Maxim MAX1795EUA Low supply current, step-up dc-dc converters with true-shutdown Datasheet

19-1798; Rev 0; 12/00
Low Supply Current, Step-Up DC-DC Converters
with True-Shutdown
Features
♦ >95% Efficiency
________________________Applications
True-Shutdown is a trademark of Maxim Integrated Products.
♦ 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
MAX1795 EUA
-40°C to +85°C
8 µMAX
MAX1796 EUA
-40°C to +85°C
8 µMAX
MAX1797 EUA
-40°C to +85°C
8 µMAX
Portable Digital Audio Players
PDAs/Palmtops
Wireless Handsets
Portable Terminals
Pin Configuration
Typical Operating Circuit
IN
0.7V TO
5.5V
TOP VIEW
LBI
1
FB
2
LBO
3
SHDN
4
8
MAX1795
MAX1796
MAX1797
µMAX
BATT
BATT
7
OUT
6
LX
5
GND
LBI
LX
MAX1795
MAX1796
MAX1797
OUT
OUT
FB
LBO
OFF
SHDN
ON
GND
________________________________________________________________ Maxim Integrated Products
1
For price, delivery, and to place orders, please contact Maxim Distribution at 1-888-629-4642,
or visit Maxim’s website at www.maxim-ic.com.
MAX1795/MAX1796/MAX1797
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 8pin µMAX package that is only 1.09mm tall and half the
size of an 8-pin SO.
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)
1.0
TA = +25°C, RL = 3kΩ
0.85
VOUT
3.3
3.43
5.0
5.20
IOUT
VFB
VOUT = +2V to +5.5V
Feedback Input Current
IFB
VFB = +1.24V
Internal NFET, PFET OnResistance
RDS(ON)
VOUT = +3.3V,
ILX = 100mA
ILIM
ILEAK
2
5.5
100
180
200
300
MAX1797
400
550
MAX1795
50
120
MAX1796
100
200
MAX1797
250
370
1.20
1.24
1.28
V
4
100
nA
mA
NFET
0.17
0.3
PFET
0.27
0.45
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
100
VFB = +1.4V
V
MAX1796
MAX1795
RDAMP
V
MAX1795
Synchronous Rectifier Turn-Off
Current Limit
Operating Current into OUT
(Note 2)
mV/°C
4.80
Feedback Set-Point Voltage
(Adjustable Mode)
Damping Switch On-Resistance
V
3.17
BATT = +2V,
FB = GND
(VOUT = +5.0V)
LX Leakage Current
V
1.0
FB = OUT
BATT = +2V,
FB = OUT
(VOUT = +3.3V)
LX Switch Current Limit (NFET
only)
5.5
FB = GND
2.0
UNITS
V
-2.2
Adjustable Output Voltage
Range
Steady-State Output Current
MAX
0.7
Startup Voltage Tempco
Output Voltage
TYP
Ω
A
0.2
µA
25
mA
200
400
Ω
25
45
µA
_______________________________________________________________________________________
Low Supply Current, Step-Up DC-DC Converters
with True-Shutdown
(VBATT = +2V, OUT = FB (VOUT = +3.3V), SHDN = LBI = GND, TA = 0°C to +85°C, unless otherwise noted. Typical values are
TA = +25°C.)
PARAMETER
SYMBOL
Operating Current into BATT
CONDITIONS
MIN
VFB = +1.4V, VLBI = +1V
Shutdown Current into BATT
tON
VFB = +1V, if current limit not reached
LX Switch Minimum Off-Time
tOFF
VFB = +1V
LBI Threshold Voltage Falling
VLBI
MAX
UNITS
2
4
µA
2
4
µA
3
4
5
µs
0.8
1
1.2
µs
SHDN = BATT, VLBI = +1V
LX Switch MaxImum On-Time
TYP
VBATT = +2V
0.8
0.85
0.90
VBATT = LBI
0.875
0.925
0.975
LBI Hysteresis
25
LBI Input Current
ILBI
VLBI = +0.8V
9
V
mV
100
VBATT = VLBI = +0.975V,
sinking 20µA (50Ω typ)
0.1
VBATT = VLBI = +1.1V,
sinking 100µA (25Ω typ)
0.1
LBO Low Output Voltage
nA
V
LBO Off-Leakage Current
VLBO = +5.5V
1
0.2 ✕
VBATT
VIL
SHDN Input Voltage
0.8 ✕
VBATT
VIH
Shutdown Input Current
100
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
Operating Voltage
SYMBOL
VBATT
Output Voltage
VOUT
CONDITIONS
MAX
UNITS
V
Note 1
1.0
5.5
FB = OUT
3.13
3.47
FB = GND
4.75
5.25
2.0
5.5
Adjustable Output Voltage
Range
FB = OUT
(VOUT = +3.3V)
Steady-State Output Current
(Note 1)
MIN
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
MAX1795
100
MAX1796
200
MAX1797
400
MAX1795
60
MAX1796
125
MAX1797
250
1.19
V
V
mA
1.29
V
100
nA
_______________________________________________________________________________________
3
MAX1795/MAX1796/MAX1797
ELECTRICAL CHARACTERISTICS (continued)
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 OnResistance
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
MAX1796
0.35
0.7
MAX1797
0.8
1.32
UNITS
0.37
100
400
Ω
45
µA
Operating Current into OUT
(Note 2)
VFB = +1.4V
Operating Current into BATT
VFB = +1.4V, VLBI = +1V
4
µA
Shutdown Current into BATT
SHDN = BATT, VLBI = +1V
4
µA
5.25
µs
µs
LX Switch Maximum On-Time
tON
VFB = +1V, if current limit not reached
LX Switch Minimum Off-Time
tOFF
VFB = +1V
0.7
1.3
VBATT = +2V
0.8
0.90
VBATT = LBI
0.875
0.975
LBI Threshold Voltage
VLBI
LBI Input Current
ILBI
2.75
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
LBO Off-Leakage Current
0.2 ✕
VBATT
VIL
SHDN Input Voltage
0.8 ✕
VBATT
VIH
Shutdown Input Current
V
nA
V
LBO Low Output Voltage
VSHDN = 0 and +5.5V
100
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.
4
A
µA
VLX = 0 and +5.5V, VOUT = +5.5V
RDAMP
Ω
_______________________________________________________________________________________
nA
V
nA
Low Supply Current, Step-Up DC-DC Converters
with True-Shutdown
VBATT = +1.2V
60
50
40
VBATT = +1.2V
60
50
40
80
50
40
30
20
10
10
0
0
100
10
L = 10µH
0
1000
0.1
1
10
100
1000
0.1
1
10
100
1000
LOAD CURRENT (mA)
LOAD CURRENT (mA)
LOAD CURRENT (mA)
MAX1796
EFFICIENCY vs. LOAD CURRENT (+3.3V)
MAX1797
EFFICIENCY vs. LOAD CURRENT (+5V)
MAX1797
EFFICIENCY vs. LOAD CURRENT (+3.3V)
60
50
40
100
70
VBATT = +2.4V
60
VBATT = +1.2V
50
40
80
70
60
40
30
30
20
20
10
10
10
0
0
10
100
1000
VBATT = +1.2V
50
20
1
VBATT = +2.4V
90
30
0
0.1
1
10
100
1000
0.1
1
10
100
LOAD CURRENT (mA)
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
3.0
2.5
VOUT = +3.3V
2.0
1.5
1.0
50
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)
2.5
2.0
1.5
1.0
0.5
0.5
0
0
0
1000
MAX1795/96/97 toc09
3.5
EFFICIENCY (%)
250
3.0
SHUTDOWN THRESHOLD (V)
4.0
MAX1795/96/97 toc07
300
MAX1795/96/97 toc08
0.1
MAX1795/96/97 toc06
80
EFFICIENCY (%)
VBATT = +1.2V
VBATT = +3.6V
90
EFFICIENCY (%)
80
70
100
MAX1795/96/97 toc05
VBATT = +2.4V
90
100
VBATT = +1.2V
60
20
10
VBATT = +2.4V
70
20
1
VBATT = +3.6V
90
30
100
EFFICIENCY (%)
70
100
30
0.1
BATTERY CURRENT (µA)
80
EFFICIENCY (%)
VBATT = +2.4V
90
MAX1795/96/97 toc04
EFFICIENCY (%)
80
VBATT = +2.4V
MAX1795/96/97 toc03
90
MAX1796
EFFICIENCY vs. LOAD CURRENT (+5V)
EFFICIENCY (%)
VBATT = +3.6V
70
100
MAX1795/96/97 toc01
100
MAX1795
EFFICIENCY vs. LOAD CURRENT (+3.3V)
MAX1795/96/97 toc02
MAX1795
EFFICIENCY vs. LOAD CURRENT (+5V)
0.1
1
10
100
LOAD CURRENT (mA)
1000
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)
_______________________________________________________________________________________
5
MAX1795/MAX1796/MAX1797
Typical Operating Characteristics
(L = 22µH, CIN = 47µF, COUT = 47µF, TA = +25°C, unless otherwise noted.)
Typical Operating Characteristics (continued)
(L = 22µH, CIN = 47µF, COUT = 47µF, TA = +25°C, unless otherwise noted.)
INCREASING VLBI
0.875
DECREASING VLBI
0.825
200
INCREASING VLBI
0.900
0.875
0.850
DECREASING VLBI
-40
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
-15
10
35
60
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
BATT VOLTAGE (V)
300
VOUT = +3.3V
200
VOUT = +5.0V
1000
800
LOAD CURRENT (mA)
LOAD CURRENT (mA)
400
VOUT = +3.3V
600
400
VOUT = +5.0V
200
100
0
0
0
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
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 (µA)
0
MAX1797
MAXIMUM OUTPUT CURRENT
vs. BATT INPUT VOLTAGE
MAX1795/96/97 toc13
500
SHDN = BATT
VOUT = +5V
VBATT = +2.4V
VLX
5V/div
1.2
IINDUCTOR
500mA/div
0.8
0.4
0
VOUT
(AC-COUPLED)
100mV/div
OUT BIASED WITH
EXTERNAL VOLTAGE
SOURCE
4.00µs/div
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)
6
VOUT = +5.0V
100
0
85
MAX1796
MAXIMUM OUTPUT CURRENT
vs. BATT INPUT VOLTAGE
1.6
150
TEMPERATURE (°C)
BATT VOLTAGE (V)
2.0
VOUT = +3.3V
50
0.825
0.800
0.800
MAX1795/96/97 toc12
0.925
MAX1795/96/97 toc14
0.850
VBATT = +3.6V
LOAD CURRENT (mA)
0.900
250
MAX1795/96/97 toc12
0.925
0.950
LOW-BATTERY INPUT THRESHOLD (V)
MAX1795/96/97 toc10
0.950
MAX1795
MAXIMUM OUTPUT CURRENT
vs. BATT INPUT VOLTAGE
LOW-BATTERY INPUT THRESHOLD
vs. TEMPERATURE
LOW-BATTERY INPUT THRESHOLD
vs. INPUT BATT VOLTAGE
LOW-BATTERY INPUT THRESHOLD (V)
MAX1795/MAX1796/MAX1797
Low Supply Current, Step-Up DC-DC Converters
with True-Shutdown
VIN = +3.6V
VOUT = +5.0V
ILOAD = 400mA
_______________________________________________________________________________________
Low Supply Current, Step-Up DC-DC Converters
with True-Shutdown
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
10µs/div
VBATT = +2.7V TO +3V
VOUT = +5.0V
NO LOAD
20µs/div
VBATT = +3.6V
VOUT = +5.0V
ILOAD = 40mA
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
IINDUCTOR
500mA/div
40µs/div
VBATT = +2.4V
VOUT = +3.3V
ILOAD = 0 TO 325mA
2ms/div
VBATT = +2.4V
VOUT = +5.0V
ILOAD = 200mA
_______________________________________________________________________________________
7
MAX1795/MAX1796/MAX1797
Typical Operating Characteristics (continued)
(L = 22µH, CIN = 47µF, COUT = 47µF, TA = +25°C, unless otherwise noted.)
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
Low-Battery Comparator Output, Open-Drain Output. LBO is low when VLBI < 0.85V. This function
remains operational in shutdown.
4
SHDN
5
GND
6
LX
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 TrueShutdown 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 cur-
VIN
22µH
BATT
LX
SHDN
VOUT = 3.3V
VIN
LBO
MAX1795
MAX1796
MAX1797
OUT
COUT*
GND
LBI
FB
*SEE TABLE 1 FOR COMPONENT VALUES.
Figure 1. Typical Application Circuit
rent of a traditional 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.
8
47µF
1M
_______________________________________________________________________________________
Low Supply Current, Step-Up DC-DC Converters
with True-Shutdown
MAX1795/MAX1796/MAX1797
BATT
R1
1M
SHDN
ZEROCROSSING
AMPLIFIER
MAX1795
MAX1796
MAX1797
LBO
LBI
+
_
S
Q
+
OUT
OUT
R
+
_
47µF
BODY
DIODE
CONTROL
0.85V
S
BATT
Q
BATT
R
22µH
TIMER BLOCK
47µF
S
Q
START
TON MAX
LX
R
TOFF
MAX
OUT
R2
FB
R3
FB SELECT
REFERENCE
+
_
+
_
CURRENT-LIMIT
AMPLIFIER
ERROR
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.
Shutdown
The device enters shutdown when V SHDN 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-imped-
ance node. The LBI/LBO comparator remains active in
shutdown.
As shown in Figure 1, the MAX1795/MAX1796/MAX1797
may be automatically shut down when the input voltage
drops below a preset threshold by connecting LBO to
SHDN (see 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 path to quickly dissi-
_______________________________________________________________________________________
9
MAX1795/MAX1796/MAX1797
Low Supply Current, Step-Up DC-DC Converters
with True-Shutdown
pate this energy, minimizing the ringing at LX.
Damping LX ringing does not reduce VOUT ripple, but
does reduce EMI (Figures 3, 4, and 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
VIN
VOUT
OUT
47µF
47µF
BATT
LX
Figure 3. Simplified Diagram of Inductor Damping Switch
R3
OUT
SHDN
LBI
R4
MAX1795
MAX1796
MAX1797
GND
OUTPUT
2V TO 5.5V
47µF
1M
R1
LOW-BATTERY
OUTPUT
LBO
FB
R2
VLX
1V/div
Figure 6. Setting an Adjustable Output
Use the following equation to calculate R1:
R1 = R2 [(VOUT / VFB) - 1]
where VFB = +1.245V, and VOUT may range from +2V
to +5.5V.
2µs/div
Figure 4. LX Ringing for Conventional Step-Up Converter
(without Damping Switch)
VLX
1V/div
2µs/div
Low-Battery Detection
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]
Figure 5. LX Waveform with Damping Switch
10
______________________________________________________________________________________
Low Supply Current, Step-Up DC-DC Converters
with True-Shutdown
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):
VOUT + ILIM
 L

 t OFF

VOUT + ILIM −

IRIP =
D =
× (RPFET + LESR ) − VBATT
(RPFET + LESR ) 
+

2

IRIP 
 (RPFET + LESR ) − VBATT
2 


I
IOUT(MAX) = ILIM − RIP  (1 − D)
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. High-frequency 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.


I
VOUT + ILIM − RIP  (RPFET − RNFET + LESR )
2 

and
______________________________________________________________________________________
11
MAX1795/MAX1796/MAX1797
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 V BATT <1.0V>, the LBI threshold
increases to 0.925V (see Typical Operating Characteristics).
MAX1795/MAX1796/MAX1797
Low Supply Current, Step-Up DC-DC Converters
with True-Shutdown
Table 1. Suggested Components for Typical Applications
COMPONENT
COMPONENT VALUE
(MAX1797, 1A CURRENT
LIMIT)
COMPONENT VALUE
(MAX1796, 0.5A CURRENT
LIMIT)
COMPONENT VALUE
(MAX1795, 0.25A CURRENT LIMIT)
Sumida CR32-220, 22µH
Sumida CDRH6D28-220, 22µH
Sumida CDRH4D28-220, 22µH
Sumida CR32-100, 10µH
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
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
Output Capacitor
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
US A 619-661-6835
Ja pan 81- 7- 20706306
US A 619-661-1055
Ja pan 81- 7- 20701174
Sprague
USA 603-224-1961
USA 603-224-1430
Sumida
USA 647-956-0666
Japan 81-3-36075111
US A 647-956-0702
Ja pan 81- 3- 36075144
Taiyo
Yuden
USA 408-573-4150
USA 408-573-4159
12
TRANSISTOR COUNT: 1100
PROCESS: BiCMOS
______________________________________________________________________________________
Low Supply Current, Step-Up DC-DC Converters
with True-Shutdown
8LUMAXD.EPS
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 _____________________13
© 2000 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products.
MAX1795/MAX1796/MAX1797
Package Information
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