MAX1795/MAX1796/
MAX1797
Low-Supply Current, Step-Up DC-DC Converters
with True Shutdown
General Description
The MAX1795/MAX1796/MAX1797 are high-efficiency,
step-up DC-DC converters intended for small portable
hand-held devices. These devices feature Maxim’s True
Shutdown™ circuitry, which fully disconnects the output from the input in shutdown, improves efficiency,
and eliminates costly external components. All three
devices also feature Maxim’s proprietary LX-damping
circuitry for reduced EMI in noise-sensitive applications.
For additional in-system flexibility, a battery monitoring
comparator (LBI/LBO) remains active even when the
DC-DC converter is in shutdown.
The input voltage range is +0.7V to VOUT, where VOUT
can be set from +2V to +5.5V. Startup is guaranteed
from +0.85V. The MAX1795/MAX1796/MAX1797 have a
preset, pin-selectable 5V or 3.3V output. The output can
also be adjusted to other voltages, using two external
resistors. The three devices differ only in their current
limits, allowing optimization of external components for
different loads: The MAX1795, MAX1796, and MAX1797
have current limits of 0.25A, 0.5A, and 1A, respectively.
All devices are packaged in a compact, 8-pin μMAX
package that is only 1.09mm tall and half the size of an
8-pin SO.
Features
●● > 95% Efficiency
●● True-Shutdown Circuitry
• Output Disconnects from Input in Shutdown
• No External Schottky Diode Needed
●● 25μA Quiescent Supply Current
●● Low-Noise Antiringing Feature
●● LBI/LBO Comparator Enabled in Shutdown
●● 2μA Shutdown Current
●● 8-Pin μMAX Package
Ordering Information
PART
TEMP RANGE
PIN-PACKAGE
MAX1795EUA
-40°C to +85°C
8 μMAX
MAX1796EUA
-40°C to +85°C
8 μMAX
MAX1797EUA
-40°C to +85°C
8 μMAX
Applications
●●
●●
●●
●●
Portable Digital Audio Players
PDAs/Palmtops
Wireless Handsets
Portable Terminals
Pin Configuration
Typical Operating Circuit
IN
0.7V TO
5.5V
TOP VIEW
BATT
LBI
1
FB
2
LBO
3
SHDN
4
MAX1795
MAX1796
MAX1797
8
BATT
7
OUT
6
LX
5
GND
µMAX
True Shutdown is a trademark of Maxim Integrated Products.
19-1798; Rev 0; 12/00
LBI
LBO
OFF
LX
MAX1795
OUT
MAX1796
MAX1797 FB
SHDN
ON
GND
OUT
MAX1795/MAX1796/
MAX1797
Low-Supply Current, Step-Up DC-DC Converters
with True Shutdown
Absolute Maximum Ratings
OUT, LX, SHDN, LBI, LBO, BATT to GND...............-0.3V to +6V
FB............................................................ -0.3V to (VOUT + 0.3V)
ILX, IOUT..............................................................................±1.5A
Output Short-Circuit Duration................................................... 5s
Continuous Power Dissipation
8-Pin μMAX (derate 4.1mW/°C above +70°C).............330mW
Operating Temperature Range............................ -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
(VBATT = +2V, OUT = FB (VOUT = +3.3V), SHDN = LBI = GND, TA = 0°C to +85°C, unless otherwise noted. Typical values are at
TA = +25°C.)
PARAMETER
SYMBOL
Minimum Input Voltage
Operating Voltage
CONDITIONS
MIN
After startup
VBATT
Startup Voltage
(Note 1)
TA = +25°C, RL = 3kΩ
0.85
FB = GND
4.80
5.0
5.20
2.0
IOUT
100
180
MAX1796
200
300
MAX1797
400
550
BATT = +2V,
FB = GND
(VOUT = +5.0V)
MAX1795
50
120
MAX1796
100
200
MAX1797
250
370
1.20
1.24
1.28
V
4
100
nA
NFET
0.17
0.3
PFET
0.27
0.45
RDS(ON)
VOUT = +3.3V,
ILX = 100mA
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mA
MAX1795
0.2
0.25
0.35
MAX1796
0.4
0.5
0.625
MAX1797
0.8
1.0
1.25
VLX = 0 and +5.5V, VOUT = +5.5V
Synchronous Rectifier Turn-Off
Current Limit
Operating Current into OUT
(Note 2)
V
MAX1795
VFB = +1.24V
ILEAK
5.5
V
BATT = +2V,
FB = OUT
(VOUT = +3.3V)
IFB
ILIM
mV/°C
3.43
Feedback Input Current
Damping Switch On-Resistance
V
3.3
VOUT = +2V to +5.5V
LX Leakage Current
1.0
3.17
VFB
LX Switch Current Limit
(NFET only)
V
FB = OUT
Feedback Set-Point Voltage
(Adjustable Mode)
Internal NFET, PFET
On-Resistance
5.5
-2.2
VOUT
UNITS
V
1.0
Adjustable Output Voltage Range
Steady-State Output Current
MAX
0.7
Startup Voltage Tempco
Output Voltage
TYP
RDAMP
100
VFB = +1.4V
Ω
A
0.2
µA
25
mA
200
400
Ω
25
45
µA
Maxim Integrated │ 2
MAX1795/MAX1796/
MAX1797
Low-Supply Current, Step-Up DC-DC Converters
with True Shutdown
Electrical Characteristics (continued)
(VBATT = +2V, OUT = FB (VOUT = +3.3V), SHDN = LBI = GND, TA = 0°C to +85°C, unless otherwise noted. Typical values are at
TA = +25°C.)
TYP
MAX
UNITS
Operating Current into BATT
PARAMETER
SYMBOL
VFB = +1.4V, VLBI = +1V
CONDITIONS
2
4
µA
Shutdown Current into BATT
SHDN = BATT, VLBI = +1V
2
4
µA
3
4
5
µs
0.8
1
1.2
µs
LX Switch MaxImum On-Time
tON
VFB = +1V, if current limit not reached
LX Switch Minimum Off-Time
tOFF
VFB = +1V
LBI Threshold Voltage Falling
VLBI
MIN
VBATT = +2V
0.8
0.85
0.90
VBATT = LBI
0.875
0.925
0.975
LBI Hysteresis
LBI Input Current
25
ILBI
LBO Low Output Voltage
LBO Off-Leakage Current
VLBI = +0.8V
9
mV
100
VBATT = VLBI = +0.975V,
sinking 20µA (50Ω typ)
0.1
VBATT = VLBI = +1.1V,
sinking 100µA (25Ω typ)
0.1
nA
V
VLBO = +5.5V
1
100
0.2 x
VBATT
VIL
SHDN Input Voltage
0.8 x
VBATT
VIH
Shutdown Input Current
V
VSHDN = 0 and +5.5V
100
nA
V
nA
Electrical Characteristics
(VBATT = +2V, OUT = FB (VOUT = +3.3V), SHDN = LBI = GND, TA = -40°C to +85°C, unless otherwise noted.) (Note 3)
PARAMETER
SYMBOL
Operating Voltage
VBATT
Output Voltage
VOUT
MIN
MAX
UNITS
Note 1
CONDITIONS
1.0
5.5
V
FB = OUT
3.13
3.47
FB = GND
4.75
5.25
2.0
5.5
Adjustable Output Voltage Range
Steady-State Output Current
(Note 1)
FB = OUT
(VOUT = +3.3V)
IOUT
FB = GND
(VOUT = +5.0V)
Feedback Set-Point Voltage
(Adjustable Mode)
VFB
VOUT = +2V to +5.5V
Feedback Input Current
IFB
VFB = +1.25V
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MAX1795
100
MAX1796
200
MAX1797
400
MAX1795
60
MAX1796
125
MAX1797
250
1.19
V
V
mA
1.29
V
100
nA
Maxim Integrated │ 3
MAX1795/MAX1796/
MAX1797
Low-Supply Current, Step-Up DC-DC Converters
with True Shutdown
Electrical Characteristics (continued)
(VBATT = +2V, OUT = FB (VOUT = +3.3V), SHDN = LBI = GND, TA = -40°C to +85°C, unless otherwise noted.) (Note 3)
PARAMETER
Internal NFET, PFET
On-Resistance
LX Switch Current Limit
(NFET only)
LX Leakage Current
Damping Switch On-Resistance
SYMBOL
RDS(ON)
ILIM
ILEAK
CONDITIONS
VOUT = +3.3V,
ILX = 100mA
MIN
MAX
NFET
0.3
PFET
0.45
MAX1795
0.19
0.37
MAX1796
0.35
0.7
MAX1797
0.8
1.32
UNITS
VLX = 0 and +5.5V, VOUT = +5.5V
RDAMP
Ω
A
µA
100
400
Ω
Operating Current into OUT
(Note 2)
VFB = +1.4V
45
µA
Operating Current into BATT
VFB = +1.4V, VLBI = +1V
4
µA
Shutdown Current into BATT
SHDN = BATT, VLBI = +1V
4
µA
LX Switch Maximum On-Time
tON
VFB = +1V, if current limit not reached
2.75
5.25
µs
LX Switch Minimum Off-Time
tOFF
VFB = +1V
0.7
1.3
µs
LBI Threshold Voltage
VLBI
VBATT = +2V
0.8
0.90
VBATT = LBI
0.875
0.975
LBI Input Current
ILBI
LBO Low Output Voltage
LBO Off-Leakage Current
VLBI = +0.8V
100
VBATT = VLBI = +0.975V,
sinking 20µA (50Ω typ)
0.1
VBATT = VLBI = +1.1V,
sinking 100µA (25Ω typ)
0.1
VLBO = +5.5V
100
0.2 x
VBATT
0.8 x
VBATT
VIH
Shutdown Input Current
VSHDN = 0 and +5.5V
nA
V
VIL
SHDN Input Voltage
V
100
nA
V
nA
Note 1: Operating Voltage: Since the regulator is bootstrapped to the output, once started it will operate down to a 0.7V input.
Note 2: Device is bootstrapped (power to IC comes from OUT). This correlates directly with the actual battery supply current.
Note 3: Specifications to -40°C are guaranteed by design, not production tested.
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Maxim Integrated │ 4
MAX1795/MAX1796/
MAX1797
Low-Supply Current, Step-Up DC-DC Converters
with True Shutdown
Typical Operating Characteristics
(L = 22μH, CIN = 47μF, COUT = 47μF, TA = +25°C, unless otherwise noted.)
60
50
40
30
50
40
30
10
10
0
0
0.1
1
10
100
1000
VBATT = +1.2V
60
20
80
60
50
40
30
20
10
L = 10µH
0.1
1
10
100
0
1000
1
10
100
1000
MAX1797
EFFICIENCY vs. LOAD CURRENT (+3.3V)
80
EFFICIENCY (%)
VBATT = +1.2V
40
30
VBATT = +3.6V
90
100
MAX1795/96/97 toc05
100
70
VBATT = +2.4V
60
VBATT = +1.2V
50
90
40
30
70
60
40
30
20
20
10
10
10
0
0
1
10
100
1000
0.1
1
10
100
0
1000
VBATT = +1.2V
50
20
0.1
1
10
100
LOAD CURRENT (mA)
LOAD CURRENT (mA)
NO-LOAD BATTERY CURRENT
vs. INPUT BATT VOLTAGE
STARTUP VOLTAGE
vs. LOAD CURRENT
SHUTDOWN THRESHOLD
vs. INPUT BATT VOLTAGE
200
150
VOUT = +5V
3.5
3.0
2.5
VOUT = +3.3V
2.0
1.5
1.0
VOUT = +3.3V
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
BATT VOLTAGE (V)
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0.5
0
0.1
1
10
100
LOAD CURRENT (mA)
1000
3.0
SHUTDOWN THRESHOLD (V)
250
MAX1795/96/97 toc08
4.0
1000
MAX1795/96/97 toc09
LOAD CURRENT (mA)
MAX1795/96/97 toc07
0.1
VBATT = +2.4V
80
EFFICIENCY (%)
VBATT = +2.4V
MAX1795/96/97 toc06
MAX1797
EFFICIENCY vs. LOAD CURRENT (+5V)
50
0
0.1
MAX1796
EFFICIENCY vs. LOAD CURRENT (+3.3V)
60
50
VBATT = +1.2V
LOAD CURRENT (mA)
70
100
VBATT = +2.4V
70
LOAD CURRENT (mA)
80
300
VBATT = +3.6V
90
LOAD CURRENT (mA)
MAX1795/96/97 toc04
90
EFFICIENCY (%)
70
20
100
BATTERY CURRENT (µA)
80
MAX1796
EFFICIENCY vs. LOAD CURRENT (+5V)
MAX1795/96/97 toc03
VBATT = +2.4V
VBATT = +1.2V
100
MAX1795/96/97 toc02
70
VBATT = +2.4V
90
EFFICIENCY (%)
EFFICIENCY (%)
80
100
MAX1795
EFFICIENCY vs. LOAD CURRENT (+3.3V)
EFFICIENCY (%)
VBATT = +3.6V
MAX1795/96/97 toc01
90
EFFICIENCY (%)
100
MAX1795
EFFICIENCY vs. LOAD CURRENT (+5V)
2.5
2.0
1.5
1.0
0.5
0
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
BATT VOLTAGE (V)
Maxim Integrated │ 5
MAX1795/MAX1796/
MAX1797
Low-Supply Current, Step-Up DC-DC Converters
with True Shutdown
Typical Operating Characteristics (continued)
(L = 22μH, CIN = 47μF, COUT = 47μF, TA = +25°C, unless otherwise noted.)
INCREASING VLBI
0.875
DECREASING VLBI
0.825
0.800
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
0.875
0.850
DECREASING VLBI
-40
BATT VOLTAGE (V)
-15
10
35
60
85
LOAD CURRENT (mA)
400
VOUT = +3.3V
VOUT = +5.0V
1.6
0
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
MAX1797
MAXIMUM OUTPUT CURRENT
vs. BATT INPUT VOLTAGE
1000
800
VOUT = +3.3V
600
400
VOUT = +5.0V
0
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
0
0
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
BATT VOLTAGE (V)
BATT VOLTAGE (V)
OUT LEAKAGE CURRENT
vs. OUTPUT VOLTAGE
HEAVY-LOAD SWITCHING WAVEFORMS
MAX1795/96/97 toc16
MAX1795/96/97 toc15
OUT LEAKAGE CURRENT (A)
2.0
0
200
100
0
VOUT = +5.0V
100
BATT VOLTAGE (V)
LOAD CURRENT (mA)
MAX1795/96/97 toc13
500
200
150
TEMPERATURE (°C)
MAX1796
MAXIMUM OUTPUT CURRENT
vs. BATT INPUT VOLTAGE
300
VOUT = +3.3V
50
0.825
0.800
SHDN = BATT
VOUT = +5V
VBATT = +2.4V
VLX
5V/div
1.2
IINDUCTOR
500mA/div
0.8
VOUT
(AC-COUPLED)
100mV/div
OUT BIASED WITH
EXTERNAL VOLTAGE
SOURCE
0.4
0
MAX1795/96/97 toc12
INCREASING VLBI
0.900
200
MAX1795/96/97 toc14
0.850
VBATT = +3.6V
0.925
250
LOAD CURRENT (mA)
0.900
MAX1795/96/97 toc11
MAX1795/96/97 toc10
0.925
MAX1795
MAXIMUM OUTPUT CURRENT
vs. BATT INPUT VOLTAGE
LOW-BATTERY INPUT THRESHOLD
vs. TEMPERATURE
0.950
LOW-BATTERY INPUT THRESHOLD (V)
LOW-BATTERY INPUT THRESHOLD (V)
0.950
LOW-BATTERY INPUT THRESHOLD
vs. INPUT BATT VOLTAGE
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
OUTPUT VOLTAGE (V)
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VIN = +3.6V
VOUT = +5.0V
ILOAD = 400mA
4.00µs/div
Maxim Integrated │ 6
MAX1795/MAX1796/
MAX1797
Low-Supply Current, Step-Up DC-DC Converters
with True Shutdown
Typical Operating Characteristics (continued)
(L = 22μH, CIN = 47μF, COUT = 47μF, TA = +25°C, unless otherwise noted.)
LINE-TRANSIENT RESPONSE
LIGHT-LOAD SWITCHING WAVEFORMS
MAX1795/96/97 toc18
MAX1795/96/97 toc17
VLX
5V/div
VBATT
+2.7V TO +3V
IINDUCTOR
500mA/div
VOUT
(AC-COUPLED)
20mV/div
VOUT
(AC-COUPLED)
100mV/div
VBATT = +3.6V
VOUT = +5.0V
ILOAD = 40mA
10µs/div
VBATT = +2.7V TO +3V
VOUT = +5.0V
NO LOAD
20µs/div
LOAD-TRANSIENT RESPONSE
STARTUP-SHUTDOWN WAVEFORMS
MAX1795/96/97 toc19
MAX1795/96/97 toc20
VSHDN
5V/div
IOUT
100mA/div
VOUT
2V/div
VOUT
100mV/div
VBATT = +2.4V
VOUT = +3.3V
ILOAD = 0 TO 325mA
IINDUCTOR
500mA/div
40µs/div
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VBATT = +2.4V
VOUT = +5.0V
ILOAD = 200mA
2ms/div
Maxim Integrated │ 7
MAX1795/MAX1796/
MAX1797
Low-Supply Current, Step-Up DC-DC Converters
with True Shutdown
Pin Description
PIN
NAME
FUNCTION
1
LBI
Low-Battery Comparator Input. Internally set to trip at +0.85V. This function remains operational in
shutdown.
2
FB
Dual-Mode™ Feedback Input. Connect to GND for preset 5.0V output. Connect to OUT for preset 3.3V
output. Connect a resistive voltage-divider from OUT to GND to adjust the output voltage from 2V to 5.5V.
3
LBO
4
SHDN
5
GND
6
LX
Low-Battery Comparator Output, Open-Drain Output. LBO is high impedance when VLBI < 0.85V.
This function remains operational in shutdown.
Shutdown Input. If SHDN is high, the device is in shutdown mode, OUT is high impedance, and LBI/LBO
are still operational. Connect shutdown to GND for normal operation.
Ground
Inductor Connection
7
OUT
Power Output. OUT provides bootstrap power to the IC.
8
BATT
Battery Input and Damping Switch Connection
Detailed Description
The MAX1795/MAX1796/MAX1797 compact step-up
DC-DC converters start up with voltages as low as
0.85V and operate with an input voltage down to +0.7V.
Consuming only 25μA of quiescent current, these devices
have an internal synchronous rectifier that reduces
cost by eliminating the need for an external diode and
improves overall efficiency by minimizing losses in the
circuit (see Synchronous Rectification section for details).
The internal N-channel MOSFET power switch resistance
is typically 0.17Ω, which minimizes losses. The LX switch
current limits of the MAX1795/MAX1796/MAX1797 are
0.25A, 0.5A, and 1A, respectively.
All three devices offer Maxim’s proprietary True Shutdown
circuitry, which disconnects the output from the input
in shutdown and puts the output in a high impedance
state. These devices also feature Maxim’s proprietary
LX-damping circuitry, which reduces EMI in noise-sensitive applications. For additional in-system flexibility, the
LBI/LBO comparator remains active in shutdown. Figure
1 is a typical application circuit.
Control Scheme
A unique minimum-off-time, current-limited control
scheme is the key to the MAX1795/MAX1796/MAX1797s’
low operating current and high efficiency over a wide
load range. The architecture combines the high output
power and efficiency of a pulse-width-modulation (PWM)
device with the ultra-low quiescent current of a traditional
VIN
22µH
1M
SHDN
VIN
LBO
LBI
BATT
47µF
LX
VOUT = 3.3V
OUT
COUT*
MAX1795
MAX1796
MAX1797 GND
FB
*SEE TABLE 1 FOR COMPONENT VALUES.
Figure 1. Typical Application Circuit
pulse-skipping controller (Figure 2). Switching frequency
depends upon the load current and input voltage, and can
range up to 500kHz. Unlike conventional pulse-skipping
DC-DC converters (where ripple amplitude varies with
input voltage), ripple in these devices does not exceed the
product of the switch current limit and the filter-capacitor
equivalent series resistance (ESR).
Dual Mode is a trademark of Maxim Integrated Products.
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Maxim Integrated │ 8
MAX1795/MAX1796/
MAX1797
Low-Supply Current, Step-Up DC-DC Converters
with True Shutdown
BATT
R1
1M
SHDN
MAX1795
MAX1796
MAX1797
LBO
LBI
+
_
ZEROCROSSING
AMPLIFIER
S
OUT
Q
OUT
R
47µF
BODY
DIODE
CONTROL
0.85V
S
Q
BATT
BATT
R
22µH
TIMER BLOCK
47F
S
Q
START
TON MAX
LX
R
TOFF
MAX
OUT
R2
FB
R3
FB SELECT
REFERENCE
ERROR
AMPLIFIER
CURRENT-LIMIT
AMPLIFIER
GND
Figure 2. Functional Diagram
Synchronous Rectification
The internal synchronous rectifier eliminates the need
for an external Schottky diode, reducing cost and board
space. During the cycle off-time, the P-channel MOSFET
turns on and shunts the MOSFET body diode. As a
result, the synchronous rectifier significantly improves
efficiency without the addition of an external component.
Conversion efficiency can be as high as 95%, as shown
in the Typical Operating Characteristics section.
Shutdown
The device enters shutdown when VSHDN is high,
reducing supply current to less than 2μA. During shutdown, the synchronous rectifier disconnects the output
from the input, eliminating the DC conduction path
that normally exists with traditional boost converters in
shutdown mode. In shutdown, OUT becomes a high-
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impedance node. The LBI/LBO comparator remains
active in shutdown.
As shown in Figure 1, the MAX1795/MAX1796/MAX1797
can be automatically shut down when the input voltage
drops below a preset threshold by connecting LBO to
SHDN (see the Low-Battery Detection section).
BATT/Damping Switch
The MAX1795/MAX1796/MAX1797 each contain an internal damping switch to minimize ringing at LX. The damping switch connects a resistor across the inductor when
the inductor’s energy is depleted (Figure 3). Normally,
when the energy in the inductor is insufficient to supply
current to the output, the capacitance and inductance at
LX form a resonant circuit that causes ringing. The ringing
continues until the energy is dissipated through the series
resistance of the inductor. The damping switch supplies a
Maxim Integrated │ 9
MAX1795/MAX1796/
MAX1797
Low-Supply Current, Step-Up DC-DC Converters
with True Shutdown
path to quickly dissipate this energy, minimizing the ringing
at LX. Damping LX ringing does not reduce VOUT ripple,
but does reduce EMI (Figure 3, Figure 4, and Figure 5).
VIN
Setting the Output Voltage
BATT
MAX1795
MAX1796
MAX1797
R1
200Ω
VOUT can be set to 3.3V or 5.0V by connecting the FB
pin to GND (5V) or OUT (3.3V). To adjust the output
voltage, connect a resistive voltage-divider from OUT to
FB to GND (Figure 6). Choose a value less than 250kΩ
for R2.
22µH
DAMPING
SWITCH
LX
OUT
VOUT
VIN
47µF
47µF
BATT
Figure 3. Simplified Diagram of Inductor Damping Switch
LX
R3
OUT
SHDN
LBI
R4
VLX
1V/div
MAX1795
MAX1796
MAX1797
GND
OUTPUT
2V TO 5.5V
47µF
1M
R1
LOW-BATTERY
OUTPUT
LBO
FB
R2
Figure 6. Setting an Adjustable Output
Use the following equation to calculate R1:
2µs/div
R1 = R2 [(VOUT/VFB) - 1]
Figure 4. LX Ringing for Conventional Step-Up Converter
(without Damping Switch)
where VFB = +1.245V, and VOUT can range from +2V to
+5.5V.
Low-Battery Detection
VLX
1V/div
2µs/div
Figure 5. LX Waveform with Damping Switch
www.maximintegrated.com
The MAX1795/MAX1796/MAX1797 each contain an onchip comparator for low-battery detection. If the voltage
at LBI is above 0.85V, LBO (an open-drain output) sinks
current to GND. If the voltage at LBI is below 0.85V, LBO
goes high impedance. The LBI/LBO function remains
active even when the part is in shutdown.
Connect a resistive voltage-divider to LBI from BATT to
GND. The low-battery monitor threshold is set by two
resistors, R3 and R4 (Figure 6). Since the LBI bias current
is typically 2nA, large resistor values (R4 up to 250kΩ)
can be used to minimize loading of the input supply.
Calculate R3 using the following equation:
R3 = R4[(VTRIP/0.85V) - 1]
Maxim Integrated │ 10
MAX1795/MAX1796/
MAX1797
Low-Supply Current, Step-Up DC-DC Converters
with True Shutdown
VTRIP is the input voltage where the low-battery detector
output goes high impedance.
For single-cell applications, LBI may be connected to the
battery. When VBATT , the LBI threshold increases to
0.925V (see the Typical Operating Characteristics section).
Connect a pullup resistor of 100kΩ or greater from LBO to
OUT for a logic output. LBO is an open-drain output and
can be pulled as high as 6V regardless of the voltage at
OUT. When LBI is below the threshold, the LBO output
is high impedance. If the low-battery comparator is not
used, ground LBI and LBO.
Applications Information
Inductor Selection
An inductor value of 22μH performs well in most applications. The MAX1795/MAX1796/MAX1797 will also
work with inductors in the 10μH to 47μH range. Smaller
inductance values typically offer a smaller physical size
for a given series resistance, allowing the smallest overall
circuit dimensions, but have lower output current capability. Circuits using larger inductance values exhibit higher
output current capability, but are physically larger for the
same series resistance and current rating.
The inductor’s incremental saturation current rating should
be greater than the peak switch-current limit, which is
0.25A for the MAX1795, 0.5A for the MAX1796, and 1A for
the MAX1797. However, it is generally acceptable to bias
the inductor into saturation by as much as 20% although
this will slightly reduce efficiency. Table 1 lists some suggested components for typical applications.
The inductor’s DC resistance significantly affects efficiency. Calculate the maximum output current (IOUT(MAX))
as follows, using inductor ripple current (IRIP) and duty
cycle (D):
IRIP =
VOUT + ILIM × (R PFET + L ESR ) − VBATT
L
(R
+ L ESR )
+ PFET
2
t OFF
I
VOUT + ILIM − RIP × (R PFET + L ESR ) − VBATT
2
D=
IRIP
VOUT + ILIM −
× (R PFET − R NFET + L ESR )
2
and
www.maximintegrated.com
IRIP
=
I OUT(MAX
) ILIM +
2
where: IRIP = Inductor ripple current (A)
VOUT = Output voltage (V)
ILIM = Device current limit (0.25A, 0.5A, or 1A)
RPFET = On-resistance of P-channel MOSFET
(Ω) (typ 0.27Ω)
LESR = ESR of Inductor (Ω) (typ 0.095Ω)
VBATT = Input voltage (V)
L = Inductor value in μH
tOFF = LX switch’s off-time (μs) (typ 1μs)
D = Duty cycle
RNFET = On-resistance of N-channel MOSFET
(Ω) (typ 0.17Ω)
IOUT(MAX) = Maximum output current (A)
Capacitor Selection
Table 1 lists suggested tantalum or polymer capacitor
values for typical applications. The ESR of both input
bypass and output filter capacitors affects efficiency and
output ripple. Output voltage ripple is the product of the
peak inductor current and the output capacitor ESR. Highfrequency output noise can be reduced by connecting a
0.1μF ceramic capacitor in parallel with the output filter
capacitor. See Table 2 for a list of suggested component
suppliers.
PC Board Layout and Grounding
Careful printed circuit layout is important for minimizing
ground bounce and noise. Keep the IC’s GND pin and the
ground leads of the input and output filter capacitors less
than 0.2in (5mm) apart. In addition, keep all connections
to the FB and LX pins as short as possible. In particular,
when using external feedback resistors, locate them
as close to FB as possible. To maximize output power
and efficiency and minimize output ripple voltage, use a
ground plane and solder the IC’s GND pin directly to the
ground plane.
Maxim Integrated │ 11
MAX1795/MAX1796/
MAX1797
Low-Supply Current, Step-Up DC-DC Converters
with True Shutdown
Table 1. Suggested Components for Typical Applications
COMPONENT VALUE
(MAX1797,
1A CURRENT LIMIT)
COMPONENT
COMPONENT VALUE
(MAX1796,
0.5A CURRENT LIMIT)
COMPONENT VALUE
(MAX1795, 0.25A CURRENT LIMIT)
Sumida CR32-220, 22µH
Sumida CDRH6D28-220, 22µH
Sumida CR32-100, 10µH
Sumida CDRH4D28-220, 22µH
Murata CQH3C100K34, 10µH
Inductor
Murata CQH4N100K(J)04, 10µH
Coilcraft DS3316P-223, 22µH
Coilcraft DS1608C-223, 22µH
Coilcraft DS1608C-223, 22µH
Coilcraft DS1608C-103, 10µH
Input Capacitor
Output Capacitor
Sanyo POSCAP 6TPA47M,
47µF
Sanyo POSCAP 6TPA47M,
47µF
Sanyo POSCAP 6TPA47M, 47µF
AVX TPSD476M016R0150,
47µF
AVX TPSD226M016R0150,
22µF
AVX TPSD106M016R0150, 10µF
Taiyo Yuden UMK316BI150KH,
0.1µF
Taiyo Yuden UMK316BI150KH,
0.1µF
Taiyo Yuden UMK316BI150KH, 0.1µF
Chip Information
Table 2. Component Suppliers
COMPANY
PHONE
FAX
AVX
USA 803-946-0690
USA 803-626-3123
Coilcraft
USA 847-639-6400
USA 847-639-1238469
Coiltronics
USA 561-241-7876
USA 561-241-9339
Murata
USA 814-237-1431
1-800-831-9172
USA 814-238-0490
Nihon
USA 805-867-2555
Japan 81-3-34947411
USA 805-867-2556
Japan 81-3-34947414
Sanyo
USA 619-661-6835
Japan 81-7-20706306
USA 619-661-1055
Japan 81-7-20701174
Sprague
USA 603-224-1961
USA 603-224-1430
Sumida
USA 647-956-0666
Japan 81-3-36075111
USA 647-956-0702
Japan 81-3-36075144
Taiyo
Yuden
USA 408-573-4150
USA 408-573-4159
www.maximintegrated.com
TRANSISTOR COUNT: 1100
PROCESS: BiCMOS
Maxim Integrated │ 12
MAX1795/MAX1796/
MAX1797
Low-Supply Current, Step-Up DC-DC Converters
with True Shutdown
Package Information
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.
© 2000 Maxim Integrated Products, Inc. │ 13
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