_______________General Description
The MAX761/MAX762 step-up switching regulators
provide high efficiency over a wide range of load currents,
delivering up to 150mA. A unique, current-limited
pulse-frequency-modulated (PFM) control scheme gives
the devices the benefits of pulse-width-modulated (PWM)
converters (high efficiency with heavy loads), while using
less than 110A of supply current (vs. 2mA to 10mA for
PWM converters). The result is high efficiency over a wide
range of loads.
The MAX761/MAX762 input voltage range is 2V to 16.5V.
Output voltages are preset to 12V (MAX761) and 15V
(MAX762), or they can be set with two external resistors.
With a 5V input, the MAX761 guarantees a 12V, 150mA
output. Its high efficiency, low supply current, fast start-up
time, SHDN controlling capability, and small size make the
MAX761 ideal for powering flash memory.
The MAX761/MAX762 have an internal 1A power MOS-
FET, making them ideal for minimum-component, low- and
medium-power applications. These devices use tiny exter-
nal components, and their high switching frequencies (up
to 300kHz) allow for small surface-mount magnetics.
For increased output drive capability or higher output volt-
ages, use the MAX770MAX773, which are similar in
design to the MAX761/MAX762, but drive external power
MOSFETs. For stepping up to 5V, see the MAX756/
MAX757 and MAX856-MAX859 data sheets.
_________________________Applications
Flash Memory Programming
PCMCIA Cards
Battery-Powered Applications
High-Efficiency DC-DC Converters
____________________________Features
o
High Efficiency for a Wide Range of Load Currents
o
12V/150mA Flash Memory Programming Supply
o
110A Max Supply Current
o
5A Max Shutdown Supply Current
o
2V to 16.5V Input Voltage Range
o
12V (MAX761), 15V (MAX762) or Adjustable Output
o
Current-Limited PFM Control Scheme
o
300kHz Switching Frequency
o
Internal, 1A, N-Channel Power FET
o
LBI/LBO Low-Battery Comparator
______________Ordering Information
MAX761/MAX762
12V/15V or Adjustable, High-Efficiency,
Low I
Q
, Step-Up DC-DC Converters
________________________________________________________________
Maxim Integrated Products
1
1
2
3
4
8
7
6
5
V+
LX
GND
REF
SHDN
FB
LBI
LBO
MAX761
MAX762
DIP/SO
TOP VIEW
__________________Pin Configuration
MAX761
LBI
SHDN
V+
FB
REF
LX
OUTPUT
12V
150mA
LOW-BATTERY
DETECTOR OUTPUT
LOW-BATTERY
DETECTOR INPUT
INPUT
4.75V
TO 12V
ON/OFF
GND
LBO
33F
33F
18H
__________Typical Operating Circuit
Call toll free 1-800-998-8800 for free samples or literature.
19-0201; Rev 0; 11/93
PART
TEMP. RANGE
PIN-PACKAGE
MAX761
CPA
0C to +70C
8 Plastic DIP
MAX761CSA
0C to +70C
8 SO
MAX761C/D
0C to +70C
Dice*
MAX761ESA
-40C to +85C
8 SO
MAX761EPA
-40C to +85C
8 Plastic DIP
MAX761MJA
-55C to +125C
8 CERDIP**
MAX762
CPA
0C to +70C
8 Plastic DIP
MAX762CSA
0C to +70C
8 SO
MAX762C/D
0C to +70C
Dice*
MAX762EPA
-40C to +85C
8 Plastic DIP
MAX762ESA
-40C to +85C
8 SO
MAX762MJA
-55C to +125C
8 CERDIP**
* Contact factory for dice specifications.
** Contact factory for availability and processing to MIL-STD-883.
Evaluation Kit
Available
MAX761/MAX762
12V/15V or Adjustable, High-Efficiency,
Low I
Q
, Step-Up DC-DC Converters
2
_______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
Supply Voltage V+ to GND .......................................-0.3V to 17V
REF, LBO, LBI, SHDN, FB ............................-0.3V to (V+ + 0.3V)
LX..............................................................................-0.3V to 17V
LX Peak Current ....................................................................1.5A
LBO Current ..........................................................................5mA
Continuous Power Dissipation (T
A
= +70C)
Plastic DIP (derate 9.09mW/C above +70C) ............727mW
SO (derate 5.88mW/C above +70C) .........................471mW
CERDIP (derate 8.00mW/C above +70C) .................640mW
Operating Temperature Ranges:
MAX76_C_A ........................................................0C to +70C
MAX76_E_A .....................................................-40C to +85C
MAX76_MJA ..................................................-55C to +125C
Junction Temperatures:
MAX76_C_A/E_A..........................................................+150C
MAX76_MJA.................................................................+175C
Storage Temperature Range .............................-65C to +160C
Lead Temperature (soldering, 10sec) .............................+300C
ELECTRICAL CHARACTERISTICS
(V+ = 5V, I
LOAD
= 0mA, C
REF
= 0.1F, T
A
= T
MIN
to T
MAX,
typical values are at T
A
= +25C, unless otherwise noted.)
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.
PARAMETER
SYMBOL
MIN
TYP
MAX
88
110
Minimum Start-Up Voltage
1.7
2.0
V
Minimum Operating Voltage
1.7
V
Supply Current
300
A
Shutdown Current
1
5
A
11.52
12.0
12.48
2
16.5
3
16.5
Supply Voltage
V+
3.1
16.5
V
11.52
12.0
12.48
14.4
15.0
15.6
Output Voltage
(Note 1)
V
OUT
14.4
15.0
15.6
V
Peak Current at LX
I
PEAK
0.75
1.0
1.25
A
Maximum Switch-On Time
t
ON
6
8
10
s
Minimum Switch-Off Time
t
OFF
1.0
1.3
1.6
s
Load Regulation
0.0042
%/mA
Line Regulation
0.08
%/V
Efficiency
86
%
1.4700
1.50
1.5300
1.4625
1.50
1.5375
Reference Voltage
V
REF
1.4550
1.50
1.5450
V
CONDITIONS
Figure 2,
MAX761,
bootstrapped
V+ = 16.5V, normal operation, SHDN = 0V,
non-bootstrapped
Figure 2, bootstrapped
Figure 2, bootstrapped
Figure 2, MAX761, V
IN
= 5V, SHDN = 0V,
normal operation
Figure 2,
MAX762,
bootstrapped
V+ = 10.0V, shutdown mode, SHDN = V+
See Figure 4b
Figure 2, 0mA
I
LOAD
200mA, bootstrapped
Figure 2, bootstrapped
Figure 2, 4V
V
IN
6V, bootstrapped
Figure 2, bootstrapped, V
OUT
= 12V,
60mA
I
LOAD
120mA
Figure 3 or 5 with
external resistors.
MAX76_C
MAX76_E
MAX76_M
MAX76_C/E
MAX76_M
UNITS
0mA
I
LOAD
75mA,
3V
V+
12V
0mA
I
LOAD
150mA,
4.75V
V+
12V
0mA
I
LOAD
50mA,
3V
V+
15V
0mA
I
LOAD
100mA,
4.75V
V+
15V
MAX761/MAX762
12V/15V or Adjustable, High-Efficiency,
Low I
Q
, Step-Up DC-DC Converters
_______________________________________________________________________________________
3
ELECTRICAL CHARACTERISTICS (continued)
(V+ = 5V, I
LOAD
= 0mA, C
REF
= 0.1F, T
A
= T
MIN
to T
MAX
, typical values are at T
A
= +25C, unless otherwise noted.)
Note 1:
See
Typical Operating Characteristics for output current capability versus input voltage. Guarantees based on correlation
to switching on and off times, on-resistance, and peak-current ratings.
PARAMETER
Voltage Trip Point
SYMBOL
MIN
TYP
MAX
V
FB
1.4550
1.50
1.5450
UNITS
V
-20
20
LX On Resistance
LX Leakage Current
1.0
2.2
-30
30
A
-10
10
-5
5
SHDN Input High Voltage
-40
40
FB Leakage Current
I
FB
-60
60
nA
V
IH
1.6
V
SHDN Input Low Voltage
V
IL
1.4700
1.50
1.5300
0.4
V
1.4625
1.50
1.5375
SHDN Leakage Current
-1
1
A
Reference Load Regulation
1.4700
1.50
1.5300
1.4625
1.50
1.5375
10
LBI Threshold Voltage
1.4550
1.50
1.5450
V
LBI Hysteresis
20
mV
LBI Leakage Current
Reference Line Regulation
-20
20
nA
15
mV
LBO Leakage Current
-1
1
A
LBO Voltage
V
OL
0.4
V
LBI to LBO Delay
2.5
s
CONDITIONS
MAX76_M
MAX76_C
V+ > 5.0V
V+ = 16.5V,
LX = 17V
MAX76_E
MAX76_M
2.0V
V+
16.5V
2.0V
V+
16.5V
MAX76_C
MAX76_E
V+ = 16.5V, SHDN = 0V or V+
LBI falling
0A
I
LOAD
100A
V+ = 16.5V, V
LBI
= 1.5V
3.0V
V+
16.5V
V+ = 16.5V, V
LBO
= 16.5V
V+ = 5.0V, I
SINK
= 1mA
Overdrive = 5mV
MAX76_C
MAX76_M
MAX76_E
30
100
V/V
MAX76_C/E
MAX76_M
MAX76_C
MAX76_M
MAX76_E
MAX761/MAX762
12V/15V or Adjustable, High-Efficiency,
Low I
Q
, Step-Up DC-DC Converters
4
_______________________________________________________________________________________
__________________________________________Typical Operating Characteristics
(Circuit of Figure 2, T
A
= +25C, unless otherwise noted.)
100
0
0.1
10
1000
EFFICIENCY vs. OUTPUT CURRENT
BOOTSTRAPPED
20
MAX761-01
OUTPUT CURRENT (mA)
EFFICIENCY (%)
40
60
80
1
100
V
IN
= 10V
10
30
50
70
90
V
IN
= 5V
V
IN
= 2V
VOUT = 12V
100
0
0.1
10
1000
EFFICIENCY vs. OUTPUT CURRENT
NON-BOOTSTRAPPED
20
MAX761-02
OUTPUT CURRENT (mA)
EFFICIENCY (%)
40
60
80
1
100
V
IN
= 10V
10
30
50
70
90
V
IN
= 5V
VOUT = 12V
2.00
0
0
1
3
6
QUIESCENT CURRENT vs.
INPUT VOLTAGE
0.50
1.50
MAX761-03
INPUT VOLTAGE (V)
QUIESCENT CURRENT (mA)
2
4
1.00
5
1.75
0.25
1.25
0.75
0.5
1.5
3.5
2.5
4.5
5.5
V
OUT
= 12V
BOOTSTRAPPED
(INTERNAL RESISTORS)
BOOTSTRAPPED
(EXTERNAL RESISTORS)
NON-BOOTSTRAPPED
400
0
3.0
6.0
MAXIMUM OUTPUT CURRENT vs.
INPUT VOLTAGE
100
300
MAX761-04
SUPPLY VOLTAGE (V)
MAXIMUM OUTPUT CURRENT (mA)
4.0
200
5.0
350
50
250
150
3.5
4.5
5.5
NON-BOOTSTRAPPED
BOOTSTRAPPED
V
OUT
= 12V
3.5
0.5
-60
-20
60
140
NO-LOAD START-UP VOLTAGE
1.0
3.0
MAX761-07
TEMPERATURE (
C)
NO-LOAD START-UP VOLTAGE (V)
20
100
2.0
-40
0
80
40
120
1.5
2.5
V
OUT
= 12V
BOOTSTRAPPED
(INTERNAL RESISTORS)
BOOTSTRAPPED
(EXTERNAL RESISTORS)
NON-BOOTSTRAPPED
(EXTERNAL RESISTORS)
250
0
-60
-20
60
140
REFERENCE OUTPUT RESISTANCE vs.
TEMPERATURE
50
MAX761-05
TEMPERATURE (
C)
REFERENCE OUTPUT RESISTANCE (
)
20
100
150
-40
0
80
40
120
100
200
100A
50A
10A
1.502
-60
-20
60
140
REFERENCE vs.TEMPERATURE
COEFFICIENT
MAX761-06
TEMPERATURE (
C)
REFERENCE OUTPUT (V)
20
100
-40
0
80
40
120
1.500
1.498
1.496
1.494
1.492
1.504
1.506
2.2
1.3
0.1
10
1000
MAX761
START-UP VOLTAGE vs. R
LOAD
MAX761-08
R
LOAD
(k
)
START-UP VOLTAGE (V)
1.4
1.5
1.6
1.7
2.1
2.0
1.8
1.9
1
100
V
OUT
= 12V
BOOTSTRAPPED
INTERNAL RESISTORS
1.6
0.4
-60
-20
60
140
LX ON-RESISTANCE vs.
TEMPERATURE
0.6
1.4
MAX761-09
TEMPERATURE (
C)
LX ON-RESISTANCE (
)
20
100
1.0
-40
0
80
40
120
0.8
1.2
V+ = 12V
V+ = 5V
MAX761/MAX762
12V/15V or Adjustable, High-Efficiency,
Low I
Q
, Step-Up DC-DC Converters
_______________________________________________________________________________________
5
1000
0.01
20
120
LX LEAKAGE vs. TEMPERATURE
10
100
MAX761-10
LX LEAKAGE (nA)
1
0.1
140
100
80
40
60
TEMPERATURE (
C)
V+ = 15V
V
LX
= 16.5V
1.5
-60
-20
60
140
PEAK CURRENT AT LX vs. TEMPERATURE
MAX761-11
TEMPERATURE (
C)
I
PEAK
(A)
20
100
-40
0
80
40
120
1.4
1.3
1.2
1.1
1.0
0.9
0.8
0.7
0.6
0.5
V+ = 12V
V+ = 5V
4.0
-60
-20
60
140
SHUTDOWN CURRENT
vs. TEMPERATURE
MAX761-12
TEMPERATURE (
C)
I
CC
(A)
20
100
-40
0
80
40
120
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
V+ = 15V
V+ = 4V
V+ = 8V
8.5
7.5
-60
60
SWITCH-ON TIME vs. TEMPERATURE
8.0
MAX761-13
TEMPERATURE (
C)
t on
(s)
0
120
V+ = 5V
2.0
1.0
-60
60
SWITCH-OFF TIME vs. TEMPERATURE
1.5
MAX761-14
TEMPERATURE (
C)
t off
(s)
0
120
V+ = 5V
100
80
-60
60
POWER-SUPPLY CURRENT
vs. TEMPERATURE
90
MAX761-15
TEMPERATURE (
C)
I
CC
(A)
0
120
V+ = 3V
V+ = 16.5V
7
5
-60
60
SWITCH-ON/SWITCH-OFF TIME RATIO
vs.TEMPERATURE
6
MAX761-16
TEMPERATURE (
C)
t on
/t
off
RATIO (s/s)
0
120
V+ = 5V
SHDN RESPONSE TIME
I
LOAD
= 100mA, V
IN
= 5V
A: V
OUT
, 2V/div
B: SHDN (0V to 4V)
2ms/div
4V
0V
12V
5V
____________________________Typical Operating Characteristics (continued)
(Circuit of Figure 2, T
A
= +25C, unless otherwise noted.)
MAX761/MAX762
12V/15V or Adjustable, High-Efficiency,
Low I
Q
, Step-Up DC-DC Converters
6
_______________________________________________________________________________________
LOADTRANSIENT RESPONSE
A: I
LOAD
, (0mA to 200mA)
B: V
OUT
, AC COUPLED, 100mV/div
V
IN
= 5V, V
OUT
= 12V
5s/div
200mA
0mA
A
B
_____________________________Typical Operating Characteristics (continued)
(Circuit of Figure 2, T
A
= +25C, unless otherwise noted.)
LINETRANSIENT RESPONSE
A: V
IN
(4V to 6V)
B: V
OUT
, AC COUPLED, 20mV/div
I
OUT
= 50mA, V
OUT
= 12V
5ms/div
6V
4V
A
B
NAME
FUNCTION
LBO
Low-battery output is an open-drain output that goes low when LBI is less than 1.5V.
Connect to V+ through a pull-up resistor. Leave LBO floating if not used.
LBI
Input to the internal low-battery comparator. Tie to GND or V+ if not used.
PIN
1
2
FB
Feedback input. For fixed-output bootstrapped operation, connect FB to GND. For
adjustable-output bootstrapped operation, connect a resistor divider between V+, FB and
GND. For non-bootstrapped operation, there is no fixed-output option. Connect a resistor
divider network between V
OUT
, FB and GND. See
Bootstrapped/Non-Bootstrapped
Modes section.
SHDN
Active-high TTL/CMOS logic-level input. In shutdown mode (SHDN = V+), the internal
switch is turned off and the output voltage equals V+ minus a diode drop (due to the DC
path from the input to the output). Tie to GND for normal operation.
REF
1.5V reference output that can source 100A for external loads. Bypass with 0.1F
or larger capacitor.
GND
Ground
3
4
LX
Drain of the internal N-channel FET. LX has an output resistance of 1
and a peak current
limit of 1A.
V+
Power-supply input. In bootstrapped mode, V+ is also the output voltage sense input.
5
6
7
8
______________________________________________________________Pin Description
________________Detailed Description
Operating Principle
The MAX761/MAX762 BiCMOS step-up switch-mode
power supplies provide fixed outputs of 12V and 15V,
respectively. They have a unique control scheme that
combines the advantages of pulse-frequency modulation
(low supply current) and pulse-width modulation (high
efficiency at high loads). The internal N-channel power
MOSFET allows 1A peak currents, increasing the output
current capability over previous pulse-frequency-modu-
lation (PFM) devices. Figure 1 shows the MAX761/
MAX762 block diagram.
The MAX761/MAX762 offer three main improvements
over prior solutions: (1) the converters operate with tiny
surface-mount inductors (less than 5mm diameter)
because of their 300kHz switching frequency, (2) the
current-limited PFM control scheme allows 86% efficien-
cies over a wide range of load currents, and (3) the max-
imum supply current is only 110A.
Bootstrapped/Non-Bootstrapped Modes
Figures 2 and 3 show the standard application circuits
for bootstrapped and non-bootstrapped modes. In boot-
strapped mode, the IC is powered from the output
(V
OUT
). In other words, the current needed to power the
bootstrapped circuit is different from the V+ current the
chip consumes. The voltage applied to the gate of the
internal N-channel FET is switched from V
OUT
to ground,
providing more switch-gate drive and increasing the effi-
ciency of the DC-DC converter compared with non-boot-
strapped operation.
MAX761/MAX762
12V/15V or Adjustable, High-Efficiency,
Low I
Q
, Step-Up DC-DC Converters
_______________________________________________________________________________________
7
N
N
N
LBI
LBI
LBO
V+
FB
DUAL-MODE
TM
COMPARATOR
REF
1.5V
REFERENCE
UNDER VOLTAGE
COMPARATOR
LOW INPUT
VOLTAGE
OSCILLATOR
CURRENT CONTROL
CIRCUITRY
CURRENT
COMPARATOR
Q
S
R
Q
TRIG
ONE-SHOT
Q
TRIG
ONE-SHOT
0.1V
0.2V
2.5V
100mV
V+
LX
GND
ERROR
COMPARATOR
MAX761
MAX762
Figure 1. Simple Block Diagram
MAX761/MAX762
In non-bootstrapped mode, the IC is powered from the
supply voltage, V
IN
, and operates with minimum supply
current. Since the voltage applied to the gate of the inter-
nal FET is reduced, efficiency declines with low input
voltages.
Note: In non-bootstrapped mode, there is no
fixed-output operation; external resistors must be
used to set the output voltage.
Use 1% external feed-
back resistors when operating in non-bootstrapped
mode (Figure 3).
Use bootstrapped mode when V
IN
is below approxi-
mately 4V. For V
IN
between 4V and 6V, the trade-off is
lower supply current in non-bootstrapped mode versus
higher output current in bootstrapped mode (see
Typical Operating Characteristics).
Pulse-Frequency Modulation
(PFM) Control Scheme
The MAX761/MAX762 use a proprietary current-limited
PFM control scheme. This control scheme combines
the ultra-low supply current of pulse-skipping PFM con-
verters with the high full-load efficiency characteristic of
current-mode pulse-width-modulation (PWM) convert-
ers. It allows the devices to achieve high efficiency over
a wide range of loads, while the current-sense function
and high operating frequency allow the use of tiny
external components.
As with traditional PFM converters, the internal power
MOSFET is turned on when the voltage comparator
senses the output is out of regulation (Figure 1).
However, unlike traditional PFM converters, switching is
accomplished through the combination of a peak cur-
rent limit and a pair of one-shots that set the maximum
on-time (8s) and minimum off-time (1.3s) for the
switch. Once off, the minimum off-time one-shot holds
the switch off for 1.3s. After this minimum time, the
switch either (1) stays off if the output is in regulation, or
(2) turns on again if the output is out of regulation.
The MAX761/MAX762 also limit the peak inductor cur-
rent, allowing the devices to run in continuous-conduc-
tion mode (CCM) and maintain high efficiency with
heavy loads (Figure 4a). This current-limiting feature is
a key component of the control circuitry. Once turned
on, the switch stays on until either (1) the maximum on-
time one-shot turns it off (8s later), or (2) the current
limit is reached.
To increase light-load efficiency, the current limit for the
first two pulses is set to half the peak current limit. If
those pulses bring the output voltage into regulation,
the voltage comparator holds the MOSFET off, and the
current limit remains at half the peak current limit. If the
output voltage is still out of regulation after two pulses,
the current limit for the next pulse is raised to the full
current limit of 1A (Figure 4b).
Internal vs. External Resistors
When external feedback resistors are used, an internal
undervoltage lockout system prevents start-up until V+
rises to about 2.7V. When external feedback resistors are
12V/15V or Adjustable, High-Efficiency,
Low I
Q
, Step-Up DC-DC Converters
8
_______________________________________________________________________________________
Figure 2. Bootstrapped Operating Circuit
Figure 3. Non-Bootstrapped Operating Circuit
V
IN
=
+5V
LX
LBO
GND
MAX761
SHDN
V+
REF
L1
18H
LBI
FB
D1
1N5817
C1
33F
C3
0.1F
C4
33F
C2
0.1F
R4
R3
100k
+12V at
150mA
LOW-BATTERY
OUTPUT
1
8
7
5
4
2
3
6
V
IN
LX
LBO
GND
MAX761
MAX762
SHDN
V+
REF
L1
18H
LBI
FB
D1
1N5817
C1
C2
C4
R4
R3
100k
ADJUSTABLE
OUTPUT (V
OUT
)
LOW-BATTERY
DETECT OUTPUT
C3
R2
8
2
5
4
7
6
1
3
LOW-BATTERY
DETECT
R4 = R3
( )
VTRIP - VREF
VREF
R2 = R1
(
-1
)
VOUT
VREF
V
REF
= 1.5V NOMINAL
C1 = 33F
C2 = 0.1F
C3 = 0.1F
C4 = 33F
R1
7
used in a bootstrapped circuit (Figure 5), undervoltage
lockout prevents start-up at low input voltages; but
once started, operation can continue down to a lower
voltage that depends on the load.
There is no undervoltage lockout when the internal feed-
back resistors are used (Figure 2), and special circuitry
guarantees start-up at 2.0V. The start-up circuitry fixes
the duty cycle at 50% until V+ is driven to 2.5V, above
which the normal control system takes over.
Shutdown Mode
The MAX761/MAX762 enter shutdown mode when
SHDN is high. In this mode, the internal biasing circuitry
is turned off (including the reference) and V
OUT
equals
V+ minus a diode drop (due to the DC path from the
input to the output). In shutdown mode, the supply cur-
rent drops to less than 5A. SHDN is a TTL/CMOS logic
level input. Connect SHDN to GND for normal operation.
LBO is high impedance during shutdown.
Modes of Operation
When delivering high output currents, the MAX761/
MAX762 operate in CCM. In this mode, current always
flows in the inductor, and the control circuit adjusts the
switch's duty cycle on a cycle-by-cycle basis to maintain
regulation without exceeding the switch-current capabili-
ty. This provides excellent load-transient response and
high efficiency.
In discontinuous-conduction mode (DCM), current
through the inductor starts at zero, rises to a peak value,
then ramps down to zero on each cycle. Although effi-
ciency is still excellent, the switch waveforms contain
ringing (the inductor's self-resonant frequency). This
ringing is normal and poses no operational problems.
Low-Battery Detector
The MAX761/MAX762 provide a low-battery comparator
that compares the voltage on LBI to the 1.5V reference
voltage. When the LBI voltage is below V
REF
, LBO (an
open-drain output) goes low. The low-battery compara-
tor's 20mV of hysteresis adds noise immunity, prevent-
ing repeated triggering of LBO. Use a resistor-divider
network between V+, LBI, and GND to set the desired
trip voltage V
TRIP
(Figure 3). When SHDN is high, LBI is
ignored and LBO is high impedance. The value of
resistor R3 should be no larger than 500k
to ensure
the LBI leakage current does not cause inaccuracies in
V
TRIP
.
__________________Design Procedure
Setting the Output Voltage
The MAX761/MAX762's output voltage can be adjusted
from 5V to 16.5V using external resistors R1 and R2
configured as shown in Figures 3 and 5. For adjustable-
output operation, select feedback resistor R1 in the
10k
to 250k
range. Higher R1 values within this
range give lowest supply current and best light-load
efficiency. R2 is given by:
R2 = (R1)( V
OUT
- 1)
V
REF
where V
REF
= 1.5V.
Note: Tie FB to GND for fixed-output operation
(bootstrapped mode only).
MAX761/MAX762
12V/15V or Adjustable, High-Efficiency,
Low I
Q
, Step-Up DC-DC Converters
_______________________________________________________________________________________
9
Figure 4a. CCM, Heavy Load Current Waveform (500mA/div)
Figure 4b. Light/Medium Load Current Waveform (500mA/div)
1A
500mA
1A
500mA
0A
MAX761/MAX762
Selecting the Inductor (L)
In both CCM and DCM, practical inductor values range
from 10H to 50H. If the inductor value is too low, the
current in the coil will ramp up to a high level before the
current-limit comparator can turn off the switch. The mini-
mum on-time for the switch (t
ON(min)
) is approximately
2.5s, so select an inductance that allows the current to
ramp up to I
LIM
/2 in no less than 2.5s. Choosing a value
of I
LIM
/2 allows the half-size pulses to occur, giving high-
er light-load efficiency and minimizing ripple. Hence, cal-
culate the minimum inductance value as:
L
(V
IN(max)
)(t
ON(min)
)
I
LIM/2
OR
L
(V
IN(max)
)(5)
where V
IN(max)
is in volts and L is in microhenries.
The coil's inductance need not satisfy this criterion
exactly, as the circuit can tolerate a wide range of val-
ues. Larger inductance values tend to produce physical-
ly larger coils and increase the start-up time, but are oth-
erwise acceptable. Smaller inductance values allow the
coil current to ramp up to higher levels before the switch
can turn off, producing higher ripple at light loads. In
general, an 18H inductor is sufficient for most applica-
tions (V
IN
5V). An 18H inductor is appropriate for
input voltages up to 3.6V, as calculated above. However,
the same 18H coil can be used with input voltages up
to 5V with only small increases in peak current, as shown
in Figures 4a and 4b.
Inductors with a ferrite core or equivalent are recom-
mended. The inductor's incremental saturation-current
rating should be greater than the 1A peak current limit. It
is generally acceptable to bias the inductor into satura-
tion by approximately 20% (the point where the induc-
tance is 20% below the nominal value). For highest effi-
ciency, use a coil with low DC resistance, preferably
under 100m
. To minimize radiated noise, use a toroid,
a pot core, or a shielded coil.
Table 1 lists inductor types and suppliers for various
applications. The listed surface-mount inductors' efficien-
cies are nearly equivalent to those of the larger through-
hole inductors.
Diode Selection
The MAX761/MAX762's high switching frequency
demands a high-speed rectifier. Use a Schottky diode
with a 1A average current rating, such as a 1N5817. For
high-temperature applications, use a high-speed silicon
diode, such as the MUR105 or the EC11FS1. These
diodes have lower high-temperature leakage than
Schottky diodes (Table 1).
Capacitor Selection
Output Filter Capacitor
The primary criterion for selecting the output filter capac-
itor (C4) is low effective series resistance (ESR). The
product of the inductor current variation and the output
filter capacitor's ESR determines the amplitude of the
high-frequency ripple seen on the output voltage. A
33F, 16V Sanyo OS-CON capacitor with 100m
ESR
typically provides 100mV ripple when stepping up from
5V to 12V at 150mA.
Because the output filter capacitor's ESR affects efficien-
cy, use low-ESR capacitors for best performance. The
smallest low-ESR SMT tantalum capacitors currently
available are the Sprague 595D series. Sanyo OS-CON
organic semiconductor through-hole capacitors and
Nichicon PL series also exhibit very low ESR. Table 1
lists some suppliers of low-ESR capacitors.
Input Bypass Capacitors
The input bypass capacitor, C1, reduces peak currents
drawn from the voltage source, and also reduces noise
at the voltage source caused by the MAX761/MAX762's
switching action. The input voltage source impedance
determines the size of the capacitor required at the V+
input. As with the output filter capacitor, a low-ESR
capacitor is recommended. For output currents up to
250mA, 33F (C1) is adequate, although smaller bypass
capacitors may also be acceptable. Bypass the IC sepa-
rately with a 0.1F ceramic capacitor, C2, placed close
to the V+ and GND pins.
12V/15V or Adjustable, High-Efficiency,
Low I
Q
, Step-Up DC-DC Converters
10
______________________________________________________________________________________
V
IN
LX
GND
MAX761
MAX762
SHDN
V+
REF
L1
18H
LBI
FB
D1
1N5817
C2
C4
V
OUT
C3
R2
8
2
5
4
7
6
3
R2 = R1
(
-1
)
VOUT
VREF
C1 = 33F
C2 = 0.1F
C3 = 0.1F
C4 = 33F
C1
R1
V
REF
= 1.5V NOMINAL
Figure 5. Bootstrapped Operation with Adjustable Output
Reference Capacitor
Bypass REF with a 0.1F capacitor. REF can source up
to 100A.
Setting the Low-Battery Detector Voltage
To set the low-battery detector's falling trip voltage
(V
TRIP
), select R3 between 10k
and 500k
(Figures 2
and 3), and calculate R4 as follows:
R4 = R3 [
(V
TRIP
- V
REF
)
]
V
REF
where V
REF
= 1.5V.
The rising trip voltage is higher because of the compara-
tor's hysteresis of approximately 20mV, and can be cal-
culated by:
V
TRIP
(rising) = (V
REF
+ 20mV)(1 + R4/R3).
Connect a high-value resistor (larger than R3 + R4)
between LBI and LBO if additional hysteresis is required.
Connect a pull-up resistor (e.g., 100k
) between LBO
and V
OUT
. Tie LBI to GND or V+ and leave LBO floating
if the low-battery detector is not used.
___________Applications Information
Layout Considerations
Proper PC board layout is essential because of high cur-
rent levels and fast switching waveforms that radiate
noise. Minimize ground noise by connecting GND, the
input bypass-capacitor ground lead, and the output filter-
capacitor ground lead to a single point (star ground con-
figuration). Also minimize lead lengths to reduce stray
capacitance, trace resistance, and radiated noise. The
traces connected to FB and LX, in particular, must be
short. Place bypass capacitor C2 as close as possible to
V+ and GND.
MAX761/MAX762
12V/15V or Adjustable, High-Efficiency,
Low I
Q
, Step-Up DC-DC Converters
______________________________________________________________________________________
11
INDUCTORS
DIODES
Sumida
CD54-180 (22H)
Coiltronics
CTX 100-series
Matsuo
267 series
Surface Mount
Sanyo
OS-CON series
Low-ESR organic
semiconductor
Sumida
RCH855-180M
Miniature Through-Hole
PRODUCTION METHOD
Nichicon
PL series
Low-ESR electrolytics
United Chemi-Con
LXF series
Renco
RL 1284-18
Low-Cost Through-Hole
CAPACITORS
Nihon
EC10 series
Motorola
1N5817,
MUR105
Table 1. Component Suppliers
Coiltronics
(USA)
(407) 241-7876
FAX (407) 241-9339
Matsuo
(USA)
(714) 969-2491
FAX (714) 960-6492
Matsuo
(Japan)
81-6-337-6450
FAX 81-6-337-6456
Nichicon
(USA)
(708) 843-7500
FAX (708) 843-2798
Nihon
(USA)
(805) 867-2555
FAX (805) 867-2556
Renco
(USA)
(516) 586-5566
FAX (516) 586-5562
Sanyo
(USA)
(619) 661-6835
FAX (619) 661-1055
Sanyo
(Japan)
(0720) 70-1005
FAX (0720) 70-1174
Sumida
(USA)
(708) 956-0666
Sumida
(Japan)
81-3-607-5111
FAX 81-3-607-5144
United Chem-Con
(USA)
(714) 255-9500
FAX (714) 255-9400
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.
12
__________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 (408) 737-7600
1993 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products.
MAX761/MAX762
12V/15V or Adjustable, High-Efficiency,
Low I
Q
, Step-Up DC-DC Converters
TRANSISTOR COUNT: 492;
SUBSTRATE CONNECTED TO V+.
___________________Chip Topography
REF
LBO
0.142"
(3.607mm)
0.080"
(2.030mm)
FB
SHDN
GND
LX
V+
LBI
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