SHDN
FB
RDY
SUPL
LDOOUT
MAIN
(8V)
R8
49.9kΩ
IN
SUPP
R7
150kΩ
C2
0.1µF
SUPB
SUPN
MAX1778
FBL
C3
1.0µF
R5
200kΩ
R6
49.9kΩ
CREF
0.22µF
FBN
R2
274kΩ
R2
49.9kΩ
C4
0.1µF
C5
1.0µF
DRVP
C4
0.1µF
DRVN
NEGATIVE
VNEG = -5V
MAIN
VMAIN = 8V
COUT
(2) 4.7µF
FBP
C7
1.0µF
POSITIVE
VPOS = 20V
R3
750kΩ
R4
49.9kΩ
REF
INTG
FLTSET
PGND
BUFOUT
BUFBUF+
GND
CBUF
1.0µF
BUFFER OUTPUT
VBUFOUT = VSUPB/2
TGND
Figure 1. Typical Application Circuit
Detailed Description
The MAX1778/MAX1880–MAX1885 are highly efficient
multiple-output power supplies for thin-film transistor
(TFT) liquid crystal display (LCD) applications. The
devices contain one high-power step-up converter, two
low-power charge pumps, an operational transconductance amplifier (VCOM buffer), and a low-dropout linear
regulator. The primary step-up converter uses an internal
N-channel MOSFET to provide maximum efficiency and
to minimize the number of external components. The output voltage of the main step-up converter (VMAIN) can be
set from VIN to 13V with external resistors.
The dual charge pumps (MAX1778/MAX1880–MAX1882
only) independently regulate a positive output (VPOS) and
a negative output (VNEG). These low-power outputs use
external diode and capacitor stages (as many stages as
required) to regulate output voltages from - 40V to +40V.
A unique control scheme minimizes output ripple as well
as capacitor sizes for both charge pumps.
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A resistor-programmable 40mA linear regulator (MAX1778/
MAX1881/MAX1883/MAX1884 only) can provide preregulation or postregulation for any of the supplies. For higher
current applications, an external transistor can be added.
Additionally, the VCOM buffer provides a high current
output that is ideal for driving capacitive loads, such as
the backplane of a TFT LCD panel. The positive feedback
input features dual-mode operation, allowing this input to
be connected to an internal 50% resistive-divider between
the buffer’s supply voltage and ground, or externally
adjusted for other voltages.
Also included in the MAX1778/MAX1880–MAX1885 is a
precision 1.25V reference that sources up to 50μA, logic
shutdown, soft-start, power-up sequencing, adjustable
fault detection, thermal shutdown, and an active-low,
open-drain ready output.
Maxim Integrated │ 19
MAX1778/MAX1880–MAX1885
Quad-Output TFT LCD DC/DC
Converters with Buffer
Main Step-up Controller
During normal pulse-width modulation (PWM) operation,
the MAX1778/MAX1880–MAX1885 main step-up controllers switch at a constant frequency of 500kHz or 1MHz
(see the Selector Guide), allowing the use of low-profile
inductors and output capacitors. Depending on the inputto-output voltage ratio, the controller regulates the output
voltage and controls the power transfer by modulating the
duty cycle (D) of each switching cycle:
V
-V
D ≈ MAIN IN
VMAIN
On the rising edge of the internal clock, the controller sets
a flip-flop when the output voltage is too low, which turns
on the n-channel MOSFET (Figure 2). The inductor current ramps up linearly, storing energy in a magnetic field.
Once the sum of the feedback voltage error amplifier,
slope-compensation, and current-feedback signals trip
the multi-input comparator, the MOSFET turns off, the flipflop resets, and the diode (D1) turns on. This forces the
current through the inductor to ramp back down, transferring the energy stored in the magnetic field to the output
capacitor and load. The MOSFET remains off for the rest
of the clock cycle.
Changes in the feedback voltage-error signal shift the
switch-current trip level, consequently modulating the
MOSFET duty cycle.
Under very light loads, an inherent switchover to pulseskipping takes place (Figure 3). When this occurs, the
controller skips most of the oscillator pulses in order to
reduce the switching frequency and gate charge losses.
When pulse-skipping, the step-up controller initiates a
new switching cycle only when the output voltage drops
too low. The n-channel MOSFET turns on, allowing the
inductor current to ramp up until the multi-input comparator trips. Then, the MOSFET turns off and the diode turns
on, forcing the inductor current to ramp down. When the
inductor current reaches zero, the diode turns off, so
the inductor stops conducting current. This forces the
threshold between pulse-skipping and PWM operation
to coincide with the boundary between continuous and
discontinuous inductor-current operation:
2
ILOAD(CROSSOVER) ≈
1 VIN VMAIN - VIN
2 VMAIN f OSCL
L1
MAX1778
MAX1880
MAX1881
MAX1882
MAX1883
MAX1884
MAX1885
CIN
LX
D1
VMAIN
(UP TO 13V)
S
R
PWM
COMPARATOR
VIN
(2.7V TO 5.5V)
OSC
(80% DUTY)
COUT
Q
ILIM
R1
PGND
ILIM
COMPARATOR
RCOMP
(OPTIONAL)
FB
gm
ERROR
AMPLIFIER
INTG
CCOMP
(OPTIONAL)
REF
VREF
1.25V
R2
CREF
GND
( )
VMAIN = 1 +R1
R2
VREF = 1.25V
VREF
CINTG
Figure 2. Main Step-Up Converter Block Diagram
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Maxim Integrated │ 20
MAX1778/MAX1880–MAX1885
Quad-Output TFT LCD DC/DC
Converters with Buffer
Dual Charge-Pump Regulator (MAX1778/
MAX1880–MAX1882 Only)
The switching waveforms appear noisy and asynchronous when light loading causes pulse-skipping operation;
this is a normal operating condition that improves lightload efficiency.
The MAX1778/MAX1880–MAX1882 controllers contain
two independent low-power charge pumps (Figure 4).
One charge pump inverts the input voltage and provides
a regulated negative output voltage. The second charge
pump doubles the input voltage and provides a regulated
positive output voltage. The controllers contain internal
p-channel and n-channel MOSFETs to control the power
transfer. The internal MOSFETs switch at a constant
frequency (fCHP = fOSC/2).
INDUCTOR CURRENT
IPEAK
Positive Charge Pump
ILOAD
tON
During the first half-cycle, the n-channel MOSFET turns
on and charges flying capacitor CX(POS) (Figure 4). This
initial charge is controlled by the variable n-channel onresistance. During the second half-cycle, the n-channel
MOSFET turns off and the p-channel MOSFET turns
on, level shifting CX(POS) by VSUPP volts. This connects CX(POS) in parallel with the reservoir capacitor
COUT(POS). If the voltage across COUT(POS) plus a diode
drop (VPOS + VDIODE) is smaller than the level-shifted flying capacitor voltage (VCX(POS) + VSUPP), charge flows
from CX(POS) to COUT(POS) until the diode (D3) turns off.
TIME
tOFF
Figure 3. Discontinuous-to-Continuous Conduction Crossover
Point
MAX1778
MAX1880
MAX1881
MAX1882
SUPP
VSUPP
2.7V TO 13V
SUPN
VSUPN
2.7V TO 13V
OSC
D2
VSUPD
CX(POS)
DRVP
DRVN
CX(NEG)
D3
D5
R3
VPOS
D4
FBP
R5
FBN
VNEG
COUT(POS)
COUT(NEG)
R4
R6
VREF
1.25V
REF
( )V
VPOS = 1 + R3
R4
VREF = 1.25V
REF
GND
PGND
CREF
0.22µF
( )
VNEG = - R5 VREF
R6
VREF = 1.25V
Figure 4. Low-Power Charge Pump Block Diagram
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Maxim Integrated │ 21
MAX1778/MAX1880–MAX1885
Quad-Output TFT LCD DC/DC
Converters with Buffer
Negative Charge Pump
feedback voltage is too low, the device increases the pass
transistor base current, which allows more current to pass
to the output and increases the output voltage. However,
the linear regulator also includes an output current limit to
protect the internal pass transistor against short circuits.
During the first half-cycle, the p-channel MOSFET turns
on, and flying capacitor CX(NEG) charges to VSUPN minus
a diode drop (Figure 4). During the second half-cycle, the
p-channel MOSFET turns off, and the n-channel MOSFET
turns on, level shifting CX(NEG). This connects CX(NEG) in
parallel with reservoir capacitor COUT(NEG). If the voltage
across COUT(NEG) minus a diode drop is greater than the
voltage across CX(NEG), charge flows from COUT(NEG)
to CX(NEG) until the diode (D5) turns off. The amount
of charge transferred to the output is controlled by the
variable n-channel on-resistance.
The low-dropout linear regulator monitors and controls the
pass transistor’s base current, limiting the output current
to 130mA (typ). In conjunction with the thermal overload
protection, this current limit protects the output, allowing
it to be shorted to ground for an indefinite period of time
without damaging the part.
VCOM Buffer
Low-Dropout Linear Regulator (MAX1778/
MAX1881/MAX1883/MAX1884 Only)
The MAX1778/MAX1880–MAX1885 include a VCOM
buffer, which uses an operational transconductance
amplifier (OTA) to provide a current output that is ideal for
driving capacitive loads, such as the backplane of a TFT
LCD panel. The unity-gain bandwidth of this currentoutput buffer is:
GBW = gm/COUT
The MAX1778/MAX1881/MAX1883/MAX1884 contain a
low-dropout linear regulator (Figure 5) that uses an
internal pnp pass transistor (QP) to supply loads up to
40mA. As illustrated in Figure 5, the 1.25V reference is
connected to the error amplifier, which compares this
reference with the feedback voltage and amplifies the
difference. If the feedback voltage is higher than the reference voltage, the controller lowers the base current of QP,
which reduces the amount of current to the output. If the
MAX1778
MAX1881
MAX1883
MAX1884
where gm is the amplifier’s transconductance. The bandwidth is inversely proportional to the output capacitor, so
large capacitive loads improve stability; however, lower
bandwidth decreases the buffer’s transient response time.
SUPL
CSUPL
VSUPL
4.5V TO 15V
THERMAL
SENSOR
CURRENT
LIMIT
QP
VLDOOUT
1.25V TO (VSUPL - 0.3V)
LDOOUT
R7
CLDOOUT
FBL
ERROR
AMPLIFIER
R8
VREF
1.25V
GND
(
VLDOOUT = 1 +
VREF = 1.25V
)
R7
VREF
R8
Figure 5. Low-Dropout Linear Regulator Block Diagram
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Maxim Integrated │ 22
MAX1778/MAX1880–MAX1885
MAX1778
MAX1880
MAX1881
MAX1882
MAX1883
MAX1884
MAX1885
Quad-Output TFT LCD DC/DC
Converters with Buffer
SUPB
gm
VSUPB
4.5V TO 13V
VBUFOUT
1.2V TO (VSUPB - 1.2V)
CBUF
BUFOUT
BUF-
R
R11
BUF+
R
125mV
R12
GND
VBUFOUT =
( R11R12+ R12)V
SUPB
Figure 6. VCOM Buffer Block Diagram
To improve the transient response times, the amplifier’s
transconductance increases as the output current increases (see the Typical Operating Characteristics).
A logic-low level on SHDN shuts down all of the converters and the reference. When shut down, the supply
current drops to 0.1μA to maximize battery life, and the
reference is pulled to ground. The output capacitance,
feedback resistors, and load current determine the rate
at which each output voltage decays. A logic-level high
on SHDN power activates the MAX1778/MAX1880–
MAX1885 (see the Power-Up Sequencing section). Do
not leave SHDN floating. If unused, connect SHDN to IN.
A logic-level transition on SHDN clears the fault latch.
put signal are not affected by the regulation of the linear
regulator. While the main step-up converter powers up,
the output of the PWM comparator remains low (Figure
2), and the step-up converter charges the output capacitors, limited only by the maximum duty cycle and currentlimit comparator. When the step-up converter approaches
its nominal regulation value and the PWM comparator’s
output changes states for the first time, the negative
charge pump turns on. When the negative output voltage
reaches approximately 90% of its nominal value (VFBN <
110mV), the positive charge pump starts up. Finally, when
the positive output voltage reaches 90% of its nominal
value (VFBP > 1.125V), the active-low ready signal (RDY)
goes low (see the Power Ready section), and the VCOM
buffer powers up. The MAX1883–MAX1885 do not contain the charge pumps, but the power-up sequence still
contains the charge pumps’ startup logic, which appears
as a delay (2 x 4096/fOSC) between the step-up converter
reaching regulation and when the ready signal and VCOM
buffer are activated.
Power-Up Sequencing
Soft-Start
The VCOM buffer’s positive feedback input features
dual mode operation. The buffer’s output voltage can be
internally set by a 50% resistive divider connected to the
buffer’s supply voltage (SUPB), or the output voltage can
be externally adjusted for other voltages.
Shutdown (SHDN)
Upon power-up or exiting shutdown, the MAX1778/
MAX1880–MAX1885 start a power-up sequence. First,
the reference powers up. Then, the main DC-DC step-up
converter powers up with soft-start enabled. The linear
regulator powers up at the same time as the main step-up
converter; however, the power sequence and ready out-
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For the main step-up regulator, soft-start allows a gradual
increase of the current-limit level during startup to reduce
input surge currents. The MAX1778/MAX1880–MAX1885
divide the soft-start period into four phases. During the
first phase, the controller limits the current limit to only
0.38A (see the Electrical Characteristics), approximately
Maxim Integrated │ 23
MAX1778/MAX1880–MAX1885
a quarter of the maximum current limit (ILX(MAX)). If
the output does not reach regulation within 1ms, softstart enters phase II, and the current limit is increased
by another 25%. This process is repeated for phase III.
The maximum 1.5A (typ) current limit is reached within
3072 clock cycles or when the output reaches regulation,
whichever occurs first (see the startup waveforms in the
Typical Operating Characteristics).
For the charge pumps (MAX1778/MAX1880–MAX1882
only), soft-start is achieved by controlling the rate of rise
of the output voltage. Both charge-pump output voltages are controlled to be in regulation within 4096 clock
cycles, regardless of output capacitance and load, limited
only by the charge pump’s output impedance. Although
the MAX1883–MAX1885 controllers do not include the
charge pumps, the soft-start logic still contains the 4096
clock cycle startup periods for both charge pumps.
Fault Trip Level (FLTSET)
The MAX1778/MAX1880–MAX1885 feature dual-mode
operation to allow operation with either a preset fault trip
level or an adjustable trip level for the step-up converter
and positive charge-pump outputs. Connect FLTSET to
GND to select the preset 0.9 x VREF fault threshold. The
fault trip level can also be adjusted by connecting a voltage-divider from REF to FLTSET (Figure 8). For greatest
accuracy, the total load on the reference (including current
through the negative charge-pump feedback resistors)
should not exceed 50μA so that VREF is guaranteed
to be in regulation (see the Electrical Characteristics).
Therefore, select R10 in the 100kΩ to 1MΩ range, and
calculate R9 with the following equation:
R9 = R10 [(VREF/VFLTSET) - 1]
where VREF = 1.25V, and VFLTSET can range from 0.67
x VREF to 0.85 x VREF. FLTSET’s input bias current has
a maximum value of 50nA. For 1% error, the current
through R10 should be at least 100 times the FLTSET
input bias current (IFLTSET).
Fault Condition
Once RDY is low, if the output of the main regulator
or either low-power charge pump falls below its fault
detection threshold, or if the input drops below its undervoltage threshold, then RDY goes high impedance and
all outputs shut down; however, the reference remains
active. After removing the fault condition, toggle shutdown
(below 0.8V) or cycle the input voltage (below 0.2V) to
clear the fault latch and reactivate the device.
The reference fault threshold is 1.05V. For the step-up
converter and positive charge-pump, the fault trip level is
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Quad-Output TFT LCD DC/DC
Converters with Buffer
set by FLTSET (see the Fault Trip Level (FLTSET) section). For the negative charge pump, the fault threshold
measured at the charge-pump’s feedback input (FBN) is
140mV (typ).
Power Ready (RDY)
RDY is an open-drain output. When the power-up
sequence for the main step-up converter and low-power
charge pumps has properly completed, the 14V MOSFET
turns on and pulls RDY low with a 125Ω (typ) onresistance. If a fault is detected on any of these three
outputs, the internal open-drain MOSFET appears as
a high impedance. Connect a 100kΩ pullup resistor
between RDY and IN for a logic-level output.
Voltage Reference (REF)
The voltage at REF is nominally 1.25V. The reference
can source up to 50μA with good load regulation (see
the Typical Operating Characteristics). Connect a 0.22μF
ceramic bypass capacitor between REF and GND.
Thermal-Overload Protection
Thermal-overload protection limits total power dissipation
in the MAX1778/MAX1880–MAX1885. When the junction
temperature exceeds TJ = +160°C, a thermal sensor activates the fault protection, which shuts down the controller,
allowing the IC to cool. Once the device cools down by
15°C, toggle shutdown (below 0.8V) or cycle the input
voltage (below 0.2V) to clear the fault latch and reactivate
the controller. Thermal-overload protection protects the
controller in the event of fault conditions. For continuous
operation, do not exceed the absolute maximum junctiontemperature rating of TJ = +150°C.
Operating Region and Power Dissipation
The MAX1778/MAX1880–MAX1885s’ maximum power
dissipation depends on the thermal resistance of the IC
package and circuit board, the temperature difference
between the die junction and ambient air, and the rate of
any airflow. The power dissipated in the device depends on
the operating conditions of each regulator and the buffer.
The step-up controller dissipates power across the internal n-channel MOSFET as the controller ramps up the
inductor current. In continuous conduction, the power dissipated internally can be approximated by:
2
2
I
V
1 V D
PSTEP−UP ≈ MAIN MAIN + IN
VIN
12 f OSCL
× R DS(ON)D
Maxim Integrated │ 24
MAX1778/MAX1880–MAX1885
where IMAIN includes the primary load current and the
input supply currents for the charge pumps (see the
Charge-Pump Input Power and Efficiency Considerations
section), linear regulator, and VCOM buffer.
The linear regulator generates an output voltage by dissipating power across an internal pass transistor, so the
power dissipation is simply the load current times the
input-to-output voltage differential:
PLDO(INT) = ILDO (VSUPL - VLDO )
When driving an external transistor, the internal linear
regulator provides the base drive current. Depending on
the external transistor’s current gain (β) and the maximum
load current, the power dissipated by the internal linear
regulator can still be significant:
I
=
PLDO(INT) LDO VSUPL - (VLDO + 0.7V )
β
= ILDOOUT (VSUPL - VLDOOUT )
The charge pumps provide regulated output voltages by
dissipating power in the low-side n-channel MOSFET, so
they could be modeled as linear regulators followed by
unregulated charge pumps. Therefore, their power dissipation is similar to a linear regulator:
PNEG = INEG (VSUPN - 2VDIODE )N - VNEG
PPOS
IPOS (VSUPP - 2VDIODE )N + VSUPD - VPOS
where N is the number of charge-pump stages, VDIODE
is the diodes’ forward voltage, and VSUPD is the positive
charge-pump diode supply (Figure 4).
The VCOM buffer’s power dissipation depends on the
capacitive load (CLOAD) being driven, the peak-to-peak
voltage change (VP-P) across the load, and the load’s
switching rate:
PBUF = VP - PC LOADf LOAD VSUPB
To find the total power dissipated in the device, the power
dissipated by each regulator and the buffer must be
added together:
Quad-Output TFT LCD DC/DC
Converters with Buffer
=
PMAX (TJ(MAX) − T A ) / (θ JB + θ BA )
where TJ - TA is the temperature difference between the
controller’s junction and the surrounding air, θJB (or θJC)
is the thermal resistance of the package to the board,
and θBA is the thermal resistance from the PCB to the
surrounding air.
Design Procedure
Main Step-Up Converter
Output-Voltage Selection
Adjust the output voltage by connecting a voltage-divider
from the output (VMAIN) to FB to GND (see the Typical
Operating Circuit). Select R2 in the 10kΩ to 50kΩ range.
Calculate R1 with the following equations:
R1 = R2 [(VMAIN/VREF) - 1]
where VREF = 1.25V. VMAIN can range from VIN to 13V.
Inductor Selection
Inductor selection depends upon the minimum required
inductance value, saturation rating, series resistance, and
size. These factors influence the converter’s efficiency,
maximum output load capability, transient response time,
and output-voltage ripple. For most applications, values
between 4.7μH and 22μH work best with the controller’s
switching frequency (Tables 1 and 2).
The inductor value depends on the maximum output load
the application must support, input voltage, output voltage, and switching frequency. With high inductor values,
the MAX1778/MAX1880–MAX1885 source higher output
currents, have less output ripple, and enter continuous
conduction operation with lighter loads; however, the
circuit’s transient response time is slower. On the other
hand, low-value inductors respond faster to transients,
remain in discontinuous conduction operation, and typically offer smaller physical size for a given series resistance and current rating. The equations provided here
include a constant LIR, which is the ratio of the peakto-peak AC inductor current to the average DC inductor
current. For a good compromise between the size of the
inductor, power loss, and output-voltage ripple, select an
LIR of 0.3 to 0.5. The inductance value is then given by:
=
PTOTAL PSTEP - UP + PLDO(INT)
L MIN
=
+ PNEG + PPOS + PBUF
VIN(MIN)
VMAIN
2
VMAIN - VIN(MIN) 1
η
IMAIN(MAX)f OSC LIR
The maximum allowed power dissipation is 975mW
(24-pin TSSOP)/879mW (20-pin TSSOP) or:
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Maxim Integrated │ 25
MAX1778/MAX1880–MAX1885
where η is the efficiency, fOSC is the oscillator frequency
(see the Electrical Characteristics), and IMAIN includes
the primary load current and the input supply currents for
the charge pumps (see the Charge-Pump Input Power
and Efficiency Considerations section), linear regulator,
and VCOM buffer. Considering the typical application
circuit, the maximum average DC load current (IMAIN(MAX))
is 300mA with an 8V output. Based on the above
equations and assuming 85% efficiency, the inductance
value is then chosen to be 4.7μH.
The inductor’s saturation current rating should exceed the
peak inductor current throughout the normal operating
range. The peak inductor current is then given by:
IMAIN(MAX)VMAIN LIR 1
1+
IPEAK
=
VIN(MIN)
2 η
Under fault conditions, the inductor current can reach up
to 1.85A (ILIM(MAX)), see the Electrical Characteristics).
However, the controller’s fast current-limit circuitry allows
the use of soft-saturation inductors while still protecting
the IC.
The inductor’s DC resistance can significantly affect
efficiency due to the power loss in the inductor. The power
loss due to the inductor’s series resistance (PLR) can be
approximated by the following equation:
I
X VMAIN
PLR ≅ R L MAIN
V
IN
2
where RL is the inductor’s series resistance. For best performance, select inductors with resistance less than the
internal n-channel MOSFET on-resistance (0.35Ω typ).
Use inductors with a ferrite core or equivalent. To
minimize radiated noise in sensitive applications, use a
shielded inductor.
Output Capacitor
Output capacitor selection depends on circuit stability and
output-voltage ripple. A 10μF ceramic capacitor works
well in most applications (Tables 1 and 2). Additional
feedback compensation is required (see the Feedback
Compensation section) to increase the margin for stability
by reducing the bandwidth further. In cases where the output capacitance is sufficiently large, additional feedback
compensation is not necessary.
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Quad-Output TFT LCD DC/DC
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Output-voltage ripple has two components: variations in
the charge stored in the output capacitor with each LX
pulse, and the voltage drop across the capacitor’s equivalent series resistance (ESR) caused by the current into
and out of the capacitor:
=
VRIPPLE VRIPPLE(C) + VRIPPLE(ESR)
VRIPPLE(ESR) ≈ IPEAKR ESR(COUT), AND
V
− VIN IMAIN
VRIPPLE(C) ≈ MAIN
V
MAIN
C OUT f OSC
where IPEAK is the peak inductor current (see the Inductor
Selection section). For ceramic capacitors, the outputvoltage ripple is typically dominated by VRIPPLE( C). The
voltage rating and temperature characteristics of the output capacitor must also be considered.
Feedback Compensation
For stability, add a pole-zero pair from FB to GND in the
form of a compensation resistor (RCOMP) in series with
a compensation capacitor (CCOMP), as shown in Figure
2. Select RCOMP to be half the value of R2, the low-side
feedback resistor.
Integrator Capacitor
The MAX1778/MAX1880–MAX1885 contain an internal
current integrator that improves the DC load regulation
but increases the peak-to-peak transient voltage (see
the load-transient waveforms in the Typical Operating
Characteristics). For highly accurate DC load regulation,
enable the current integrator by connecting a 470pF
(ƒOSC = 1MHz)/1000pF (ƒOSC = 500kHz) capacitor to
INTG. To minimize the peak-to-peak transient voltage
at the expense of DC regulation, disable the integrator
by connecting INTG to REF. When using the MAX1883–
MAX1885, connect a 100kΩ resistor to GND when disabling the integrator.
Input Capacitor
The input capacitor (CIN) in step-up designs reduces the
current peaks drawn from the input supply and reduces
noise injection. The value of CIN is largely determined by
the source impedance of the input supply. High source
impedance requires high input capacitance, particularly
as the input voltage falls. Since step-up DC-DC converters
act as “constant-power” loads to their input supply, input
current rises as input voltage falls. A good starting point is
to use the same capacitance value for CIN as for COUT.
Maxim Integrated │ 26
MAX1778/MAX1880–MAX1885
Quad-Output TFT LCD DC/DC
Converters with Buffer
Rectifier Diode
Use a Schottky diode with an average current rating equal
to or greater than the peak inductor current, and a voltage
rating at least 1.5 times the main output voltage (VMAIN).
Charge Pumps (MAX1778/ MAX1880/
MAX1881/MAX1882 Only)
Selecting the Number of Charge-Pump Stages
The number of charge-pump stages required to regulate
the output voltage depends on the supply voltage, output
voltage, load current, switching frequency, the diode’s forward voltage drop, and ceramic capacitor values.
For positive charge-pump outputs, the number of required
stages can be determined by:
VPOS - VSUPD
NPOS ≥
V
1.1(2V
R
I
)
+
DIODE
TX LOAD
SUPP
where VSUPD is the positive charge-pump diode supply
(Figure 4), VDIODE is the diode’s forward voltage drop,
and RTX is the charge pump’s output impedance. The
charge pump’s output impedance can be approximated
using the following equation:
1
R TX = 2(R PCH(ON) + R NCH(ON) ) +
C
f
X CHP
1
+
C
f
OUT CHP
where the charge pump’s switching frequency (fCHP) is
equal to 0.5 x fOSC, the p-channel MOSFET’s on-resistance (RPCH(ON)) is 10Ω, and the n-channel MOSFET’s
on-resistance (RNCH(ON)) is 4Ω (see the Electrical
Characteristics).
For negative charge-pump outputs, the number of required
stages can be determined by:
VNEG
NNEG ≥
VSUPN - 1.1(2VDROP + R TXILOAD )
where NNEG is rounded up to the nearest integer.
Table 1. MAX1778/MAX1880/MAX1883 Component Values (fOSC = 1MHz)
CIRCUIT 1
CIRCUIT 2
CIRCUIT 3
CIRCUIT 4
CIRCUIT 5
VIN
3.3V
3.3V
3.3V
5V
5V
VMAIN
9V
9V
9V
12V
12V
IMAIN(MAX)
100mA
200mA
200mA
220mA
220mA
VNEG
-5V
-5V
-5V
-5V
-5V
INEG
2mA
5mA
5mA
5mA
5mA
VPOS
24V
24V
24V
24V
24V
IPOS
2mA
5mA
5mA
5mA
5mA
L
2.2µH
4.7µH
4.7µH
6.8µH
6.8µH
IPEAK
>1A
>1A
>1A
>1A
>1A
COUT
4.7µF
10µF
20µF
10µF
20µF
R1
309kΩ
309kΩ
309kΩ
429kΩ
429kΩ
R2
49.9kΩ
49.9kΩ
49.9kΩ
49.9kΩ
49.9kΩ
RCOMP
None
None
39kΩ*
None
20kΩ*
CCOMP
None
None
100pF*
None
200pF*
*RCOMP and CCOMP are connected between the step-up converter’s output (VMAIN) and FB.
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Maxim Integrated │ 27
MAX1778/MAX1880–MAX1885
Quad-Output TFT LCD DC/DC
Converters with Buffer
Table 2. MAX1881/MAX1882/MAX1884/MAX1885 Component Values (fOSC = 500kHz)
CIRCUIT 6
CIRCUIT 7
CIRCUIT 8
CIRCUIT 9
VIN
3.3V
3.3V
3.3V
3.3V
VMAIN
9V
9V
9V
9V
IMAIN(MAX)
100mA
100mA
200mA
200mA
VNEG
-5V
-5V
-5V
-5V
INEG
2mA
2mA
5mA
5mA
VPOS
24V
24V
24V
24V
IPOS
2mA
2mA
5mA
5mA
L
4.7µH
10µH
10µH
10µH
IPEAK
>1A
>1A
>1A
>1A
COUT
4.7µF
10µF
10µF
20µF
R1
309kΩ
309kΩ
309kΩ
309kΩ
R2
49.9kΩ
49.9kΩ
49.9kΩ
49.9kΩ
RCOMP
None
None
None
20kΩ*
CCOMP
None
None
None
200pF*
*RCOMP and CCOMP are connected between the step-up converter’s output (VMAIN) and FB.
Table 3. Component Suppliers
SUPPLIER
PHONE
FAX
INDUCTORS
Coilcraft
847-639-6400
847-639-1469
Coiltronics
561-241-7876
561-241-9339
Sumida USA
847-956-0666
847-956-0702
TOKO
847-297-0070
847-699-1194
AVX
803-946-0690
803-626-3123
KEMET
408-986-0424
408-986-1442
SANYO
619-661-6835
619-661-1055
Taiyo Yuden
408-573-4150
408-573-4159
CAPACITORS
Charge-Pump Input Power and
Efficiency Considerations
The charge pumps in the MAX1778/MAX1880–MAX1882
provide regulated output voltages by controlling the voltage drop across the low-side n-channel MOSFET, so
they can be modeled as linear regulators followed by an
unregulated charge pump when determining the input
power requirements and efficiency.
The charge pump only provides charge to the output
capacitor during half the period (50% duty cycle), so the
input current is a function of the number of stages and the
load current:
=
I SUPP IPOS (N + 1)
for the positive charge pump, and:
DIODES
Central
Semiconductor
516-435-1110
516-435-1824
I=
SUPP IPOS (N + 1)
International
Rectifier
310-322-3331
310-322-3332
for the negative charge pump, where N is the number of
charge-pump stages.
Motorola
602-303-5454
602-994-6430
Nihon
847-843-7500
847-843-2798
Zetex
516-543-7100
516-864-7630
www.maximintegrated.com
The efficiency characteristics of the MAX1778/MAX1880–
MAX1882 regulated charge pumps are similar to a linear
regulator. It is dominated by quiescent current at low
Maxim Integrated │ 28
MAX1778/MAX1880–MAX1885
output currents and by the input voltage at higher output
currents (see the Typical Operating Characteristics). So
the maximum efficiency can be approximated by:
η
POS
≅
VPOS
VSUPD + VSUPPN
for the positive charge pump, and:
η
NEG
≅
VNEG
VSUPNN
for the negative charge pump, where VSUPD is the
positive charge pump’s diode supply (Figure 4).
Output-Voltage Selection
Adjust the positive output voltage by connecting a
voltage-divider from the output (VPOS) to FBP to GND
(see the Typical Operating Circuit). Adjust the negative
output voltage by connecting a voltage-divider from the
output (VNEG) to FBN to REF. Select R4 and R6 in the
50kΩ to 100kΩ range. Higher resistor values improve
efficiency at low output current but increase outputvoltage error due to the feedback input bias current. For the
negative charge pump, higher resistor values also reduce
the load on the reference, which should not exceed
50μA for greatest accuracy (including current through
the FLTSET resistors) to guarantee that VREF remains in
regulation (see the Electrical Characteristics). Calculate
the remaining resistors with the following equations:
R3 = R4 [(VPOS/VREF) - 1]
R5 = R6 |VNEG/VREF|
where VREF = 1.25V. VPOS can range from VSUPP to
40V, and VNEG can range from 0V to -40V.
Flying Capacitor
Increasing the flying capacitor (CX) value increases the
output current capability. Above a certain point, increasing
the capacitance has a negligible effect because the output current capability becomes dominated by the internal
switch resistance and the diode impedance. The flying
capacitor’s voltage rating must exceed the following:
VCXN(POS) > 1.5VSUPD + VSUPP (N -1)
for the positive charge pump, and:
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Quad-Output TFT LCD DC/DC
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VCXN(NEG) > 1.5(VSUPNN)
for the negative charge pump, where N is the stage
number in which the flying capacitor appears, and VSUPD
is the positive charge pump’s diode supply (Figure 4).
For example, the two-stage positive charge pump in the
typical application circuit (Figure 1) where VSUPP =
VSUPD = 8V contains two flying capacitors. The flying
capacitor in the first stage (C4) requires a voltage rating
over 12V. The flying capacitor in the second stage (C6)
requires a voltage rating over 24V.
Charge-Pump Output Capacitor
Increasing the output capacitance or decreasing the ESR
reduces the output ripple voltage and the peak-to-peak
transient voltage. With ceramic capacitors, the outputvoltage ripple is dominated by the capacitance value.
Use the following equation to approximate the required
capacitor value:
C OUT ≥
ILOAD
f CHP VRIPPLE
where fCHP is typically fOSC/2 (see the Electrical
Characteristics).
Charge-Pump Input Capacitor
Use a bypass capacitor with a value equal to or greater than
the flying capacitor. Place the capacitor as close as possible to the IC. Connect directly to power ground (PGND).
Charge-Pump Rectifier Diodes
Use Schottky diodes with a current rating equal to or
greater than two times the average charge-pump input
current, and a voltage rating at least 1.5 times VSUPP
for the positive charge pump and VSUPN for the negative
charge pump.
Low-Dropout Linear Regulator (MAX1778/
MAX1881/MAX1883/MAX1884 Only)
Output-Voltage Selection
Adjust the linear-regulator output voltage by connecting a
voltage-divider from LDOOUT to FBL to GND (Figure 5).
Select R8 in the 5kΩ to 50kΩ range. Calculate R7 with the
following equation:
R7 = R8 [(VLDOOUT/VFBL) - 1]
where VFBL = 1.25V, and VLDOOUT can range from 1.25V
to (VSUPL - 300mV). FBL’s input bias current is 0.8μA
(max). For less than 0.5% error due to FBL input bias
current (IFBL), R8 must be less than 8kΩ.
Maxim Integrated │ 29
MAX1778/MAX1880–MAX1885
Capacitor Selection and Regulator Stability
Capacitors are required at the input and output of
the MAX1778/MAX1881/MAX1883/MAX1884 for stable
operation over the full temperature range and with
load currents up to 40mA. Connect a 1μF input bypass
capacitor (CSUPL) between SUPL and ground to lower
the source impedance of the input supply. Connect a
ceramic capacitor between LDOOUT and ground, using
the following equation to determine the lowest value
required for stable operation:
ILDOOUT(MAX)
C LDOOUT ≥ 0.5ms X
VLDOOUT
For example, with a 5V linear regulator output voltage
and a maximum 40mA load, use at least 4μF of output
capacitance. Applications that experience high-current
load pulses may require more output capacitance.
The ESR of the linear regulator’s output capacitor
(CLDOOUT) affects stability and output noise. Use output
capacitors with an ESR of 0.1Ω or less to ensure stability
and optimum transient response. Surface-mount ceramic
capacitors are good for this purpose. Place CSUPL and
CLDOOUT as close as possible to the linear regulator to
minimize the impact of PCB trace inductance.
External Pass Transistor
Quad-Output TFT LCD DC/DC
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For stable operation, place a capacitor (CLDOOUT) and
a minimum load resistor (R5) at the output of the internal
linear regulator (the base of the external transistor) to set
the dominant pole:
1
C LDOOUT ≥ 0.5ms
VLDO
V
+ 0.7V ILOAD(MAX)
x LDO
+
R5
β MIN
Since the LDO cannot sink current, a minimum pulldown resistor (R5) is required at the base of the npn
transistor to sink leakage currents and improve the highto-low load-transient response. Under no-load conditions,
leakage currents from the internal pass transistor supply
the output capacitor (CLDOOUT), even when the transistor
is off. As the leakage currents increase over temperature,
charge can build up on CLDOOUT, making the linear
regulator’s output rise above its set point. Therefore, R5
must sink at least 100μA to guarantee proper regulation.
Additionally, the minimum load current provided by R5
improves the high-to-low load transients by lowering the
impedance seen by CLDOOUT after the transient occurs.
Therefore, if large load transients are expected, select R5
so that the minimum load current is 10% of the transistor’s
maximum base current:
For applications where the linear regulator currents
(V
VLDO + 0.7V
+ 0.7V)β MIN
exceed 40mA or where the power dissipation in the IC=
R5 =
0.1 LDO
ILDOOUT(MIN)
ILOAD(MAX)
needs to be reduced, an external npn transistor can be
used. In this case, the internal LDO only provides the
Alternatively, output capacitance placed on the external
necessary base drive while the external npn transistor
linear regulator’s output (the emitter) adds a second pole
supports the load, so most of the power dissipation occurs
that could destabilize the regulator. A capacitive-divider
across the external transistor’s collector and emitter.
from the transistor’s base to the feedback input (C2 and
Selection of the external npn transistor is based on three
C3, Figure 7) circumvents this second pole by adding a
factors: the package’s power dissipation, the current
pole-zero pair. Furthermore, to minimize excessive overgain (β), and the collector-to-emitter saturation voltage
shoot, the capacitive-divider’s ratio must be the same as
(VCE(SAT)). First, the maximum power dissipation should
the resistive-divider’s ratio. Once the output capacitor is
not exceed the transistor’s package rating:
selected, using the following equations to determine the
required capacitive-divider values:
=
P (VCOLLECTOR − VLDO ) x ILOAD(MAX)
Once the appropriate package type is selected, consider
the npn transistor’s current gain. Since the internal LDO
cannot source more than 40mA (min), the transistor’s
current gain must be high enough at the lowest collectorto-emitter voltage to support the maximum output load:
β MIN ≥
C LDO R4
1+
100 R3
VREF
C2
R4
= =
C2 + C3 R3 + R4 VLDO
C2 + C3 ≥
ILOAD(MAX) - 40mA
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40mA
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MAX1778/MAX1880–MAX1885
Quad-Output TFT LCD DC/DC
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Input-to-Output (Dropout) Voltage and Startup
A linear regulator’s minimum input-to-output voltage
differential (dropout voltage) determines the lowest
useable supply voltage. Because the MAX1778/MAX1881/
MAX1883/MAX1884 use an internal pnp transistor (or
external npn transistor), their dropout voltage is a function
of the transistor’s collector-to-emitter saturation voltage
(see the Typical Operating Characteristics). The linear
regulator’s quiescent current increases when in dropout.
The internal linear regulator tries to start up once its
supply voltage (VSUPL) exceeds 4V. When the linear
regulator powers up, the linear regulator may be in
dropout if the linear regulator’s output set voltage is
higher than its input supply voltage. Therefore, during this
brief period, the linear regulator draws additional supply
current until the input supply voltage exceeds the output
set voltage plus the pass transistor’s saturation voltage
(VLDO(SET) + VCE(SAT)).
VCOM Buffer (Operational Transconductance
Amplifier)
Buffer Output Voltage and Capacitor Selection
The positive input (BUF+) features dual-mode operation.
Connect BUF+ to GND for the preset VSUPB/2 output
voltage, set by an internal 50% resistive-divider. Adjust
the amplifier’s output voltage by connecting a voltage-
INPUT
VIN = 3.3V
divider from SUPB to BUF+ to GND (Figure 6). Select
R12 in the 10kΩ to 100kΩ range. Calculate R11 with the
following equation:
V
R11 = R12 SUPB - 1
V
BUF+
where VSUPB can range from 4.5V to 13V, and VBUF+
can range from 1.2V to (VSUPB - 1.2V). Connect a
minimum 1μF ceramic capacitor from BUFOUT to ground.
PCB Layout and Grounding
Careful PCB layout is extremely important for proper
operation. Follow the following guidelines for good PCB
layout:
1) Place the main step-up converter output diode and
output capacitor less than 0.2in (5mm) from the LX
and PGND pins with wide traces and no vias.
2) Separate analog ground and power ground. The
ground connections for the step-up converter’s and
charge pump’s input and output capacitors should be
connected to the power ground plane. The linear regulator’s and VCOM buffer’s input and output capacitors
should be connected to a separate power-ground
path, star-connected to the PGND pin to minimize
voltage drops. When using multi-layer boards, the top
L1
6.8µH
CIN
4.7µF
LX
IN
C1
0.22µF
SHDN
MAX1778
MAX1883
(MAX1881)*
(MAX1884)*
INTG
CREF
0.22µF
R1
274kΩ
FB
R2
49.9kΩ
SUPL
LDOOUT
R5
1.5kΩ
REF
FBL
PGND
MAIN
VMAIN = 8V
COUT
(2) 4.7µF
GND
CLDOIN
1µF
CLDOOUT
4.7µF
C2
0.01µF
C3
0.01µF
Q1
R3
49.9kΩ
CLDO
1µF
LDO
VLDO = 2.5V
R4
49.9kΩ
Figure 7. External Linear Regulator
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Maxim Integrated │ 31
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Quad-Output TFT LCD DC/DC
Converters with Buffer
layer should contain the boost regulator and chargepump power ground plane, and the inner layer should
contain the analog ground plane and power-ground
plane/path for the VCOM buffer and LDO. Connect all
three ground planes together at one place near the
PGND pin.
charging nodes on the top layer and high-impedance
nodes on the bottom layer. The fast-charging nodes,
such as the LX and charge-pump driver nodes, should
not have any other traces or ground planes near by.
5) Keep the charge-pump circuitry as close as possible
to the IC, using wide traces and avoiding vias when
possible. Place 0.1μF ceramic bypass capacitors near
the charge-pump input pins (SUPP and SUPN) to the
PGND pin.
3) Locate all feedback resistive-dividers as close as possible to their respective feedback pins. The voltagedivider’s center trace should be kept short. Avoid running any feedback trace near the LX switching node or
the charge-pump drivers. The resistive-dividers’ ground
connections should be to analog ground (GND).
6) To maximize output power and efficiency and minimize
output ripple voltage, use extra-wide, power-ground
traces, and solder the IC’s power-ground pin directly
to it.
4) When using multilayer boards, separate the top signal
layer and bottom signal layer with a ground plane
between to eliminate capacitive coupling between fast-
INPUT
VIN = 5V
Refer to the MAX1778/MAX1880–MAX1885 evaluation
kit for an example of proper board layout.
L1
10µH
CIN
(2) 4.7µF
C1
0.22µF
RRDY
100kΩ
TO LOGIC
IN
LX
SHDN
FB
RDY
SUPL
Q1
LDO
VLDO = 3.3V
C6
1µF
R8
10kΩ
LDOOUT
CLDOOUT
4.7µF
R7
16.4kΩ
C7
0.01µF
NEGATIVE
VNEG = -8V
R1
86.6kΩ
R8
1.5kΩ
MAIN
VMAIN = 12V
CCOMP
470pF
R2
10kΩ
SUPB
SUPN
SUPP
C4
0.1µF
MAX1778
DRVP
C6
0.01µF
FBL
FBP
R4
49.9kΩ
DRVN
C2
0.1µF
C3
1.0µF
RCOMP
4.7kΩ
COUT
(2) 10µF
FBN
R5
316kΩ
R6
49.9kΩ
REF
CREF
0.22µF
INTG
PGND
BUFOUT
BUF-
R3
750kΩ
CBUF
1.0µF
FLTSET
BUF+
GND
TGND
C5
1.0µF
POSITIVE
VPOS = 20V
BUFFER OUTPUT
VBUFOUT = VSUPB/2
R9
30kΩ
REF
R10
100kΩ
Figure 8. 5V Input Monitor Application
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Maxim Integrated │ 32
MAX1778/MAX1880–MAX1885
Quad-Output TFT LCD DC/DC
Converters with Buffer
Applications Information
with output voltages above 10V. Additionally, physically
larger inductors with less series resistance and higher
saturation ratings provide more output current and higher
efficiency.
Low-Profile Components
Notebook applications generally require low-profile components, potentially limiting the circuit’s performance. For
example, low-profile inductors typically have lower saturation ratings and more series resistance, limiting output
current and efficiency. Low-profile capacitors have lower
voltage ratings for a given capacitance value, so 3.3μF
low-profile capacitors with voltage ratings greater than
10V were not available at the time of publication.
Input Voltage Above and
Below the Output Voltage
Combining the step-up converter and linear regulator
as shown in Figure 9 provides output-voltage regulation above and below the input voltage. Supplied by the
step-up converter, the linear regulator output provides a
constant output voltage (VLDO). When the input voltage
exceeds the main step-up converter’s nominal output voltage, the controller stops switching but the linear regulator maintains the output voltage. When the input voltage
drops below the output voltage, the step-up converter
Desktop Monitors
Monitor applications do not have the same component
height restrictions associated with laptops, allowing more
flexibility in component selection (Figure 8). Larger output
capacitors with higher voltage ratings allow configurations
L1
6.8µH
POWER INPUT
VBATT = 10V TO 15V
CIN
4.7µF
INPUT
VIN = 3.3V TO 5V
C1
0.1µF
LX
IN
SHDN
RRDY
100kΩ
TO LOGIC
SUPL
BUFOUT
CBUF
1.0µF
BUFC2
0.1µF
C3
1.0µF
LDOOUT
FBN
R5
475kΩ
SUPB
SUPN
SUPP
CINTG
470pF
R7
470kΩ
R9
6.8kΩ
LDO
VLDO = 13V
CLDO
(2) 3.3µF
R8
49.9kΩ
C4
0.1µF
REF
DRVP
R9
30kΩ
R10
100kΩ
C7
0.1µF
FBL
R6
49.9kΩ
CREF
0.22µF
Q1
CLDOOUT
3.3µF
C6
0.1µF
MAX1778
DRVN
NEGATIVE
VNEG = -12V
R2
49.9kΩ
FB
RDY
BUFFER OUTPUT
VBUFOUT = VSUPB/2
COUT
(3) 3.3µF
R1
511kΩ
FLTSET
INTG
PGND
FBP
BUF+
GND
TGND
R4
49.9kΩ
R3
909kΩ
POSITIVE
VPOS = 24V
C5
1.0µF
Figure 9. Input Voltage Above and Below the Output Voltage
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Maxim Integrated │ 33
MAX1778/MAX1880–MAX1885
INPUT
VIN = 3.3V
Quad-Output TFT LCD DC/DC
Converters with Buffer
L1
6.8µH
CIN
4.7µF
LX
IN
C1
0.22µF
STARTUP MAIN
VMAIN(START) = 8V
SHDN
C8
3.3µF
COUT
(2) 3.3µF
R1
274kΩ
FB
R2
49.9kΩ
SUPP
MAX1778
C10
0.1µF
C4
0.1µF
C5
1.0µF
DRVP
C6
0.1µF
INTG
CREF
0.22µF
SYSTEM MAIN
VMAIN(SYS) = 8V
R7
10kΩ
C7
1.0µF
REF
R9
30kΩ
R3
750kΩ
FBP
FLTSET
R4
49.9kΩ
R10
100kΩ
RDY
TGND
PGND
GND
STARTUP
POSITIVE
VPOS(START) = 20V
RRDY
5.1kΩ
Q3
Q2
SYSTEM
POSITIVE
VPOS(SYS) = 20V
INPUT
VIN = 3.3V
R8
100kΩ
Figure 10. Power-Up Sequencing and Fault Protection
steps up the input voltage so that the linear regulator will
not drop out. Therefore, to guarantee that the external
pass transistor does not saturate, the step-up converter’s
output voltage must be set above the linear regulator’s
output voltage plus the transistor’s saturation rating
(VMAIN ≥ VLDO + VSAT).
Power-Up Sequencing and Fault Protection
The MAX1778/MAX1880–MAX1885’s fault protection cannot be activated until the power-up sequence
is successfully completed and the power-ready output goes low. Therefore, faults on the main output or
positive charge-pump output could damage the controller
or external components. Additional fault protection can be
added as shown in Figure 10. The external MOSFET and
pnp transistor isolate the positive outputs during startup.
When the controller finishes the power-up sequence,
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the power-ready output goes low, turning on the pnp
transistor. Any fault on the positive charge-pump output
pulls down the charge pump’s output voltage and triggers
the fault protection; otherwise, the MOSFET’s gate slow
charges. Once the MOSFET turns on, any faults on the
main step-up converter’s output pull down the main output
voltage and trigger the fault protection.
VCOM Buffer Startup
The VCOM buffer does not include soft-start. Therefore,
once the VCOM buffer turns on, it draws high surge currents while charging the output capacitance. In some
applications, the buffer’s high startup surge current could
potentially trip the fault-detection circuit, forcing the controller to shut down. In these cases, adding a soft-start
resistive-divider between SUPB and BUFOUT reduces the
startup surge current and voltage drops associated with
Maxim Integrated │ 34
MAX1778/MAX1880–MAX1885
INPUT
VIN = 3.3V
Quad-Output TFT LCD DC/DC
Converters with Buffer
L1
6.8µH
CIN
4.7µF
LX
IN
C1
0.22µF
SHDN
CREF
0.22µF
R1
274kΩ
FB
R2
49.9kΩ
MAX1778
INTG
SUPB
REF
BUF-
R3
10kΩ
BUFOUT
BUF+
PGND
MAIN
VMAIN = 8V
COUT
(2) 4.7µF
GND
R4
10kΩ
CSUPB
1.0µF
BUFFER OUTPUT
VBUFOUT = VSUPB/2
CBUF
1.0µF
[( VV
R3 = R4
SUPB
BUFOUT
) -1]
Figure 11. VCOM Buffer Soft-Start
this load (Figure 11), as shown in the Typical Operating
Characteristics. Set the resistive divider to precharge
BUFOUT, matching the buffer’s output set voltage:
V
=
R3 R4 SUPB − 1
VBUFOUT
These resistor values are selected to charge the output
capacitor close to the output set voltage before the buffer
starts up:
C BUFOUT (R3 || R4) ≈
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5000
f OSC
Selector Guide
PART
STEP-UP
SWITCHING
FREQUENCY
(Hz)
DUAL
CHARGE
PUMPS
LINEAR
REGULATOR
MAX1778
1M
Yes
Yes
MAX1880
1M
Yes
No
MAX1881
500k
Yes
Yes
MAX1882
500k
Yes
No
MAX1883
1M
No
Yes
MAX1884
500k
No
Yes
MAX1885
500k
No
No
Maxim Integrated │ 35
MAX1778/MAX1880–MAX1885
Quad-Output TFT LCD DC/DC
Converters with Buffer
Typical Operating Circuit
MAIN
INPUT
TO LOGIC
LDO OUTPUT
IN
LX
SHDN
FB
RDY
SUPL
LDOOUT
SUPB
SUPN
SUPP
FBL
MAX1778
DRVN
NEGATIVE
FBP
POSITIVE
FBN
REF
INTG
PGND
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DRVP
BUFOUT
BUF-
BUFFER OUTPUT
BUF+
FLTSET
GND
TGND
Maxim Integrated │ 36
MAX1778/MAX1880–MAX1885
Quad-Output TFT LCD DC/DC
Converters with Buffer
Pin Configurations
TOP VIEW
FB 1
+
24 RDY
INTG 2
23 TGND
IN 3
22 LX
BUF+ 4
FB 1
+
INTG 2
23 TGND
IN 3
22 LX
21 PGND
BUF+ 4
20 DRVP
BUF- 5
19 SUPP
SUPB 6
BUFOUT 7
18 DRVN
BUFOUT 7
18 DRVN
GND 8
17 SUPN
GND 8
17 SUPN
REF 9
16 FLTSET
REF 9
16 FLTSET
FBP 10
15 FBL
FBP 10
15 N.C.
FBN 11
14 LDOOUT
FBN 11
14 N.C.
SHDN 12
13 N.C.
MAX1778
MAX1881
BUF- 5
SUPB 6
SHDN 12
13 SUPL
21 PGND
MAX1880
MAX1882
TSSOP
TOP VIEW
FB 1
+
19 TGND
18 LX
IN 3
BUF+ 4
BUF- 5
MAX1883
MAX1884
SUPB 6
BUFOUT 7
20 DRVP
19 SUPP
TSSOP
20 RDY
INTG 2
17 PGND
FB 1
+
20 RDY
19 TGND
INTG 2
18 LX
IN 3
BUF+ 4
16 N.C.
BUF- 5
15 N.C.
SUPB 6
MAX1885
17 PGND
16 N.C.
15 N.C.
14 FLTSET BUFOUT 7
14 FLTSET
GND 8
13 FBL
GND 8
13 N.C.
REF 9
12 LDOOUT
REF 9
12 N.C.
SHDN 10
11 N.C.
SHDN 10
11 SUPL
TSSOP
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24 RDY
TSSOP
Maxim Integrated │ 37
MAX1778/MAX1880–MAX1885
Chip Information
TRANSISTOR COUNT: 3739
www.maximintegrated.com
Quad-Output TFT LCD DC/DC
Converters with Buffer
Package Information
For the latest package outline information and land patterns
(footprints), go to www.maximintegrated.com/packages. Note
that a “+”, “#”, or “-” in the package code indicates RoHS status
only. Package drawings may show a different suffix character, but
the drawing pertains to the package regardless of RoHS status.
PACKAGE
TYPE
PACKAGE
CODE
OUTLINE
NO.
LAND
PATTERN NO.
20 TSSOP
U20-2
21-0066
90-0116
24 TSSOP
U24-1
21-0066
90-0118
Maxim Integrated │ 38
MAX1778/MAX1880–MAX1885
Quad-Output TFT LCD DC/DC
Converters with Buffer
Revision History
REVISION
NUMBER
REVISION
DATE
2
10/12
Added MAX1880EUG/V+ to Ordering Information
1
3
4/15
Deleted MAX1880EUG/V+ from Ordering Information
1
DESCRIPTION
PAGES
CHANGED
For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642, or visit Maxim Integrated’s website at www.maximintegrated.com.
Maxim Integrated cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim Integrated product. No circuit patent licenses
are implied. Maxim Integrated reserves the right to change the circuitry and specifications without notice at any time. The parametric values (min and max limits)
shown in the Electrical Characteristics table are guaranteed. Other parametric values quoted in this data sheet are provided for guidance.
Maxim Integrated and the Maxim Integrated logo are trademarks of Maxim Integrated Products, Inc.
© 2015 Maxim Integrated Products, Inc. │ 39
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