19-2224; Rev 3; 12/03
500kHz Multi-Output, Low-Noise Power-Supply
Controllers for Notebook Computers
The MAX1901/MAX1902/MAX1904 include two PWM regulators, adjustable from 2.5V to 5.5V with fixed 5.0V and
3.3V modes. All these devices include secondary feedback regulation, and the MAX1902 contains a 12V/120mA
linear regulator. The MAX1901/MAX1904 include a secondary feedback input (SECFB), plus a control pin
(STEER) that selects which PWM (3.3V or 5V) receives the
secondary feedback signal. SECFB provides a method
for adjusting the secondary winding voltage regulation
point with an external resistor divider, and is intended to
aid in creating auxiliary voltages other than fixed 12V.
The MAX1901/MAX1902 contain internal output overvoltage and undervoltage protection features.
Features
♦
♦
♦
♦
♦
♦
♦
♦
♦
♦
♦
♦
♦
♦
♦
♦
97% Efficiency
4.2V to 30V Input Range
2.5V to 5.5V Dual Adjustable Outputs
Selectable 3.3V and 5V Fixed or Adjustable
Outputs (Dual Mode™)
12V Linear Regulator
Adjustable Secondary Feedback
(MAX1901/MAX1904)
5V/50mA Linear Regulator Output
Precision 2.5V Reference Output
Programmable Power-Up Sequencing
Power-Good (RESET) Output
Output Overvoltage Protection
(MAX1901/MAX1902)
Output Undervoltage Shutdown
(MAX1901/MAX1902)
333kHz/500kHz Low-Noise, Fixed-Frequency
Operation
Low-Dropout, 98% Duty-Factor Operation
2.5mW Typical Quiescent Power (12V input, both
SMPSs on)
4µA Typical Shutdown Current
Ordering Information
PART
TEMP RANGE
PIN-PACKAGE
MAX1901EAI
-40°C to +85°C
28 SSOP
MAX1901ETJ
-40°C to +85°C
32 Thin QFN 5mm x 5mm
MAX1902EAI
-40°C to +85°C
28 SSOP
MAX1902ETJ
-40°C to +85°C
32 Thin QFN 5mm x 5mm
MAX1904EAI
-40°C to +85°C
28 SSOP
MAX1904ETJ
-40°C to +85°C
32 Thin QFN 5mm x 5mm
Functional Diagram
________________________Applications
INPUT
Notebook and Subnotebook Computers
PDAs and Mobile Communicators
12V
5V (RTC)
Desktop CPU Local DC-DC Converters
3.3V
Idle Mode is a trademark of Maxim Integrated Products, Inc.
Dual Mode is a trademark of Maxim Integrated Products, Inc.
ON/OFF
5V
LINEAR
12V
LINEAR
3.3V
SMPS
5V
SMPS
POWER-UP
SEQUENCE
POWERGOOD
5V
RESET
Pin Configurations appear at end of data sheet.
________________________________________________________________ Maxim Integrated Products
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at
1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.
1
MAX1901/MAX1902/MAX1904
General Description
The MAX1901/MAX1902/MAX1904 are buck-topology,
step-down, switch-mode, power-supply controllers that
generate logic-supply voltages in battery-powered systems. These high-performance, dual/triple-output devices
include on-board power-up sequencing, power-good signaling with delay, digital soft-start, secondary winding
control, low dropout circuitry, internal frequency-compensation networks, and automatic bootstrapping.
Up to 97% efficiency is achieved through synchronous
rectification and Maxim’s proprietary Idle Mode™ control
scheme. Efficiency is greater than 80% over a 1000:1
load-current range, which extends battery life in system
suspend or standby mode. Excellent dynamic response
corrects output load transients within five clock cycles.
Strong 1A on-board gate drivers ensure fast external Nchannel MOSFET switching.
These devices feature a logic-controlled and synchronizable, fixed-frequency, pulse-width modulation (PWM)
operating mode. This reduces noise and RF interference
in sensitive mobile communications and pen-entry applications. Asserting the SKIP pin enables fixed-frequency
mode, for lowest noise under all load conditions.
MAX1901/MAX1902/MAX1904
500kHz Multi-Output, Low-Noise Power-Supply
Controllers for Notebook Computers
ABSOLUTE MAXIMUM RATINGS
V+ to GND ..............................................................-0.3V to +36V
PGND to GND.....................................................................±0.3V
VL to GND ................................................................-0.3V to +6V
BST3, BST5 to GND ..............................................-0.3V to +36V
CSH3, CSH5 to GND................................................-0.3V to +6V
FB3 to GND ..............................................-0.3V to (CSL3 + 0.3V)
FB5 to GND ...............................................-0.3V to (CSL5 +0.3V)
LX3 to BST3..............................................................-6V to +0.3V
LX5 to BST5..............................................................-6V to +0.3V
REF, SYNC, SEQ, STEER, SKIP,
TIME/ON5, SECFB, RESET to GND ........-0.3V to (VL + 0.3V)
VDD to GND. ...........................................................-0.3V to +20V
RUN/ON3, SHDN to GND.............................-0.3V to (V+ + 0.3V)
12OUT to GND ..........................................-0.3V to (VDD + 0.3V)
DL3, DL5 to PGND........................................-0.3V to (VL + 0.3V)
DH3 to LX3 ..............................................-0.3V to (BST3 + 0.3V)
DH5 to LX5 ..............................................-0.3V to (BST5 + 0.3V)
VL, REF Short to GND ................................................Momentary
12OUT Short to GND..................................................Continuous
REF Current...........................................................+5mA to -1mA
VL Current.........................................................................+50mA
12OUT Current . .............................................................+200mA
VDD Shunt Current. ...........................................................+15mA
Continuous Power Dissipation (TA = +70°C)
28-Pin SSOP (derate 9.52mW/°C above +70°C) ......762mW
32-Pin Thin QFN (derate 21.3mW/°C above +70°C) ..1702mW
Operating Temperature Range ...........................-40°C to +85°C
Storage Temperature Range ............................-65°C to +160°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
(V+ = 15V, both PWMs on, SYNC = VL, VL load = 0, REF load = 0, SKIP = 0, TA = 0°C to +85°C, unless otherwise noted. Typical
values are at TA = +25°C.)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
30.0
V
MAIN SMPS CONTROLLERS
Input Voltage Range
4.2
3V Output Voltage in Adjustable Mode
V+ = 4.2V to 30V, CSH3 - CSL3 = 0,
CSL3 tied to FB3
2.42
2.5
2.58
V
3V Output Voltage in Fixed Mode
V+ = 4.2V to 30V, 0 < CSH3 - CSL3
< 80mV, FB3 = 0
3.20
3.39
3.47
V
5V Output Voltage in Adjustable Mode
V+ = 4.2V to 30V, CSH5 - CSL5 = 0,
CSL5 tied to FB5
2.42
2.5
2.58
V
5V Output Voltage in Fixed Mode
V+ = 5.3V to 30V, 0 < CSH5 - CSL5
< 80mV, FB5 = 0
4.85
5.13
5.25
V
Output Voltage Adjust Range
Either SMPS
REF
5.5
V
Adjustable-Mode Threshold Voltage
Dual-mode comparator
0.5
1.1
V
Load Regulation
Either SMPS, 0 < CSH_ - CSL_ < 80mV
Line Regulation
Either SMPS, 5.2V < V+ < 30V
-2
%
0.03
%/V
CSH3 - CSL3 or CSH5 - CSL5
80
100
120
SKIP = VL or VDD < 13V or SECFB < 2.44V
-50
-100
-150
Idle Mode Threshold
SKIP = 0, not tested
10
25
40
Soft-Start Ramp Time
From enable to 95% full current limit with
respect to fOSC (Note 1)
Current-Limit Threshold
Oscillator Frequency
2
512
mV
clks
SYNC = VL
450
500
550
SYNC = 0
283
333
383
_______________________________________________________________________________________
mV
kHz
500kHz Multi-Output, Low-Noise Power-Supply
Controllers for Notebook Computers
(V+ = 15V, both PWMs on, SYNC = VL, VL load = 0, REF load = 0, SKIP = 0, TA = 0°C to +85°C, unless otherwise noted. Typical
values are at TA = +25°C.)
PARAMETER
MIN
TYP
95
97
SYNC = 0 (Note 2)
96.5
98
SYNC Input High-Pulse Width
Not tested
200
SYNC Input Low-Pulse Width
Not tested
200
SYNC Rise/Fall Time
Not tested
Maximum Duty Factor
CONDITIONS
SYNC = VL
SYNC Input Frequency Range
Current-Sense Input Leakage Current
UNITS
%
ns
ns
400
V+ = VL = 0,
CSL3 = CSH3 = CSL5 = CSH5 = 5.5V
MAX
0.01
200
ns
583
kHz
10
µA
FLYBACK CONTROLLER
VDD Regulation Threshold
Falling edge (Note 3)
SECFB Regulation Threshold
Falling edge (MAX1901/MAX1904)
13
14
V
2.44
2.60
V
DL Pulse Width
VDD < 13V or SECFB < 2.44V
VDD Shunt Threshold
Rising edge, hysteresis = 1% (Note 3)
18
VDD Shunt Sink Current
VDD = 20V (Note 3)
10
VDD Leakage Current
VDD = 5V, off mode (Notes 3, 4)
0.75
µs
20
V
mA
30
µA
12.50
V
12V LINEAR REGULATOR (Note 3)
12OUT Output Voltage
13V < VDD < 18V, 0 < ILOAD < 120mA
11.65
12.10
12OUT Current Limit
12OUT forced to 11V, VDD = 13V
150
Quiescent VDD Current
VDD = 18V, run mode, no 12OUT load
50
mA
100
µA
5.1
V
INTERNAL REGULATOR AND REFERENCE
SHDN = V+, RUN/ON3 = TIME/ON5 = 0,
5.4V < V+ < 30V, 0mA < ILOAD < 50mA
4.7
VL Undervoltage Lockout-Fault Threshold
Falling edge, hysteresis = 1%
3.5
3.6
3.7
V
VL Switchover Threshold
Rising edge of CSL5, hysteresis = 1%
4.2
4.5
4.7
V
REF Output Voltage
No external load (Note 5)
2.45
2.5
2.55
V
VL Output Voltage
REF Load Regulation
0 < ILOAD < 50µA
12.5
0 < ILOAD < 5mA
100.0
REF Sink Current
10
REF Fault-Lockout Voltage
Falling edge
V+ Operating Supply Current
VL switched over to CSL5, 5V SMPS on
V+ Standby Supply Current
mV
µA
2.4
V
5
50
µA
V+ = 5.5V to 30V, both SMPSs off, includes
current into SHDN
30
60
µA
V+ Standby Supply Current in Dropout
V+ = 4.2V to 5.5V, both SMPSs off, includes
current into SHDN
50
200
µA
V+ Shutdown Supply Current
V+ = 4.0V to 30V, SHDN = 0
4
10
µA
2.5
4
Quiescent Power Consumption
Both SMPSs enabled,
FB3 = FB5 = 0,
CSL3 = CSH3 = 3.5V,
CSL5 = CSH5 = 5.3V
1.8
(Note 3)
mW
MAX1901/MAX1904
1.5
4
_______________________________________________________________________________________
3
MAX1901/MAX1902/MAX1904
ELECTRICAL CHARACTERISTICS (continued)
MAX1901/MAX1902/MAX1904
500kHz Multi-Output, Low-Noise Power-Supply
Controllers for Notebook Computers
ELECTRICAL CHARACTERISTICS (continued)
(V+ = 15V, both PWMs on, SYNC = VL, VL load = 0, REF load = 0, SKIP = 0, TA = 0°C to +85°C, unless otherwise noted. Typical
values are at TA = +25°C.)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
4
7
10
%
FAULT DETECTION (MAX1901/MAX1902)
Overvoltage Trip Threshold
With respect to unloaded output voltage
Overvoltage Fault Propagation Delay
CSL_ driven 2% above overvoltage trip
threshold
Output Undervoltage Threshold
With respect to unloaded output voltage
Output Undervoltage Lockout Time
From each SMPS enabled, with respect to
fOSC
Thermal-Shutdown Threshold
Typical hysteresis = 10°C
1.5
µs
60
70
80
%
5,000
6,144
7,000
clks
150
°C
RESET
RESET Trip Threshold
With respect to unloaded output voltage,
falling edge; typical hysteresis = 1%
RESET Propagation Delay
Falling edge, CSL_ driven 2% below RESET
trip threshold
RESET Delay Time
With respect to fOSC
-7
-5.5
-4
1.5
27,000
%
µs
32,000
37,000
clks
1
50
nA
0.6
V
INPUTS AND OUTPUTS
Feedback-Input Leakage Current
FB3, FB5; SECFB = 2.6V
Logic Input-Low Voltage
RUN/ON3, SKIP, TIME/ON5 (SEQ = REF),
SHDN, STEER, SYNC
Logic Input-High Voltage
RUN/ON3, SKIP, TIME/ON5 (SEQ = REF),
SHDN, STEER, SYNC
Input Leakage Current
RUN/ON3, SKIP, TIME/ON5 (SEQ = REF),
SHDN, STEER, SYNC, SEQ; VPIN = 0V or 3.3V
±1
µA
Logic Output-Low Voltage
RESET, ISINK = 4mA
0.4
V
2.4
Logic Output-High Current
RESET = 3.5V
1
TIME/ON5 Input Trip Level
SEQ = 0 or VL
2.4
TIME/ON5 Source Current
TIME/ON5 = 0, SEQ = 0 or VL
2.5
V
mA
2.6
V
3
3.5
µA
80
Ω
TIME/ON5 On-Resistance
TIME/ON5; RUN/ON3 = 0, SEQ = 0 or VL
15
Gate-Driver Sink/Source Current
DL3, DH3, DL5, DH5; forced to 2V
1
Gate-Driver On-Resistance
4
High or low (Note 6)
A
SSOP package
1.5
7
QFN package
1.5
8
_______________________________________________________________________________________
Ω
500kHz Multi-Output, Low-Noise Power-Supply
Controllers for Notebook Computers
(V+ = 15V, both PWMs on, SYNC = VL, VL load = 0, REF load = 0, SKIP = 0, TA = -40°C to +85°C, unless otherwise noted.) (Note 7)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
4.2
30.0
V
MAIN SMPS CONTROLLERS
Input Voltage Range
3V Output Voltage in Adjustable Mode
V+ = 4.2V to 30V, CSH3 - CSL3 = 0,
CSL3 tied to FB3
2.42
2.58
V
3V Output Voltage in Fixed Mode
V+ = 4.2V to 30V, 0 < CSH3 - CSL3
< 80mV, FB3 = 0
3.20
3.47
V
5V Output Voltage in Adjustable Mode
V+ = 4.2V to 30V, CSH5 - CSL5 = 0,
CSL5 tied to FB5
2.42
2.58
V
5V Output Voltage in Fixed Mode
V+ = 5.3V to 30V, 0 < CSH5 - CSL5
< 80mV, FB5 = 0
4.85
5.25
V
Output Voltage Adjust Range
Either SMPS
REF
5.5
V
Adjustable-Mode Threshold Voltage
Dual-mode comparator
0.5
1.1
V
Current-Limit Threshold
Oscillator Frequency
Maximum Duty Factor
CSH3 - CSL3 or CSH5 - CSL5
80
120
SKIP = VL or VDD < 13V or SECFB < 2.44V
-50
-150
SYNC = VL
450
550
SYNC = 0
283
383
SYNC = VL
95
SYNC = 0 (Note 2)
97
SYNC Input Frequency Range
mV
kHz
%
400
583
kHz
13
14
V
V
V
FLYBACK CONTROLLER
VDD Regulation Threshold
Falling edge (Note 3)
SECFB Regulation Threshold
Falling edge (MAX1901/MAX1904)
2.44
2.60
VDD Shunt Threshold
Rising edge, hysteresis = 1% (Note 3)
18
20
VDD Shunt Sink Current
VDD = 20V (Note 3)
10
mA
12V LINEAR REGULATOR (Note 3)
12OUT Output Voltage
13V < VDD < 18V, 0mA < ILOAD < 100mA
Quiescent VDD Current
VDD = 18V, run mode, no 12OUT load
11.65
12.50
V
100
µA
INTERNAL REGULATOR AND REFERENCE
VL Output Voltage
SHDN = V+, RUN/ON3 = TIME/ON5 = 0,
5.4V < V+ < 30V, 0 < ILOAD < 50mA
4.7
5.1
V
VL Undervoltage Lockout-Fault Threshold
Falling edge, hysteresis = 1%
3.5
3.7
V
VL Switchover Threshold
Rising edge of CSL5, hysteresis = 1%
4.2
4.7
V
REF Output Voltage
No external load (Note 5)
2.45
2.55
V
REF Load Regulation
0 < ILOAD < 50µA
12.5
0 < ILOAD < 5mA
100.0
REF Sink Current
10
REF Fault Lockout Voltage
Falling edge
V+ Operating Supply Current
VL switched over to CSL5, 5V SMPS on
1.8
mV
µA
2.4
V
50
µA
_______________________________________________________________________________________
5
MAX1901/MAX1902/MAX1904
ELECTRICAL CHARACTERISTICS
MAX1901/MAX1902/MAX1904
500kHz Multi-Output, Low-Noise Power-Supply
Controllers for Notebook Computers
ELECTRICAL CHARACTERISTICS (continued)
(V+ = 15V, both PWMs on, SYNC = VL, VL load = 0, REF load = 0, SKIP = 0, TA = -40°C to +85°C, unless otherwise noted.) (Note 7)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
V+ Standby Supply Current
V+ = 5.5V to 30V, both SMPSs off, includes
current into SHDN
60
µA
V+ Standby Supply Current in Dropout
V+ = 4.2V to 5.5V, both SMPSs off, includes
current into SHDN
200
µA
V+ Shutdown Supply Current
V+ = 4.0V to 30V, SHDN = 0
10
µA
(Note 3)
4
Quiescent Power Consumption
Both SMPSs enabled,
FB3 = FB5 = 0,
CSL3 = CSH3 = 3.5V,
CSL5 = CSH5 = 5.3V
MAX1901/MAX1904
4
mW
FAULT DETECTION (MAX1901/MAX1902)
Overvoltage Trip Threshold
With respect to unloaded output voltage
4
10
%
Output Undervoltage Threshold
With respect to unloaded output voltage
60
80
%
Output Undervoltage Lockout Time
From each SMPS enabled, with respect to
fOSC
5,000
7,000
clks
-7
-4
%
clks
27,000
37,000
RESET
RESET Trip Threshold
With respect to unloaded output voltage,
falling edge; typical hysteresis = 1%
RESET Delay Time
With respect to fOSC
INPUTS AND OUTPUTS
Feedback-Input Leakage Current
FB3, FB5; SECFB = 2.6V
50
nA
Logic Input-Low Voltage
RUN/ON3, SKIP, TIME/ON5 (SEQ = REF),
SHDN, STEER, SYNC
0.6
V
Logic Input-High Voltage
RUN/ON3, SKIP, TIME/ON5 (SEQ = REF),
SHDN, STEER, SYNC
Logic Output-Low Voltage
RESET, ISINK = 4mA
Logic Output-High Current
RESET = 3.5V
1
TIME/ON5 Input Trip Level
SEQ = 0 or VL
2.4
2.6
V
TIME/ON5 Source Current
TIME/ON5 = 0, SEQ = 0 or VL
2.5
3.5
µA
TIME/ON5 On-Resistance
TIME/ON5; RUN/ON3 = 0, SEQ = 0 or VL
80
Ω
Note 3:
Note 4:
Note 5:
Note 6:
Note 7:
6
V
0.4
V
mA
SSOP package
7
Ω
QFN package
8
Each of the four digital soft-start levels is tested for functionality; the steps are typically in 20mV increments.
High duty-factor operation supports low input-to-output differential voltages, and is achieved at a lowered operating frequency
(see the Dropout Operation section).
MAX1902 only.
Off mode for the 12V linear regulator occurs when the SMPS that has flyback feedback (VDD) steered to it is disabled. In situations where the main outputs are being held up by external keep-alive supplies, turning off the 12OUT regulator prevents a leakage path from the output-referred flyback winding, through the rectifier, and into VDD.
Since the reference uses VL as its supply, the reference’s V+ line-regulation error is insignificant.
Production testing limitations due to package handing require relaxed maximum on-resistance specifications for the thin
QFN package. The SSOP and thin QFN package contain the same die, and the thin QFN package imposes no additional
resistance incircuit.
Specifications from to 0°C to -40°C are guaranteed by design, not production tested.
Gate-Driver On-Resistance
Note 1:
Note 2:
2.4
High or low (Note 6)
_______________________________________________________________________________________
500kHz Multi-Output, Low-Noise Power-Supply
Controllers for Notebook Computers
V+ = 15V
75
70
65
55
50
0.001
75
70
60
ON5 = ON3 = 5V
f = 500kHz
MAX1901/MAX1904
55
50
0.01
0.1
1
10
0.01
0.1
1
0
10
5
10
15
20
INPUT VOLTAGE (V)
NO LOAD INPUT CURRENT
vs. INPUT VOLTAGE
V+ STANDBY INPUT CURRENT
vs. INPUT VOLTAGE
SHUTDOWN INPUT CURRENT
vs. INPUT VOLTAGE
INPUT CURRENT (µA)
1
SKIP = 0V
1000
100
SHDN = 0V
8
10
0
1
15
20
25
4
ON5 = ON3 = 0V
NO LOAD
0.01
10
6
2
ON5 = ON3 = 5V
NO LOAD
5
MAX1901 toc06
10
MAX1901 toc05
10,000
INPUT CURRENT (µA)
MAX1901 toc04
10
30
0
5
10
15
20
25
0
30
5
10
15
20
25
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
MINIMUM VIN TO VOUT DIFFERENTIAL
vs. 5V OUTPUT CURRENT
SWITCHING FREQUENCY
vs. LOAD CURRENT
VL REGULATOR OUTPUT VOLTAGE
vs. OUTPUT CURRENT
SWITCHING FREQUENCY (kHz)
f = 500kHz
100
1000
f = 333kHz
3.3V, VIN = 15V
100
5V, VIN = 15V
10
3.3V, VIN = 6.5V
5V, VIN = 6.5V
1
30
5.00
VL OUTPUT VOLTAGE (V)
MAX1901 toc07
1000
MAX1901 toc03
0
0.001
0.1
MINIMUM VIN TO VOUT DIFFERENTIAL (mV)
200
3.3V OUTPUT CURRENT (A)
SKIP = VL
0
5V LOAD = 0
400
5V OUTPUT CURRENT (A)
100
INPUT CURRENT (mA)
V+ = 15V
80
65
ON5 = 5V
ON3 = 0V
f = 500kHz
MAX1901/MAX1904
60
85
5V LOAD = 3A
600
MAX1901 toc09
80
V+ = 6V
90
800
MAX1901 toc08
EFFICIENCY (%)
85
95
EFFICIENCY (%)
90
MAX1901 toc02
V+ = 6V
95
100
MAX1901 toc01
100
MAXIMUM VDD OUTPUT CURRENT
vs. INPUT VOLTAGE
EFFICIENCY vs. 3.3V OUTPUT CURRENT
MAXIMUM VDD OUTPUT CURRENT (mA)
EFFICIENCY vs. 5V OUTPUT CURRENT
VIN = 15V
ON3 = ON5 = 0V
4.98
4.96
4.94
4.92
VOUT > 4.8V
10
0.001
0.1
0.01
0.1
1
5V OUTPUT CURRENT (A)
10
4.90
0.001
0.01
0.1
LOAD CURRENT (A)
1
10
0
10
20
30
40
50
OUTPUT CURRENT (mA)
_______________________________________________________________________________________
7
MAX1901/MAX1902/MAX1904
Typical Operating Characteristics
(Circuit of Figure 1, Table 1, 6A/500kHz components, TA = +25°C, unless otherwise noted.)
Typical Operating Characteristics (continued)
(Circuit of Figure 1, Table 1, 6A/500kHz components, TA = +25°C, unless otherwise noted.)
REF OUTPUT VOLTAGE
vs. OUTPUT CURRENT
STARTUP WAVEFORMS
MAX1901 toc11
MAX1901 toc10
2.505
VIN = 15V
ON3 = ON5 = 0
REF OUTPUT VOLTAGE (V)
MAX1901/MAX1902/MAX1904
500kHz Multi-Output, Low-Noise Power-Supply
Controllers for Notebook Computers
RUN
5V/div
0
2.500
3.3V OUTPUT
2V/div
0
2.495
TIME
2V/div
0
2.490
5V OUTPUT
5V/div
0
2.485
0
1
2
3
4
5
2ms/div
SEQ = VL, O.O1µF CAPACITOR ON TIME
6
OUTPUT CURRENT (mA)
3.3V LOAD TRANSIENT RESPONSE
5V LOAD TRANSIENT RESPONSE
MAX1901 toc13
MAX1901 toc12
3.3V OUTPUT
5OmV/div
AC-COUPLED
5V OUTPUT
5OmV/div
AC-COUPLED
10V
VLX5
10V/div
0
5A
ILX5
5A/div
0
20µs/div
VIN = 8V, IOUT = 1A TO 5A
8
10V
VLX3
10V/div
0
5A
ILX3
5A/div
0
20µs/div
VIN = 8V, IOUT = 1A TO 5A
_______________________________________________________________________________________
500kHz Multi-Output, Low-Noise Power-Supply
Controllers for Notebook Computers
PIN
SSOP
1
QFN
29
2
30
3
4
5
31
1
NAME
FUNCTION
CSH3
Current-Sense Input for the 3.3V SMPS. Current-limit level is 100mV referred to CSL3.
CSL3
Current-Sense Input. Also serves as the feedback input in fixed-output mode.
FB3
Feedback Input for the 3.3V SMPS. Regulates at FB3 = REF (approx. 2.5V) in
adjustable mode. FB3 is a dual-mode input that also selects the 3.3V fixed output
voltage setting when connected to GND. Connect FB3 to a resistor-divider for
adjustable-output mode.
12OUT
(MAX1902)
12V/120mA Linear-Regulator Output. Input supply comes from VDD. Bypass 12OUT to
GND with 1µF (min).
STEER
(MAX1901/
MAX1904)
Logic-Control Input for Secondary Feedback. Selects the PWM that uses a transformer
and secondary feedback signal (SECFB):
STEER = GND: 3.3V SMPS uses transformer
STEER = VL: 5V SMPS uses transformer
VDD
(MAX1902)
Supply Voltage Input for the 12OUT Linear Regulator. Also connects to an internal
resistor-divider for secondary winding feedback and to an 18V overvoltage shunt
regulator clamp.
SECFB
(MAX1901/
MAX1904)
Secondary Winding Feedback Input. Normally connected to a resistor-divider from an
auxiliary output. SECFB regulates at VSECFB = 2.5V (see the Secondary Feedback
Regulation Loop section). Connect to VL if not used.
Oscillator Synchronization and Frequency Select. Connect to VL for 500kHz operation;
connect to GND for 333kHz operation. Can be driven at 400kHz to 583kHz for external
synchronization.
2
6
3
SYNC
7
4
TIME/ON5
8
5
GND
Low-Noise Analog Ground and Feedback Reference Point
9
7
REF
2.5V Reference Voltage Output. Bypass to GND with 1µF (min).
10
8
SKIP
Logic-control input that disables idle mode when high. Connect to GND for normal use.
11
9
RESET
Active-Low Timed Reset Output. RESET swings GND to VL. Goes high after a fixed
32,000 clock-cycle delay following power-up.
Dual-Purpose Timing Capacitor Pin and ON/OFF Control Input. See the Power-Up
Sequencing and ON/OFF Controls section.
Feedback Input for the 5V SMPS. Regulates at FB5 = REF (approx. 2.5V) in adjustable
mode. FB5 is a dual-mode input that also selects the 5V fixed output voltage setting
when connected to GND. Connect FB5 to a resistor-divider for adjustable-output
mode.
12
10
FB5
13
11
CSL5
Current-Sense Input for the 5V SMPS. Also serves as the feedback input in fixed-output
mode, and as the bootstrap supply input when the voltage on CSL5/VL is > 4.5V.
14
12
CSH5
Current-Sense Input for the 5V SMPS. Current-limit level is 100mV referred to CSL5.
_______________________________________________________________________________________
9
MAX1901/MAX1902/MAX1904
Pin Description
500kHz Multi-Output, Low-Noise Power-Supply
Controllers for Notebook Computers
MAX1901/MAX1902/MAX1904
Pin Description (continued)
PIN
QSOP
10
QFN
NAME
FUNCTION
15
13
SEQ
Pin-strap input that selects the SMPS power-up sequence:
SEQ = GND: 5V before 3.3V, RESET output determined by both outputs
SEQ = REF: Separate ON3/ON5 controls, RESET output determined by 3.3V
output
SEQ = VL: 3.3V before 5V, RESET output determined by both outputs
16
14
DH5
Gate-Drive Output for the 5V, High-Side N-Channel Switch. DH5 is a floating driver
output that swings from LX5 to BST5, riding on the LX5 switching node voltage.
17
15
LX5
Switching-Node (Inductor) Connection. Can swing 2V below ground without hazard.
18
17
BST5
Boost Capacitor Connection for High-Side Gate Drive (0.1µF)
19
18
DL5
Gate-Drive Output for the Low-Side Synchronous-Rectifier MOSFET. Swings 0 to VL.
20
19
PGND
Power Ground
21
20
VL
5V Internal Linear-Regulator Output. VL is also the supply-voltage rail for the chip.
After the 5V SMPS output has reached 4.5V (typ), VL automatically switches to the
output voltage through CSL5 for bootstrapping. Bypass to GND with 4.7µF. VL
supplies up to 25mA for external loads.
22
21
V+
Battery Voltage Input, 4.2V to 30V. Bypass V+ to PGND close to the IC with a 0.22µF
capacitor. Connects to a linear regulator that powers VL.
23
22
SHDN
24
23
DL3
Gate-Drive Output for the Low-Side Synchronous-Rectifier MOSFET. Swings 0 to VL.
25
24
BST3
Boost Capacitor Connection for High-Side Gate Drive (0.1µF)
26
26
LX3
Switching-Node (Inductor) Connection. Can swing 2V below ground without hazard.
27
27
DH3
Gate-Drive Output for the 3.3V, High-Side N-Channel Switch. DH3 is a floating driver
output that swings from LX3 to BST3, riding on the LX3 switching-node voltage.
28
28
RUN/ON3
—
6, 16, 25, 32
N.C.
Shutdown Control Input, Active Low. Logic threshold is set at approximately 1V. For
automatic startup, connect SHDN to V+ through a 220kΩ resistor and bypass SHDN to
GND with a 0.01µF capacitor.
ON/OFF Control Input. See the Power-Up Sequencing and ON/OFF Controls section.
No Connection
______________________________________________________________________________________
500kHz Multi-Output, Low-Noise Power-Supply
Controllers for Notebook Computers
5V ALWAYS ON
7V TO 24V
C3
10Ω
4.7µF
0.1µF
0.1µF
V+ SHDN SECFB VL
4.7µF
Q1
C4
0.1µF
R1
5V OUTPUT
C1
BST3
DH5
C5
0.1µF
0.1µF
LX5
Q2
Q3
DH3
0.1µF
L1
*
SYNC
BST5
L2
LX3
DL5 MAX1901
MAX1904 DL3
GND
PGND
CSH5
CSH3
CSL5
CSL3
R2
3.3V OUTPUT
*
C2
Q4
FB3
FB5
RESET
5V ON/OFF
TIME/ON5
3.3V ON/OFF
RUN/ON3
RESET OUTPUT
SKIP
STEER
SEQ
REF
2.5V ALWAYS ON
1µF
*1A SCHOTTKY DIODE REQUIRED
FOR THE MAX1901 (SEE THE OUTPUT
OVERVOLTAGE PROTECTION SECTION).
Figure 1. Standard 3.3V/5V Application Circuit (MAX1901/MAX1904)
______________________________________________________________________________________
11
MAX1901/MAX1902/MAX1904
ON/OFF
INPUT
MAX1901/MAX1902/MAX1904
500kHz Multi-Output, Low-Noise Power-Supply
Controllers for Notebook Computers
Standard Application Circuit
Detailed Description
The basic MAX1901/MAX1904 dual-output 3.3V/5V
buck converter (Figure 1) is easily adapted to meet a
wide range of applications with inputs up to 28V by
substituting components from Table 1. These circuits
represent a good set of tradeoffs between cost, size,
and efficiency, while staying within the worst-case
specification limits for stress-related parameters, such
as capacitor ripple current. Don’t change the frequency
of these circuits without first recalculating component
values (particularly inductance value at maximum battery voltage). Adding a Schottky rectifier across each
synchronous rectifier improves the efficiency of these
circuits by approximately 1%, but this rectifier is otherwise not needed because the MOSFETs required for
these circuits typically incorporate a high-speed silicon
diode from drain to source. Use a Schottky rectifier
rated at a DC current equal to at least one-third of the
load current.
The MAX1901/MAX1902/MAX1904 are dual, BiCMOS,
switch-mode power-supply controllers designed primarily for buck-topology regulators in battery-powered
applications where high-efficiency and low-quiescent
supply current are critical. Light-load efficiency is
enhanced by automatic Idle-Mode operation, a variable-frequency pulse-skipping mode that reduces transition and gate-charge losses. Each step-down,
power-switching circuit consists of two N-channel
MOSFETs, a rectifier, and an LC output filter. The output voltage is the average AC voltage at the switching
node, which is regulated by changing the duty cycle of
the MOSFET switches. The gate-drive signal to the
N-channel high-side MOSFET must exceed the battery
voltage, and is provided by a flying-capacitor boost circuit that uses a 100nF capacitor connected to BST_.
Table 1. Component Selection for Standard 3.3V/5V Application
LOAD CURRENT
COMPONENT
4A/333kHz
4A/500kHz
6A/500kHz
Input Range
7V to 24V
7V to 24V
7V to 24V
Frequency
333kHz
500kHz
500kHz
Q1, Q3 High-Side
MOSFETs
1/2 Fairchild FDS6982S or
1/2 International Rectifier
IRF7901D1
1/2 Fairchild FDS6982S or
1/2 International Rectifier
IRF7901D1
Fairchild FDS6612A or
International Rectifier
IRF7807V
Q2, Q4 Low-Side
MOSFETs with Integrated
Schottky Diodes
1/2 Fairchild FDS6982S or
1/2 International Rectifier
IRF7901D1
1/2 Fairchild FDS6982S or
1/2 International Rectifier
IRF7901D1
Fairchild FDS6670S or
International Rectifier
IRF7807DV1
C3 Input Capacitor
3 ✕ 10µF, 25V ceramic
Taiyo Yuden TMK432BJ106KM
3 ✕ 10µF, 25V ceramic
Taiyo Yuden TMK432BJ106KM
4 ✕ 10µF, 25V ceramic
Taiyo Yuden TMK432BJ106KM
C1 Output Capacitor
150µF, 6V POSCAP
Sanyo 6TPC150M
150µF, 6V POSCAP
Sanyo 6TPC150M
2 ✕ 150µF, 6V POSCAP
Sanyo 6TPC150M
C2 Output Capacitor
2 ✕ 150µF, 4V POSCAP
Sanyo 4TPC150M
2 ✕ 150µF, 4V POSCAP
Sanyo 4TPC150M
2 ✕ 220µF, 4V POSCAP
Sanyo 4TPC220M
R1, R2 Resistors
0.018Ω
Dale WSL2512-R018-F
0.018Ω
Dale WSL2512-R018-F
0.012Ω
Dale WSL2512-R012-F
L1 Inductor
10µH, 4.5A Ferrite
Sumida CDRH124-100
7.0µH, 5.2A Ferrite
Sumida CEI122-H-7R0
4.2µH, 6.9A Ferrite
Sumida CEI122-H-4R2
L2 Inductor
7.0µH, 5.2A Ferrite
Sumida CEI122-H-7R0
5.6µH, 5.2A Ferrite
Sumida CEI122-H-5R6
4.2µH, 6.9A Ferrite
Sumida CEI122-H-4R2
12
______________________________________________________________________________________
500kHz Multi-Output, Low-Noise Power-Supply
Controllers for Notebook Computers
MANUFACTURER
USA PHONE
FACTORY FAX
Dale-Vishay
402-564-3131
402-563-6418
Fairchild
Semiconductor
408-721-2181
408-721-1635
International
Rectifier
310-322-3331
310-322-3332
Sanyo
619-661-6835
619-661-1055
Sumida
847-956-0666
847-956-0702
Taiyo Yuden
408-573-4150
408-573-4159
The MAX1901/MAX1902/MAX1904 contain ten major
circuit blocks (Figure 2).
The two pulse-width-modulation (PWM) controllers
each consist of a Dual Mode feedback network and
multiplexer, a multi-input PWM comparator, high-side
and low-side gate drivers, and logic. MAX1901/
MAX1902 contain fault-protection circuits that monitor
the main PWM outputs for undervoltage and overvoltage. A power-on sequence block controls the powerup timing of the main PWMs and determines whether
one or both of the outputs are monitored for undervoltage
faults. The MAX1902 includes a secondary feedback network and 12V linear regulator to generate a 12V output
from a coupled-inductor flyback winding. The
MAX1901/MAX1904 have a secondary feedback input
(SECFB) instead, which allows a quasi-regulated,
adjustable output, coupled-inductor flyback winding to be
attached to either the 3.3V or the 5V main inductor. Bias
generator blocks include the 5V IC internal rail (VL) linear
regulator, 2.5V precision reference, and automatic bootstrap switchover circuit. The PWMs share a common
333kHz/500kHz synchronizable oscillator.
These internal IC blocks aren’t powered directly from
the battery. Instead, the 5V VL linear regulator steps
down the battery voltage to supply both VL and the
gate drivers. The synchronous-switch gate drivers are
directly powered from VL, while the high-side switch
gate drivers are indirectly powered from V L via an
external diode-capacitor boost circuit. An automatic
bootstrap circuit turns off the 5V linear regulator and
powers the IC from the 5V PWM output voltage if the
output is above 4.5V.
PWM Controller Block
The two PWM controllers are nearly identical. The only
differences are fixed output settings (3.3V vs. 5V), the
VL/CSL5 bootstrap switch connected to the 5V PWM,
and SECFB. The heart of each current-mode PWM controller is a multi-input, open-loop comparator that sums
three signals: the output-voltage error signal with
respect to the reference voltage, the current-sense signal, and the slope-compensation ramp (Figure 3). The
PWM controller is a direct-summing type, lacking a traditional error amplifier and the phase shift associated
with it. This direct-summing configuration approaches
ideal cycle-by-cycle control over the output voltage.
When SKIP = low, Idle Mode circuitry automatically
optimizes efficiency throughout the load current range.
Idle Mode dramatically improves light-load efficiency
by reducing the effective frequency, which reduces
switching losses. It keeps the peak inductor current
above 25% of the full current limit in an active cycle,
allowing subsequent cycles to be skipped. Idle Mode
transitions seamlessly to fixed-frequency PWM operation as load current increases.
With SKIP = high, the controller always operates in fixedfrequency PWM mode for lowest noise. Each pulse from
the oscillator sets the main PWM latch that turns on the
high-side switch for a period determined by the duty factor (approximately VOUT / VIN). As the high-side switch
turns off, the synchronous rectifier latch sets; 60ns later,
the low-side switch turns on. The low-side switch stays on
until the beginning of the next clock cycle.
In PWM mode, the controller operates as a fixed-frequency current-mode controller where the duty ratio is
set by the input/output voltage ratio. The current-mode
feedback system regulates the peak inductor current
value as a function of the output-voltage error signal. In
continuous-conduction mode, the average inductor
current is nearly the same as the peak current, so the
circuit acts as a switch-mode transconductance amplifier. This pushes the second output LC filter pole, normally found in a duty-factor-controlled (voltage-mode)
PWM, to a higher frequency. To preserve inner-loop
stability and eliminate regenerative inductor current
“staircasing”, a slope-compensation ramp is summed
into the main PWM comparator to make the apparent
duty factor less than 50%.
The MAX1901/MAX1902/MAX1904 use a relatively low
loop gain, allowing the use of lower-cost output capacitors. The relative gains of the voltage-sense and current-sense inputs are weighted by the values of current
sources that bias three differential input stages in the
main PWM comparator (Figure 4). The relative gain of
the voltage comparator to the current comparator is
internally fixed at K = 2:1. The low loop gain results in
the 2% typical load-regulation error. The low value of
loop gain helps reduce output filter capacitor size and
cost by shifting the unity-gain crossover frequency to a
lower level.
______________________________________________________________________________________
13
MAX1901/MAX1902/MAX1904
Table 2. Component Suppliers
INPUT
7V to 24V
SHDN
V+
SYNC
CSL5
+
MAX1902
4.5V
ON/OFF
5V ALWAYS ON
12V
LINEAR
REG
5V
LINEAR
REG
VL
REF
VDD
+
-
13V
DH3
3.3V
12V
IN
SECFB
2.5V
REF
BST3
12OUT
BST5
RAW 15V
DH5
LX3
VL
DL3
3.3V
PWM
LOGIC
5V
PWM
LOGIC
333kHz
TO
500kHz
OSC
LX5
VL
5V
DL5
PGND
OV/UV
FAULT
CSH3
CSL3
+
REF
-
REF
1.75V
2.68V
LPF
50kHz
CSH5
CSL5
+
LPF
50kHz
2.388V
FB3
-
R3
FB5
OUTPUTS
UP
-
R2
-
+
+
R1
-
+
R4
+
0.6V
0.6V
VL
+
REF
-
POWER-ON
SEQUENCE
LOGIC
+
SEQ
1V
RUN/ON3
RESET
2.6V
+
TIME/ON5
TIMER
-
MAX1901/MAX1902/MAX1904
500kHz Multi-Output, Low-Noise Power-Supply
Controllers for Notebook Computers
GND
Figure 2. MAX1902 Functional Diagram
14
______________________________________________________________________________________
500kHz Multi-Output, Low-Noise Power-Supply
Controllers for Notebook Computers
1X
CSL_
REF
FROM
FEEDBACK
DIVIDER
MAIN PWM
COMPARATOR
BST_
R
LEVEL
SHIFT
Q
S
DH_
LX_
SLOPE COMP
OSC
30mV
SKIP
CURRENT
LIMIT
DAC
SHOOTTHROUGH
CONTROL
CK
COUNTER
SHDN
SOFT-START
SYNCHRONOUS
RECTIFIER CONTROL
R
-100mV
S
VL
Q
LEVEL
SHIFT
DL_
PGND
REF
0.75µs
SINGLE-SHOT
SECFB
Figure 3. PWM Controller Functional Block Diagram
______________________________________________________________________________________
15
MAX1901/MAX1902/MAX1904
CSH_
MAX1901/MAX1902/MAX1904
500kHz Multi-Output, Low-Noise Power-Supply
Controllers for Notebook Computers
Table 3. SKIP PWM Table
SKIP
LOAD CURRENT
MODE
Low
Light
Idle
DESCRIPTION
Pulse-skipping, supply current = 250µA at VIN =12V, discontinuous inductor
Low
Heavy
PWM
Constant-frequency PWM continuous-inductor current
High
Light
PWM
Constant-frequency PWM continuous-inductor current
High
Heavy
PWM
Constant-frequency PWM continuous-inductor current
VL
R1
R2
TO PWM
LOGIC
UNCOMPENSATED
HIGH-SPEED
LEVEL TRANSLATOR
AND BUFFER
OUTPUT DRIVER
FB_
I1
I2
I3
VBIAS
REF
CSH_
CSL_
SLOPE COMPENSATION
Figure 4. Main PWM Comparator Block Diagram
The output filter capacitors (Figure1, C1 and C2) set a
dominant pole in the feedback loop that must roll off the
loop gain to unity before encountering the zero introduced by the output capacitor’s parasitic resistance
(ESR) (see the Design Procedure section). A 50kHz
pole-zero cancellation filter provides additional rolloff
above the unity-gain crossover. This internal 50kHz
low-pass compensation filter cancels the zero due to filter capacitor ESR. The 50kHz filter is included in the
loop in both fixed-output and adjustable-output modes.
Synchronous Rectifier Driver (DL)
Synchronous rectification reduces conduction losses in
the rectifier by shunting the normal Schottky catch
diode with a low-resistance MOSFET switch. Also, the
synchronous rectifier ensures proper startup of the
boost gate-driver circuit.
If the circuit is operating in continuous-conduction
mode, the DL drive waveform is simply the complement
of the DH high-side drive waveform (with controlled
dead time to prevent cross-conduction or “shoot
through”). In discontinuous (light-load) mode, the syn16
chronous switch is turned off as the inductor current falls
through zero. The synchronous rectifier works under all
operating conditions, including Idle Mode.
The SECFB signal further controls the synchronous switch
timing in order to improve multiple-output cross-regulation
(see the Secondary Feedback Regulation Loop section).
Internal VL and REF Supplies
An internal regulator produces the 5V supply (VL) that
powers the PWM controller, logic, reference, and other
blocks within the IC. This 5V low-dropout linear regulator supplies up to 25mA for external loads, with a
reserve of 25mA for supplying gate-drive power.
Bypass VL to GND with 4.7µF.
Important: Ensure that V L does not exceed 6V.
Measure VL with the main output fully loaded. If it is
pumped above 5.5V, either excessive boost-diode
capacitance or excessive ripple at V+ is the probable
cause. Use only small-signal diodes for the boost circuit (10mA to 100mA Schottky or 1N4148 are preferred), and bypass V+ to PGND with 4.7µF directly at
the package pins.
______________________________________________________________________________________
500kHz Multi-Output, Low-Noise Power-Supply
Controllers for Notebook Computers
Boost High-Side Gate-Drive Supply
(BST3 and BST5)
Gate-drive voltage for the high-side N-channel switches
is generated by a flying-capacitor boost circuit (Figure 2).
The capacitor between BST_ and LX_ is alternately
charged from the VL supply and placed parallel to the
high-side MOSFET’s gate-source terminals. On startup,
the synchronous rectifier (low-side MOSFET) forces LX_
to 0V and charges the boost capacitors to 5V. On the
second half-cycle, the SMPS turns on the high-side MOSFET by closing an internal switch between BST_ and
DH_. This provides the necessary enhancement voltage
to turn on the high-side switch, an action that “boosts” the
5V gate-drive signal above the battery voltage.
Ringing at the high-side MOSFET gate (DH3 and DH5)
in discontinuous-conduction mode (light loads) is a natural operating condition. It is caused by residual energy in the tank circuit, formed by the inductor and stray
capacitance at the switching node, LX. The gate-drive
negative rail is referred to LX, so any ringing there is
directly coupled to the gate-drive output.
Current-Limiting and Current-Sense
Inputs (CSH and CSL)
The current-limit circuit resets the main PWM latch and
turns off the high-side MOSFET switch whenever the
voltage difference between CSH and CSL exceeds
100mV. This limiting is effective for both current flow
directions, putting the threshold limit at ±100mV. The
tolerance on the positive current limit is ±20%, so the
external low-value sense resistor (R1) must be sized for
80mV/ IPEAK, where IPEAK is the required peak-inductor
current to support the full load current, while components must be designed to withstand continuouscurrent stresses of 120mV/R1.
For breadboarding or for very-high-current applications, it may be useful to wire the current-sense inputs
with a twisted pair, rather than PC traces. (This twisted
pair need not be special; two pieces of wire-wrap wire
twisted together is sufficient.) This reduces the possible
noise picked up at CSH_ and CSL_, which can cause
unstable switching and reduced output current. The
CSL5 input also serves as the IC’s bootstrap supply
input. Whenever VCSL5 > 4.5V, an internal switch connects CSL5 to VL.
Oscillator Frequency and
Synchronization (SYNC)
The SYNC input controls the oscillator frequency. Low
selects 333kHz; high selects 500kHz. SYNC can also
be used to synchronize with an external 5V CMOS or
TTL clock generator. SYNC has a guaranteed 400kHz
to 583kHz capture range. A high-to-low transition on
SYNC initiates a new cycle.
500kHz operation optimizes the application circuit for
component size and cost. 333kHz operation provides
increased efficiency, lower dropout, and improved
load-transient response at low input-output voltage differences (see the Low-Voltage Operation section).
Shutdown Mode
Holding SHDN low puts the IC into its 4µA shutdown
mode. SHDN is logic input with a threshold of about 1V
(the VTH of an internal N-channel MOSFET). For automatic startup, bypass SHDN to GND with a 0.01µF capacitor
and connect it to V+ through a 220kΩ resistor.
Power-Up Sequencing and
ON/OFF Controls
Startup is controlled by RUN/ON3 and TIME/ON5 in
conjunction with SEQ. With SEQ tied to REF, the two
control inputs act as separate ON/OFF controls for
each supply. With SEQ tied to VL or GND, RUN/ON3
becomes the master ON/OFF control input and
TIME/ON5 becomes a timing pin, with the delay
between the two supplies determined by an external
capacitor. The delay is approximately 800µs/nF. The
3.3V supply powers up first if SEQ is tied to VL, and the
5V supply is first if SEQ is tied to GND. When driving
TIME/ON5 as a control input with external logic, always
place a resistor (>1kΩ) in series with the input. This
prevents possible crowbar current due to the internal
discharge pulldown transistor, which turns on in standby mode and momentarily at the first power-up or in
shutdown mode.
______________________________________________________________________________________
17
MAX1901/MAX1902/MAX1904
The 2.5V reference (REF) is accurate to ±2% over temperature, making REF useful as a precision system reference. Bypass REF to GND with 1µF minimum. REF
can supply up to 5mA for external loads. (Bypass REF
with a minimum 1µF/mA reference load current.)
However, if extremely accurate specifications for both
the main output voltages and REF are essential, avoid
loading REF more than 100µA. Loading REF reduces
the main output voltage slightly, because of the reference load-regulation error.
When the 5V main output voltage is above 4.5V, an
internal P-channel MOSFET switch connects CSL5 to
VL, while simultaneously shutting down the VL linear
regulator. This action bootstraps the IC, powering the
internal circuitry from the output voltage, rather than
through a linear regulator from the battery.
Bootstrapping reduces power dissipation due to gate
charge and quiescent losses by providing that power
from a 90%-efficient switch-mode source, rather than
from a much less efficient linear regulator.
MAX1901/MAX1902/MAX1904
500kHz Multi-Output, Low-Noise Power-Supply
Controllers for Notebook Computers
RESET Power-Good Voltage Monitor
The power-good monitor generates a system RESET
signal. At first power-up, RESET is held low until both
the 3.3V and 5V SMPS outputs are in regulation. At this
point, an internal timer begins counting oscillator pulses, and RESET continues to be held low until 32,000
cycles have elapsed. After this timeout period (64ms at
500kHz or 96ms at 333kHz), RESET is actively pulled
up to VL. If SEQ is tied to REF (for separate ON3/ON5
controls), only the 3.3V SMPS is monitored—the 5V
SMPS is ignored.
Output Undervoltage Shutdown
Protection (MAX1901/MAX1902)
The output undervoltage lockout circuit is similar to
foldback current limiting, but employs a timer rather
than a variable current limit. Each SMPS has an undervoltage protection circuit that is activated 6144 clock
cycles after the SMPS is enabled. If either SMPS output
is under 70% of the nominal value, both SMPSs are
latched off and their outputs are clamped to ground by
the synchronous rectifier MOSFETs (see the Output
Overvoltage Protection section). They won’t restart until
SHDN or RUN/ON3 is toggled, or until V+ power is
cycled below 1V. Note that undervoltage protection can
make prototype troubleshooting difficult, since you
have only 12ms or 18ms to figure out what might be
wrong with the circuit before both SMPSs are latched
off. In extreme cases, it may be useful to substitute the
MAX1904 into the prototype breadboard until the prototype is working properly.
Output Overvoltage Protection
(MAX1901/MAX1902)
Both SMPS outputs are monitored for overvoltage. If
either output is more than 7% above the nominal regulation point, both low-side gate drivers (DL_) are
latched high until SHDN or RUN/ON3 is toggled, or
until V+ power is cycled below 1V. This action turns on
the synchronous rectifiers with 100% duty, in turn rapidly discharging the output capacitors and forcing both
SMPS outputs to ground. The DL outputs are also kept
high whenever the corresponding SMPS is disabled,
and in shutdown if VL is sustained.
Discharging the output capacitor through the main
inductor causes the output to momentarily go below
GND. Clamp this negative pulse with a back-biased 1A
Schottky diode across the output capacitor (Figure 1).
To ensure overvoltage protection on initial power-up,
connect signal diodes from both output voltages to VL
(cathodes to VL) to eliminate the VL power-up delay.
This circuitry protects the load from accidental overvoltage caused by a short circuit across the high-side
power MOSFETs. This scheme relies on the presence
of a fuse, in series with the battery, which is blown by
the resulting crowbar current. Note that the overvoltage
circuitry will interfere with external keep-alive supplies
that hold up the outputs (such as lithium backup or hotswap power supplies); in such cases, the MAX1904
should be used.
Low-Noise Operation (PWM Mode)
PWM mode (SKIP = high) minimizes RF and audio interference in noise-sensitive applications (such as hi-fi multimedia-equipped systems), cellular phones, RF
communicating computers, and electromagnetic pen
entry systems. See the summary of operating modes in
Table 2. SKIP can be driven from an external logic signal.
Table 4. Operating Modes
18
SHDN
SEQ
RUN/ON3
TIME/ON5
MODE
Low
X
X
X
Shutdown
High
REF
Low
Low
Standby
High
REF
High
Low
Run
DESCRIPTION
All circuit blocks turned off.
Supply current = 4µA.
Both SMPSs off. Supply current = 30µA.
3.3V SMPS enabled/5V off.
High
REF
Low
High
Run
5V SMPS enabled/3.3V off.
High
REF
High
High
Run
Both SMPSs enabled.
High
GND
Low
Timing Capacitor
Standby
High
GND
High
Timing Capacitor
Run
High
VL
Low
Timing Capacitor
Standby
High
VL
High
Timing Capacitor
Run
Both SMPSs off. Supply current = 30µA.
Both SMPSs enabled. 5V enabled before 3.3V.
Both SMPSs off. Supply current = 30µA.
Both SMPSs enabled. 3.3V enabled Before 5V.
______________________________________________________________________________________
500kHz Multi-Output, Low-Noise Power-Supply
Controllers for Notebook Computers
Internal Digital Soft-Start Circuit
Soft-start allows a gradual increase of the internal current-limit level at startup to reduce input surge currents.
Both SMPSs contain internal digital soft-start circuits,
each controlled by a counter, a digital-to-analog converter (DAC), and a current-limit comparator. In shutdown or standby mode, the soft-start counter is reset to
zero. When an SMPS is enabled, its counter starts
counting oscillator pulses, and the DAC begins incrementing the comparison voltage applied to the currentlimit comparator. The DAC output increases from 0 to
100mV in five equal steps as the count increases to
512 clocks. As a result, the main output capacitor
charges up relatively slowly. The exact time of the output rise depends on output capacitance and load current, and is typically 600µs with a 500kHz oscillator.
Dropout Operation
Dropout (low input-output differential operation) is
enhanced by stretching the clock pulse width to
increase the maximum duty factor. The algorithm follows: If the output voltage (VOUT) drops out of regulation without the current limit having been reached, the
SMPS skips an off-time period (extending the on-time).
At the end of the cycle, if the output is still out of regulation, the SMPS skips another off-time period. This
action can continue until three off-time periods are
skipped, effectively dividing the clock frequency by as
much as four.
The typical PWM minimum off-time is 300ns, regardless
of the operating frequency. Lowering the operating frequency raises the maximum duty factor above 97%.
Adjustable-Output Feedback
(Dual Mode FB)
Fixed, preset output voltages are selected when FB_ is
connected to ground. Adjusting the main output voltage with external resistors is simple for any of the
MAX1901/MAX1902/MAX1904, through resistor dividers connected to FB3 and FB5 (Figure 2). Calculate
the output voltage with the following formula:
VOUT = VREF (1 + R1 / R2)
where VREF = 2.5V nominal.
The nominal output should be set approximately 1% or
2% high to make up for the MAX1901/MAX1902/
MAX1904 -2% typical load-regulation error. For example, if designing for a 3.0V output, use a resistor ratio
that results in a nominal output voltage of 3.05V. This
slight offsetting gives the best possible accuracy.
Recommended normal values for R2 range from 5kΩ to
100kΩ. To achieve a 2.5V nominal output, simply connect FB_ directly to CSL_.
Remote output-voltage sensing, while not possible in
fixed-output mode due to the combined nature of the
voltage-sense and current-sense inputs (CSL3 and
CSL5), is easy to do in adjustable mode by using the top
of the external resistor-divider as the remote sense point.
When using adjustable mode, it is a good idea to
always set the “3.3V output” to a lower voltage than the
“5V output.” The 3.3V output must always be less than
VL, so that the voltage on CSH3 and CSL3 is within the
common-mode range of the current-sense inputs. While
VL is nominally 5V, it can be as low as 4.7V when linearly regulating, and as low as 4.2V when automatically
bootstrapped to CSH5.
Secondary Feedback Regulation Loop
(SECFB or VDD)
A flyback-winding control loop regulates a secondary
winding output, improving cross-regulation when the
primary output is lightly loaded or when there is a low
input-output differential voltage. If VDD or SECFB falls
below its regulation threshold, the low-side switch is
turned on for an extra 0.75µs. This reverses the induc-
______________________________________________________________________________________
19
MAX1901/MAX1902/MAX1904
Interference due to switching noise is reduced in PWM
mode by ensuring a constant switching frequency, thus
concentrating the emissions at a known frequency outside the system audio or IF bands. Choose an oscillator
frequency for which switching frequency harmonics
don’t overlap a sensitive frequency band. If necessary,
synchronize the oscillator to a tight-tolerance external
clock generator. To extend the output-voltage regulation range, constant operating frequency is not maintained under overload or dropout conditions (see the
Dropout Operation section).
PWM mode (SKIP = high) forces two changes upon the
PWM controllers. First, it disables the minimum-current
comparator, ensuring fixed-frequency operation.
Second, it changes the detection threshold for reverse
current limit from 0 to -100mV, allowing the inductor current to reverse at light loads. This results in fixed-frequency operation and continuous inductor-current flow.
This eliminates discontinuous-mode inductor ringing and
improves cross regulation of transformer-coupled multiple-output supplies, particularly in circuits that don’t use
additional secondary regulation via SECFB or VDD.
In most applications, tie SKIP to GND to minimize quiescent supply current. VL supply current with SKIP high
is typically 30mA, depending on external MOSFET gate
capacitance and switching losses.
MAX1901/MAX1902/MAX1904
500kHz Multi-Output, Low-Noise Power-Supply
Controllers for Notebook Computers
tor (primary) current, pulling current from the output filter capacitor and causing the flyback transformer to
operate in forward mode. The low impedance presented by the transformer secondary in forward mode
dumps current into the secondary output, charging up
the secondary capacitor and bringing VDD or SECFB
back into regulation. The secondary feedback loop
does not improve secondary output accuracy in normal
flyback mode, where the main (primary) output is heavily loaded. In this condition, secondary output accuracy
is determined by the secondary rectifier drop, transformer turns ratio, and accuracy of the main output voltage. A linear post-regulator may still be needed to meet
strict output-accuracy specifications.
MAX1902 has a VDD pin that regulates at a fixed 13.5V,
set by an internal resistor-divider. The MAX1901/
MAX1904 have an adjustable secondary-output voltage
set by an external resistor-divider on SECFB (Figure 5).
Ordinarily, the secondary regulation point is set 5% to
10% below the voltage normally produced by the flyback
effect. For example, if the output voltage as determined
by turns ratio is 15V, set the feedback resistor ratio to produce 13.5V. Otherwise, the SECFB one-shot might be
triggered unintentionally, unnecessarily increasing supply
current and output noise.
12V Linear Regulator Output (MAX1902)
The MAX1902 includes a 12V linear regulator output
capable of delivering 120mA of output current.
Typically, greater current is available at the expense of
output accuracy. If an accurate output of more than
120mA is needed, an external pass transistor can be
added. The circuit in Figure 6 delivers more than
200mA. Total output current is constrained by the V+
input voltage and the transformer primary load (see
Maximum VDD Output Current vs. Input Voltage graphs
in the Typical Operating Characteristics).
Design Procedure
The three predesigned 3V/5V standard application circuits (Figure 1 and Table 1) contain ready-to-use solutions for common application needs. Also, one
standard flyback transformer circuit supports the
12OUT linear regulator in the Applications Information
section. Use the following design procedure to optimize
these basic schematics for different voltage or current
requirements. But before beginning a design, firmly
establish the following:
Maximum Input (Battery) Voltage, VIN(MAX). This value
should include the worst-case conditions, such as noload operation when a battery charger or AC adapter is
12V OUTPUT
200mA
12OUT
R2
0.1µF
SECFB
1-SHOT
TRIG
10µF
VDD
2N3906
R1
2.5V REF
V+
0.1µF
POSITIVE
SECONDARY
OUTPUT
DH_
MAX1902
V+
10Ω
0.1µF
VDD OUTPUT
MAIN
OUTPUT
MAX1901
MAX1904
DH_
2.2µF
DL_
MAIN
OUTPUT
R1
+VTRIP = VREF 1 + –––
R2
(
)
WHERE VREF (NOMINAL) = 2.5V
Figure 5. Adjusting the Secondary Output Voltage with SECFB
20
DL_
Figure 6. Increased 12V Linear Regulator Output Current
______________________________________________________________________________________
500kHz Multi-Output, Low-Noise Power-Supply
Controllers for Notebook Computers
Inductor Value
The exact inductor value isn’t critical and can be freely
adjusted to make trade-offs between size, cost, and
efficiency. Lower inductor values minimize size and
cost, but reduce efficiency due to higher peak-current
levels. The smallest inductor is achieved by lowering
the inductance until the circuit operates at the border
between continuous and discontinuous mode. Further
reducing the inductor value below this crossover point
results in discontinuous-conduction operation even at
full load. This helps lower output-filter capacitance
requirements, but efficiency suffers due to high I2R
losses. On the other hand, higher inductor values mean
greater efficiency, but resistive losses due to extra wire
turns will eventually exceed the benefit gained from
lower peak-current levels. Also, high inductor values
can affect load-transient response (see the VSAG equation in the Low-Voltage Operation section). The equations that follow are for continuous-conduction
operation, since the MAX1901/MAX1902/MAX1904 are
intended mainly for high-efficiency, battery-powered
applications. Discontinuous conduction doesn’t affect
normal idle-mode operation.
Three key inductor parameters must be specified:
inductance value (L), peak current (IPEAK), and DC
resistance (RDC). The following equation includes a
constant (LIR) which is the ratio of inductor peak-topeak AC current to DC load current. A higher LIR value
allows smaller inductance, but results in higher losses
and higher ripple. A good compromise between size
and losses is found at a 30% ripple-current to load-current ratio (LIR = 0.3), which corresponds to a peakinductor current 1.15 times higher than the DC load
current.
L=
where:
(
VOUT VIN(MAX) - VOUT
)
VIN(MAX) × f × IOUT × LIR
f = switching frequency, normally 333kHz or
500kHz
IOUT = maximum DC load current
LIR = ratio of AC to DC inductor current, typically 0.3; should be >0.15
The nominal peak-inductor current at full load is 1.15 ✕
IOUT if the above equation is used; otherwise, the peak
current can be calculated by:
IPEAK = ILOAD +
(VOUT (VIN(MAX) - VOUT )
2 × f × L × VIN(MAX)
The inductor’s DC resistance should be low enough that
RDC ✕ IPEAK < 100mV, as it is a key parameter for efficiency performance. If a standard off-the-shelf inductor is
not available, choose a core with an LI2 rating greater
than L ✕ IPEAK2 and wind it with the largest-diameter wire
that fits the winding area. Ferrite core material is strongly
preferred. Shielded-core geometries help keep noise,
EMI, and switching-waveform jitter low.
Current-Sense Resistor Value
The current-sense resistor value is calculated according to the worst-case low current-limit threshold voltage
(from the Electrical Characteristics) and the peak
inductor current:
RSENSE =
80mV
IPEAK
Use IPEAK from the second equation in the Inductor
Value section.
Use the calculated value of RSENSE to size the MOSFET switches and specify inductor saturation-current
ratings according to the worst-case high current-limit
threshold voltage:
IPEAK(MAX) =
120mV
RSENSE
Low-inductance resistors, such as surface-mount
metal-film, are recommended.
Input-Capacitor Value
The input filter capacitor is usually selected according
to input ripple current requirements and voltage rating,
rather than capacitor value. Ceramic capacitors or
Sanyo OS-CON capacitors are typically used to handle
the power-up surge-currents, especially when connecting to robust AC adapters or low-impedance batteries.
RMS input ripple current (IRMS) is determined by the
input voltage and load current, with the worst case
occurring at VIN = 2 ✕ VOUT:
______________________________________________________________________________________
21
MAX1901/MAX1902/MAX1904
connected but no battery is installed. VIN(MAX) must not
exceed 30V.
Minimum Input (Battery) Voltage, VIN(MIN).This should
be taken at full load under the lowest battery conditions. If VIN(MIN) is less than 4.2V, use an external circuit to externally hold VL above the VL undervoltage
lockout threshold. If the minimum input-output difference is less than 1.5V, the filter capacitance required to
maintain good AC load regulation increases (see the
Low-Voltage Operation section).
MAX1901/MAX1902/MAX1904
500kHz Multi-Output, Low-Noise Power-Supply
Controllers for Notebook Computers
IRMS = ILOAD ×
VOUT (VIN - VOUT )
VIN
Therefore, when VIN is 2 x VOUT:
I
IRMS = LOAD
2
Bypassing V+
Bypass the V+ input with a 4.7µF tantalum capacitor
paralleled with a 0.1µF ceramic capacitor, close to the
IC. A 10Ω series resistor to VIN is also recommended.
Bypassing VL
Bypass the VL output with a 4.7µF tantalum capacitor
paralleled with a 0.1µF ceramic capacitor, close to the
device.
Output-Filter Capacitor Value
The output-filter capacitor values are generally determined by the ESR and voltage-rating requirements,
rather than actual capacitance requirements for loop stability. In other words, the low-ESR electrolytic capacitor
that meets the ESR requirement usually has more output
capacitance than is required for AC stability. Use only
specialized low-ESR capacitors intended for switchingregulator applications, such as AVX TPS, Sanyo
POSCAP, or Kemet T510. To ensure stability, the capacitor must meet both minimum capacitance and maximum
ESR values as given in the following equations:
COUT >
VREF (1 + VOUT / VIN(MIN) )
VOUT × RSENSE × f
R
× VOUT
RESR < SENSE
VREF
These equations are worst case, with 45° of phase margin to ensure jitter-free, fixed-frequency operation and
provide a nicely damped output response for zero to
full-load step changes. Some cost-conscious designers
may wish to bend these rules with less-expensive
capacitors, particularly if the load lacks large step
changes. This practice is tolerable if some bench testing over temperature is done to verify acceptable noise
and transient response.
No well-defined boundary exists between stable and
unstable operation. As phase margin is reduced, the
first symptom is a bit of timing jitter, which shows up as
blurred edges in the switching waveforms where the
22
scope won’t quite sync up. Technically speaking, this
jitter (usually harmless) is unstable operation, since the
duty factor varies slightly. As capacitors with higher
ESRs are used, the jitter becomes more pronounced, and
the load-transient output-voltage waveform starts looking
ragged at the edges. Eventually, the load-transient waveform has enough ringing on it that the peak noise levels
exceed the allowable output-voltage tolerance. Note that
even with zero phase margin and gross instability present, the output-voltage noise never gets much worse
than IPEAK ✕ RESR (under constant loads).
The output-voltage ripple is usually dominated by the
filter capacitor’s ESR, and can be approximated as
IRIPPLE ✕ RESR. There is also a capacitive term, so the
full equation for ripple in continuous-conduction mode
is V NOISE (p-p) = I RIPPLE ✕ [R ESR + 1/(2 ✕ π ✕ f ✕
COUT)]. In idle mode, the inductor current becomes
discontinuous, with high peaks and widely spaced
pulses, so the noise can actually be higher at light load
(compared to full load). In idle mode, calculate the output ripple as follows:
0.025 × RESR
+
RSENSE
0.0003 × L × [1/ VOUT + 1/(VIN - VOUT )]
VNOISE(P-P) =
RSENSE2 × COUT
Transformer Design
(for Auxiliary Outputs Only)
Buck-plus-flyback applications, sometimes called “coupled-inductor” topologies, need a transformer to generate multiple output voltages. Performing the basic
electrical design is a simple task of calculating turns
ratios and adding the power delivered to the secondary
to calculate the current-sense resistor and primary
inductance. However, extremes of low input-output differentials, widely different output loading levels, and
high turns ratios can complicate the design due to parasitic transformer parameters such as interwinding
capacitance, secondary resistance, and leakage
inductance. For examples of what is possible with real
world transformers, see the Maximum V DD Output
Current vs. Input Voltage graph in the Typical
Operating Characteristics.
Power from the main and secondary outputs is combined
to get an equivalent current referred to the main output
voltage (see the Inductor Value section for parameter
definitions). Set the current-sense resistor value at 80mV
/ ITOTAL.
______________________________________________________________________________________
500kHz Multi-Output, Low-Noise Power-Supply
Controllers for Notebook Computers
LPRIMARY =
VOUT (VIN(MAX) - VOUT )
VIN(MAX) × f × ITOTAL × LIR
Turns Ratio N =
VSEC + VFWD
VOUT(MIN) + VRECT + VSENSE
where: VSEC = the minimum required rectified sec
ondary output voltage
VFWD = the forward drop across the secondary
rectifier
VOUT(MIN) = the minimum value of the main out
put voltage (from the Electrical
Characteristics tables)
VRECT = the on-state voltage drop across the
synchronous rectifier MOSFET
VSENSE = the voltage drop across the sense
resistor
In positive-output applications, the transformer secondary return is often referred to the main output voltage, rather than to ground, to reduce the needed turns
ratio. In this case, the main output voltage must first be
subtracted from the secondary voltage to obtain VSEC.
Selecting Other Components
MOSFET Switches
The high-current N-channel MOSFETs must be logiclevel types with guaranteed on-resistance specifications at V GS = 4.5V. Lower gate threshold
specifications are better (i.e., 2V max rather than 3V
max). Drain-source breakdown voltage ratings must at
least equal the maximum input voltage, preferably with
a 20% derating factor. The best MOSFETs will have the
lowest on-resistance per nanocoulomb of gate charge.
Multiplying RDS(ON) ✕ QG provides a good figure for
comparing various MOSFETs. Newer MOSFET process
technologies with dense cell structures generally perform best. The internal gate drivers tolerate >100nC
total gate charge, but 70nC is a more practical upper
limit to maintain best switching times.
In high-current applications, MOSFET package power
dissipation often becomes a dominant design factor.
I2R power losses are the greatest heat contributor for
both high-side and low-side MOSFETs. I2R losses are
distributed between Q1 and Q2 according to duty factor (see the following equations). Generally, switching
losses affect only the upper MOSFET, since the
Schottky rectifier clamps the switching node in most
cases before the synchronous rectifier turns on. Gate
charge losses are dissipated by the driver and don’t
heat the MOSFET. Calculate the temperature rise
according to package thermal-resistance specifications
to ensure that both MOSFETs are within their maximum
junction temperature at high ambient temperature. The
worst-case dissipation for the high-side MOSFET
occurs at both extremes of input voltage, and the
worst-case dissipation for the low-side MOSFET occurs
at maximum input voltage:
PDupperFET = ILOAD2 × RDS(ON) × DUTY
+ VIN × ILOAD × f ×
VIN × CRSS
+ 20ns
I
GATE
PDupperFET = ILOAD2 × RDS(ON) × (1- DUTY)
DUTY = (VOUT + VQ2 ) / (VIN - VQ1)
where: On-state voltage drop VQ_ = ILOAD ✕ RDS(ON)
CRSS = MOSFET reverse transfer capacitance
IGATE = DH driver peak output current capability
(1A typ)
20ns = DH driver inherent rise/fall time
Under output short-circuit, the MAX1904 synchronous
rectifier MOSFET suffers extra stress because its duty
factor can increase to greater than 0.9. It may need to
be oversized to tolerate a continuous DC short circuit.
During short circuit, the MAX1901/MAX1902’s output
undervoltage shutdown protects the synchronous rectifier under output short-circuit conditions.
To reduce EMI, add a 0.1µF ceramic capacitor from the
high-side switch drain to the low-side switch source.
Rectifier Clamp Diode
The rectifier diode is a clamp across the low-side MOSFET that catches the negative inductor swing during
the 60ns dead time between turning one MOSFET off
and each low-side MOSFET on. The latest generations
of MOSFETs incorporate a high-speed Schottky diode,
which serves as an adequate clamp diode. For
MOSFETs without integrated Schottky diodes, place a
Schottky diode in parallel with the low-side MOSFET.
______________________________________________________________________________________
23
MAX1901/MAX1902/MAX1904
PTOTAL = The sum of the output power from all outputs
ITOTAL = PTOTAL / VOUT = The equivalent output current referred to VOUT
MAX1901/MAX1902/MAX1904
500kHz Multi-Output, Low-Noise Power-Supply
Controllers for Notebook Computers
Use a Schottky diode with a DC current rating equal to
one third of the load current. The Schottky diode’s rated
reverse breakdown voltage must be at least equal to
the maximum input voltage, preferably with a 20% derating factor.
Boost-Supply Diode
A signal diode such as a 1N4148 works well in most
applications. If the input voltage can go below +6V, use
a small (20mA) Schottky diode for slightly improved
efficiency and dropout characteristics. Don’t use large
power diodes, such as 1N5817 or 1N4001, since high
junction capacitance can pump up VL to excessive
voltages.
Rectifier Diode (Transformer Secondary Diode)
The secondary diode in coupled-inductor applications
must withstand flyback voltages greater than 60V,
which usually rules out most Schottky rectifiers.
Common silicon rectifiers, such as the 1N4001, are also
prohibited because they are too slow. This often makes
fast silicon rectifiers such as the MURS120 the only
choice. The flyback voltage across the rectifier is related to the VIN - VOUT difference, according to the transformer turns ratio:
VFLYBACK = VSEC + (VIN - VOUT) ✕ N
where: N = the transformer turns ratio SEC/PRI
V SEC = the maximum secondary DC output
voltage
VOUT = the primary (main) output voltage
Subtract the main output voltage (VOUT) from VFLYBACK in this equation if the secondary winding is
returned to VOUT and not to ground. The diode reversebreakdown rating must also accommodate any ringing
due to leakage inductance. The rectifier diode’s current
rating should be at least twice the DC load current on
the secondary output.
Low-Voltage Operation
Low input voltages and low input-output differential voltages each require extra care in their design. Low
absolute input voltages can cause the VL linear regulator
to enter dropout and eventually shut itself off. Low input
voltages relative to the output (low VIN - VOUT differential)
can cause bad load regulation in multi-output flyback
applications (see the design equations in the Transformer
Design section). Also, low VIN - VOUT differentials can
also cause the output voltage to sag when the load current changes abruptly. The amplitude of the sag is a
function of inductor value and maximum duty factor (an
24
Electrical Characteristics parameter, 97% guaranteed
over temperature at f = 333kHz), as follows:
VSAG =
2 × COUT
ISTEP 2 × L
× (VIN(MIN) × DMAX - VOUT )
The cure for low-voltage sag is to increase the output
capacitor’s value. Take a 333kHz/6A application circuit
as an example, at VIN = +5.5V, VOUT = +5V, L = 6.7µH,
f = 333kHz, ISTEP = 3A (half-load step), a total capacitance of 470µF keeps the sag less than 200mV. The
capacitance is higher than that shown in the Typical
Application Circuit because of the lower input voltage.
Note that only the capacitance requirement increases,
and the ESR requirements don’t change. Therefore, the
added capacitance can be supplied by a low-cost bulk
capacitor in parallel with the normal low-ESR capacitor.
Applications Information
Heavy-Load Efficiency Considerations
The major efficiency-loss mechanisms under loads are,
in the usual order of importance:
• P(I2R) = I2R losses
• P(tran) = transition losses
• P(gate) = gate-charge losses
• P(diode) = diode-conduction losses
• P(cap) = input capacitor ESR losses
• P(IC) = losses due to the IC’s operating supply current
Inductor core losses are fairly low at heavy loads
because the inductor’s AC current component is small.
Therefore, they aren’t accounted for in this analysis.
Ferrite cores are preferred, especially at 300kHz, but
powdered cores, such as Kool-Mu, can work well:
Efficiency = POUT/PIN ✕ 100%
= POUT/(POUT + PTOTAL) ✕ 100%
PTOTAL = P(I2R) + P(tran) + P(gate) + P(diode) +
P(cap) + P(IC)
P (I2R) = ILOAD2 x (RDC + RDS(ON) + RSENSE)
where RDC is the DC resistance of the coil, RDS(ON) is
the MOSFET on-resistance, and RSENSE is the currentsense resistor value. The RDS(ON) term assumes identical MOSFETs for the high-side and low-side switches:
because they time-share the inductor current. If the
MOSFETs aren’t identical, their losses can be estimated by averaging the losses according to duty factor.
______________________________________________________________________________________
500kHz Multi-Output, Low-Noise Power-Supply
Controllers for Notebook Computers
SYMPTOM
CONDITION
ROOT CAUSE
SOLUTION
Sag or droop in VOUT
under step-load change
Low VIN - VOUT
differential,
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