Datasheet MAX761EPA, MAX761ESA, MAX761MJA, MAX762C-D, MAX761C-D Datasheet (Maxim)

19-0201; Rev 0; 11/93

Evaluation Kit

Available

12V/15V or Adjustable, High-Efficiency,

Low IQ, Step-Up DC-DC Converters

_______________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 110µA 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 MOS­FET, making them ideal for minimum-component, low- and
medium-power applications. These devices use tiny exter­nal components, and their high switching frequencies (up
to 300kHz) allow for small surface-mount magnetics.

For increased output drive capability or higher output volt­ages, use the MAX770–MAX773, 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

♦ High Efficiency for a Wide Range of Load Currents
♦ 12V/150mA Flash Memory Programming Supply
♦ 110µA Max Supply Current
♦ 5µA Max Shutdown Supply Current
♦ 2V to 16.5V Input Voltage Range
♦ 12V (MAX761), 15V (MAX762) or Adjustable Output
♦ Current-Limited PFM Control Scheme
♦ 300kHz Switching Frequency
♦ Internal, 1A, N-Channel Power FET
♦ LBI/LBO Low-Battery Comparator

______________Ordering Information

PART TEMP. RANGE PIN-PACKAGE

MAX761CPA

MAX761CSA 0°C to +70°C 8 SO
MAX761C/D 0°C to +70°C Dice*
MAX761EPA -40°C to +85°C 8 Plastic DIP
MAX761ESA -40°C to +85°C 8 SO
MAX761MJA -55°C to +125°C 8 CERDIP**
MAX762CPA
MAX762CSA 0°C to +70°C 8 SO
MAX762C/D 0°C to +70°C Dice*
MAX762EPA -40°C to +85°C 8 Plastic DIP
MAX762ESA -40°C to +85°C 8 SO
MAX762MJA -55°C to +125°C 8 CERDIP**

* Contact factory for dice specifications.
** Contact factory for availability and processing to MIL-STD-883.

0°C to +70°C 8 Plastic DIP

0°C to +70°C 8 Plastic DIP

MAX761/MAX762

__________Typical Operating Circuit

INPUT

4.75V

TO 12V

DETECTOR INPUT

ON/OFF

LOW-BATTERY

33µF

MAX761

SHDN

LBI

REF

FB

GND

________________________________________________________________

LBO

18µH

OUTPUT

12V

LX

V+

150mA

33µF

LOW-BATTERY
DETECTOR OUTPUT

__________________Pin Configuration

TOP VIEW

LBO

1
2

LBI

MAX761
MAX762

3

FB

4

SHDN

DIP/SO

Maxim Integrated Products

Call toll free 1-800-998-8800 for free samples or literature.

V+

8

LX

7

GND

6

REF

5

1

12V/15V or Adjustable, High-Efficiency,
Low IQ, Step-Up DC-DC Converters

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

Plastic DIP (derate 9.09mW/°C above +70°C) ............727mW

SO (derate 5.88mW/°C above +70°C).........................471mW

CERDIP (derate 8.00mW/°C above +70°C).................640mW

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

MAX761/MAX762

(V+ = 5V, I

Minimum Operating Voltage 1.7 V
Minimum Start-Up Voltage 1.7 2.0 V

Supply Current

Shutdown Current 15µA

Output Voltage
(Note 1)

Peak Current at LX I
Maximum Switch-On Time t
Minimum Switch-Off Time t
Load Regulation 0.0042 %/mA
Line Regulation 0.08 %/V

Efficiency 86 %

Reference Voltage V

= 0mA, C

LOAD

PARAMETER SYMBOL MIN TYP MAX

= +70°C)

A

= 0.1µF, TA= T

REF

V

OUT

PEAK

ON

OFF

REF

to T

MIN

Figure 2, bootstrapped
Figure 3 or 5 with

external resistors.
Figure 2, bootstrapped

Figure 2, bootstrapped
V+ = 16.5V, normal operation, SHDN = 0V,

non-bootstrapped
Figure 2, MAX761, VIN= 5V, SHDN = 0V,

normal operation
V+ = 10.0V, shutdown mode, SHDN = V+

Figure 2,
MAX761,
bootstrapped

Figure 2,
MAX762,
bootstrapped

See Figure 4b

Figure 2, 0mA ≤ I
Figure 2, 4V ≤ VIN≤ 6V, bootstrapped

Figure 2, bootstrapped, V
60mA ≤ I

MAX76_C
MAX76_E
MAX76_M

typical values are at TA= +25°C, unless otherwise noted.)

MAX,

LOAD

Operating Temperature Ranges:

MAX76_C_A........................................................0°C to +70°C

MAX76_E_A.....................................................-40°C to +85°C

MAX76_MJA ..................................................-55°C to +125°C

Junction Temperatures:

MAX76_C_A/E_A..........................................................+150°C

MAX76_MJA.................................................................+175°C

Storage Temperature Range.............................-65°C to +160°C

Lead Temperature (soldering, 10sec).............................+300°C

CONDITIONS

2 16.5
MAX76_C/E
MAX76_M

0mA ≤ I
3V ≤ V+ ≤ 12V

0mA ≤ I

4.75V ≤ V+ ≤ 12V
0mA ≤ I

3V ≤ V+ ≤ 15V
0mA ≤ I

4.75V ≤ V+ ≤ 15V

≤ 200mA, bootstrapped

LOAD

≤ 120mA

OUT

LOAD

LOAD

LOAD

LOAD

= 12V,

≤ 75mA,

≤ 150mA,

≤ 50mA,

≤ 100mA,

3 16.5Supply Voltage V+

3.1 16.5

88 110

300

11.52 12.0 12.48

11.52 12.0 12.48

14.4 15.0 15.6

14.4 15.0 15.6

0.75 1.0 1.25 A
6810µs

1.0 1.3 1.6 µs

1.4700 1.50 1.5300

1.4625 1.50 1.5375

1.4550 1.50 1.5450

UNITS

V

µA

V

V

2 _______________________________________________________________________________________

12V/15V or Adjustable, High-Efficiency,

Low IQ, Step-Up DC-DC Converters

ELECTRICAL CHARACTERISTICS (continued)

(V+ = 5V, I

Reference Load Regulation
Reference Line Regulation

LX Leakage Current

Voltage Trip Point

LX On Resistance
SHDN Input High Voltage
SHDN Input Low Voltage V
SHDN Leakage Current -1 1 µA

LBI Threshold Voltage

LBI Hysteresis 20 mV
LBI Leakage Current
LBO Leakage Current -1 1 µA
LBO Voltage V
LBI to LBO Delay 2.5 µs

= 0mA, C

LOAD

PARAMETER

= 0.1µF, TA= T

REF

SYMBOL MIN TYP MAX

FB

V

FB

V

IH
IL

OL

to T

MIN

0µA ≤ I

3.0V ≤ V+ ≤ 16.5V

V+ = 16.5V,
LX = 17V

MAX76_C
MAX76_E
MAX76_M
MAX76_C
MAX76_E
MAX76_M
V+ > 5.0V

2.0V ≤ V+ ≤ 16.5V

2.0V ≤ V+ ≤ 16.5V
V+ = 16.5V, SHDN = 0V or V+

LBI falling

V+ = 16.5V, V
V+ = 16.5V, V
V+ = 5.0V, I
Overdrive = 5mV

, typical values are at TA= +25°C, unless otherwise noted.)

MAX

CONDITIONS

≤ 100µA

LOAD

= 1.5V

LBI

= 16.5V

LBO

= 1mA

SINK

MAX76_C/E
MAX76_M

MAX76_C
MAX76_E
MAX76_M

MAX76_C
MAX76_E
MAX76_M

-5 5

-10 10

-30 30

-20 20

-40 40FB Leakage Current I

-60 60

1.4700 1.50 1.5300

1.4625 1.50 1.5375

1.4550 1.50 1.5450

1.6 V

1.4700 1.50 1.5300

1.4625 1.50 1.5375

1.4550 1.50 1.5450

-20 20 nA

10
15

30 100 µV/V

1.0 2.2

0.4 V

0.4 V

MAX761/MAX762

UNITS

mV

µA

nA

V

Ω

V

Note 1: See

to switching on and off times, on-resistance, and peak-current ratings.

Typical Operating Characteristics

_______________________________________________________________________________________ 3

for output current capability versus input voltage. Guarantees based on correlation

12V/15V or Adjustable, High-Efficiency,
Low IQ, Step-Up DC-DC Converters

__________________________________________Typical Operating Characteristics

(Circuit of Figure 2, TA= +25°C, unless otherwise noted.)

EFFICIENCY vs. OUTPUT CURRENT

100

90
80
70
60
50
40

EFFICIENCY (%)

30
20

MAX761/MAX762

10

0

0.1 10 1000

400
350
300
250
200
150
100

MAXIMUM OUTPUT CURRENT (mA)

50

0

3.0 6.0

BOOTSTRAPPED

VIN = 10V

VIN = 5V

VIN = 2V

V

= 12V

OUT

1 100
OUTPUT CURRENT (mA)

MAXIMUM OUTPUT CURRENT vs. 

INPUT VOLTAGE

BOOTSTRAPPED

NON-BOOTSTRAPPED

V

= 12V

OUT

4.0

3.5 4.5 5.5
SUPPLY VOLTAGE (V)

5.0

NO-LOAD START-UP VOLTAGE

3.5

3.0

2.5

NON-BOOTSTRAPPED

(EXTERNAL RESISTORS)

2.0

1.5

BOOTSTRAPPED

1.0

(INTERNAL RESISTORS)

NO-LOAD START-UP VOLTAGE (V)

0.5

-60 -20 60 140

-40 0 8040 120

20 100

TEMPERATURE (°C)

V

= 12V

OUT

BOOTSTRAPPED

(EXTERNAL RESISTORS)

EFFICIENCY vs. OUTPUT CURRENT

100

90

MAX761-01

80
70
60
50
40

EFFICIENCY (%)

30
20
10

0

250

MAX761-04

200

150

100

50

REFERENCE OUTPUT RESISTANCE (Ω)

0

2.2

MAX761-07

2.1

2.0

1.9

1.8

1.7

1.6

START-UP VOLTAGE (V)

1.5

1.4

1.3

NON-BOOTSTRAPPED

VIN = 10V

0.1 10 1000

1 100
OUTPUT CURRENT (mA)

REFERENCE OUTPUT RESISTANCE vs.

TEMPERATURE

-60 -20 60 140

-40 0 8040 120

0.1 10 1000

20 100

TEMPERATURE (°C)

MAX761

START-UP VOLTAGE vs. R

1 100

R

LOAD

VIN = 5V

V

= 12V

OUT

10µA

50µA

LOAD

V

= 12V

OUT

BOOTSTRAPPED
INTERNAL RESISTORS

(kΩ)

100µA

2.00

MAX761-02

1.75

1.50

1.25

1.00

0.75

0.50

QUIESCENT CURRENT (mA)

0.25
0

01 3 6

1.506

MAX761-05

1.504

1.502

1.500

1.498

1.496

REFERENCE OUTPUT (V)

1.494

1.492

-60 -20 60 140

1.6

MAX761-08

1.4

1.2

1.0

0.8

LX ON-RESISTANCE (Ω)

0.6

0.4

-60 -20 60 140

QUIESCENT CURRENT vs. 

INPUT VOLTAGE

V

= 12V

BOOTSTRAPPED

(INTERNAL RESISTORS)

24

0.5 1.5 3.52.5 4.5 5.5
INPUT VOLTAGE (V)

REFERENCE vs.TEMPERATURE

COEFFICIENT

20 100-40 0 8040 120

TEMPERATURE (°C)

LX ON-RESISTANCE vs.

TEMPERATURE

V+ = 5V

V+ = 12V

-40 0 8040 120

20 100

TEMPERATURE (°C)

OUT

BOOTSTRAPPED

(EXTERNAL RESISTORS)

NON-BOOTSTRAPPED

5

MAX761-03

MAX761-06

MAX761-09

4 _______________________________________________________________________________________

12V/15V or Adjustable, High-Efficiency,

Low IQ, Step-Up DC-DC Converters

____________________________Typical Operating Characteristics (continued)

(Circuit of Figure 2, TA= +25°C, unless otherwise noted.)

LX LEAKAGE vs. TEMPERATURE

1000

V+ = 15V

100

10

1

LX LEAKAGE (nA)

0.1

0.01
20 120

TEMPERATURE (°C)

SWITCH-ON TIME vs. TEMPERATURE

8.5

(µs)

8.0

on

t

VLX = 16.5V

V+ = 5V

PEAK CURRENT AT LX vs. TEMPERATURE

MAX761-11

(µA)

CC

I

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5
0

1.5

1.4

MAX761-10

1.3

1.2

1.1

(A)

1.0

PEAK

I

0.9

0.8

0.7

0.6

0.5

1401008040 60

-60 -20 60 140

V+ = 12V

V+ = 5V

20 100-40 0 8040 120

TEMPERATURE (°C)

SWITCH-OFF TIME vs. TEMPERATURE

MAX761-13

(µs)

off

t

2.0

V+ = 5V

1.5

MAX761-14

(µA)

CC

I

100

90

SHUTDOWN CURRENT

vs. TEMPERATURE

V+ = 15V

V+ = 8V

V+ = 4V

-60 -20 60 140

20 100-40 0 8040 120

TEMPERATURE (°C)

POWER-SUPPLY CURRENT 

vs. TEMPERATURE

V+ = 16.5V

V+ = 3V

MAX761/MAX762

MAX761-12

MAX761-15

7.5

-60 60

0 120

TEMPERATURE (°C)

RATIO (µs/µs)

off

/t

on

t

_______________________________________________________________________________________

1.0

SWITCH-ON/SWITCH-OFF TIME RATIO 

7

6

5

-60 60

vs.TEMPERATURE

V+ = 5V

0 120

TEMPERATURE (°C)

-60 60

0 120

TEMPERATURE (°C)

MAX761-16

I

= 100mA, VIN = 5V

LOAD

A: V

, 2V/div

OUT

B: SHDN (0V to 4V)

80

-60 60

SHDN RESPONSE TIME

2ms/div

0 120

TEMPERATURE (°C)

12V

5V
4V

0V

5

12V/15V or Adjustable, High-Efficiency,
Low IQ, Step-Up DC-DC Converters

_____________________________Typical Operating Characteristics (continued)

(Circuit of Figure 2, TA= +25°C, unless otherwise noted.)

LOAD–TRANSIENT RESPONSE



200mA

A

0mA

B

LINE–TRANSIENT RESPONSE



6V
A

4V

B

MAX761/MAX762

5µs/div

A: I

, (0mA to 200mA)

LOAD

B: V

, AC COUPLED, 100mV/div

OUT

VIN = 5V, V

= 12V

OUT

A: VIN (4V to 6V) 
B: V

, AC COUPLED, 20mV/div

OUT

I

= 50mA, V

OUT

______________________________________________________________Pin Description

PIN

1

2

3

4

5

6

7

8

NAME FUNCTION

LBO

LBI Input to the internal low-battery comparator. Tie to GND or V+ if not used.

FB

SHDN

REF

GND Ground

LX

V+ Power-supply input. In bootstrapped mode, V+ is also the output voltage sense input.

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.

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

Modes

section.

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.

1.5V reference output that can source 100µA for external loads. Bypass with 0.1µF
or larger capacitor.

Drain of the internal N-channel FET. LX has an output resistance of 1Ω and a peak current
limit of 1A.

, FB and GND. See

OUT

5ms/div

= 12V

OUT

Bootstrapped/Non-Bootstrapped

6 _______________________________________________________________________________________

12V/15V or Adjustable, High-Efficiency,

Low IQ, Step-Up DC-DC Converters

LBO V+ FB

MAX761/MAX762

LBI

N

LBI

REF

1.5V

REFERENCE

Q

TRIG

ONE-SHOT

R

CIRCUITRY

Q

CURRENT
COMPARATOR

LOW INPUT

VOLTAGE

OSCILLATOR

TRIG

ONE-SHOT

S

Q

CURRENT CONTROL

Figure 1. Simple Block Diagram

________________Detailed Description

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-modu­lation (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)

Operating Principle

100mV

GND

DUAL-MODE
COMPARATOR

ERROR
COMPARATOR

0.1V0.2V

UNDER VOLTAGE 
COMPARATOR

MAX761
MAX762

N

2.5V

N

because of their 300kHz switching frequency, (2) the
current-limited PFM control scheme allows 86% efficien­cies over a wide range of load currents, and (3) the max­imum supply current is only 110µA.

Bootstrapped/Non-Bootstrapped Modes

Figures 2 and 3 show the standard application circuits
for bootstrapped and non-bootstrapped modes. In boot­strapped mode, the IC is powered from the output
(V

). In other words, the current needed to power the

OUT

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

providing more switch-gate drive and increasing the effi­ciency of the DC-DC converter compared with non-boot­strapped operation.

V+

LX

to ground,

_______________________________________________________________________________________ 7

12V/15V or Adjustable, High-Efficiency,
Low IQ, Step-Up DC-DC Converters

V
+5V

R4

R3

= 

IN

C1
33µF

C3

0.1µF

L1

18µH

5

REF

MAX761

4

SHDN

2

LBI

3

FB

MAX761/MAX762

LX

GND

D1

1N5817

7

8

V+

1

LBO

6

+12V at

150mA

C2

0.1µF

LOW-BATTERY

OUTPUT

Figure 2. Bootstrapped Operating Circuit

In non-bootstrapped mode, the IC is powered from the
supply voltage, VIN, and operates with minimum supply
current. Since the voltage applied to the gate of the inter­nal 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 VINis below approxi­mately 4V. For VINbetween 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 con­verters with the high full-load efficiency characteristic of
current-mode pulse-width-modulation (PWM) convert­ers. 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-

C4
33µF

100k

V

IN

R4

R3

LOW-BATTERY 
DETECT

V

( )

R4 = R3

V

= 1.5V NOMINAL

REF

C1 = 33µF
C2 = 0.1µF
C3 = 0.1µF
C4 = 33µF

C1

TRIP

L1

18µH

8

V+

C2

2

LBI

5

REF

C3

4

SHDN

- V

REF

V

REF

LX

MAX761
MAX762

GND

D1

1N5817

7

7

6

C4

FB

LBO

ADJUSTABLE

OUTPUT (V

R2 = R1 ( -1)

R2

3

R1

1

LOW-BATTERY

DETECT OUTPUT

OUT

V
V

100k

)

OUT
REF

Figure 3. Non-Bootstrapped Operating Circuit

rent limit and a pair of one-shots that set the maximum
on-time (8µs) and minimum off-time (1.3µs) for the
switch. Once off, the minimum off-time one-shot holds
the switch off for 1.3µs. 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 cur­rent, allowing the devices to run in continuous-conduc­tion 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 on­time one-shot turns it off (8µs 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

8 _______________________________________________________________________________________

12V/15V or Adjustable, High-Efficiency,

Low IQ, Step-Up DC-DC Converters

MAX761/MAX762

, LBO (an

REF

1A

500mA

0A

1A
500mA

Figure 4a. CCM, Heavy Load Current Waveform (500mA/div) Figure 4b. Light/Medium Load Current Waveform (500mA/div)

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 feed­back 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
V+ minus a diode drop (due to the DC path from the
input to the output). In shutdown mode, the supply cur­rent drops to less than 5µA. SHDN is a TTL/CMOS logic
level input. Connect SHDN to GND for normal operation.
LBO is high impedance during shutdown.

OUT

equals

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 capabili­ty. 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 effi­ciency 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
open-drain output) goes low. The low-battery compara­tor’s 20mV of hysteresis adds noise immunity, prevent­ing repeated triggering of LBO. Use a resistor-divider
network between V+, LBI, and GND to set the desired
trip voltage V
ignored and LBO is high impedance. The value of
resistor R3 should be no larger than 500kΩ to ensure
the LBI leakage current does not cause inaccuracies in
V

.

TRIP

(Figure 3). When SHDN is high, LBI is

TRIP

__________________Design Procedure

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 adjustable­output operation, select feedback resistor R1 in the
10kΩ to 250kΩ range. Higher R1 values within this
range give lowest supply current and best light-load
efficiency. R2 is given by:

R2 = (R1)(

where V

Note: Tie FB to GND for fixed-output operation
(bootstrapped mode only).

REF

= 1.5V.

Setting the Output Voltage

V

OUT

- 1)

V

REF

_______________________________________________________________________________________ 9

12V/15V or Adjustable, High-Efficiency,
Low IQ, Step-Up DC-DC Converters

LX

GND

D1

1N5817

7

8

V+

R2

3

FB

R1

6

R2 = R1

( -1)

L1

V

IN

C1 = 33µF

MAX761/MAX762

C2 = 0.1µF
C3 = 0.1µF
C4 = 33µF

Figure 5. Bootstrapped Operation with Adjustable Output

18µH

C1

MAX761

5

REF

2

LBI

4

SHDN

MAX762

V

= 1.5V NOMINAL

REF

C3

Selecting the Inductor (L)

In both CCM and DCM, practical inductor values range
from 10µH to 50µH. 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 mini­mum on-time for the switch (t

) is approximately

ON(min)

2.5µs, so select an inductance that allows the current to
ramp up to I
of I

/2allows the half-size pulses to occur, giving high-

LIM

/2in no less than 2.5µs. Choosing a value

LIM

er light-load efficiency and minimizing ripple. Hence, cal­culate the minimum inductance value as:

where V

(V

L ≥

OR

L ≥ (V

is in volts and L is in microhenries.

IN(max)

IN(max)

IN(max)

I

LIM

)(t

ON(min)
/2

)(5)

)

The coil’s inductance need not satisfy this criterion
exactly, as the circuit can tolerate a wide range of val­ues. Larger inductance values tend to produce physical­ly larger coils and increase the start-up time, but are oth­erwise 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 18µH inductor is sufficient for most applica­tions (VIN≤ 5V). An 18µH inductor is appropriate for
input voltages up to 3.6V, as calculated above. However,
the same 18µH 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-

V

OUT

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-

C4

tion by approximately 20% (the point where the induc­tance is 20% below the nominal value). For highest effi­ciency, use a coil with low DC resistance, preferably
under 100mΩ. To minimize radiated noise, use a toroid,

C2

a pot core, or a shielded coil.
Table 1 lists inductor types and suppliers for various

applications. The listed surface-mount inductors’ efficien­cies are nearly equivalent to those of the larger through­hole inductors.

Diode Selection

The MAX761/MAX762’s high switching frequency
demands a high-speed rectifier. Use a Schottky diode

V

OUT

V

REF

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 capac­itor (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
33µF, 16V Sanyo OS-CON capacitor with 100mΩ ESR
typically provides 100mV ripple when stepping up from
5V to 12V at 150mA.

Because the output filter capacitor’s ESR affects efficien­cy, 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, 33µF (C1) is adequate, although smaller bypass
capacitors may also be acceptable. Bypass the IC sepa­rately with a 0.1µF ceramic capacitor, C2, placed close
to the V+ and GND pins.

10 ______________________________________________________________________________________

12V/15V or Adjustable, High-Efficiency,

Low IQ, Step-Up DC-DC Converters

Bypass REF with a 0.1µF capacitor. REF can source up

Reference Capacitor

to 100µA.

Setting the Low-Battery Detector Voltage

To set the low-battery detector’s falling trip voltage
(V

), select R3 between 10kΩ and 500kΩ (Figures 2

TRIP

and 3), and calculate R4 as follows:

(V

- V

R4 = R3 [

TRIP

)

REF

REF

]

V
where VREF = 1.5V.
The rising trip voltage is higher because of the compara-

tor’s hysteresis of approximately 20mV, and can be cal­culated by:

V

(rising) = (V

TRIP

+ 20mV)(1 + R4/R3).

REF

Connect a high-value resistor (larger than R3 + R4)
between LBI and LBO if additional hysteresis is required.

Table 1. Component Suppliers

PRODUCTION METHOD

Surface Mount

Miniature Through-Hole

Low-Cost Through-Hole

INDUCTORS DIODES

Sumida
CD54-180 (22µH)

Coiltronics
CTX 100-series

Sumida
RCH855-180M

Renco
RL 1284-18

Matsuo
267 series


Sanyo
OS-CON series
Low-ESR organic
semiconductor

Nichicon
PL series
Low-ESR electrolytics

United Chemi-Con
LXF series

Connect a pull-up resistor (e.g., 100kΩ) between LBO
and V

. Tie LBI to GND or V+ and leave LBO floating

OUT

if the low-battery detector is not used.

___________Applications Information

Proper PC board layout is essential because of high cur­rent levels and fast switching waveforms that radiate
noise. Minimize ground noise by connecting GND, the
input bypass-capacitor ground lead, and the output filter­capacitor ground lead to a single point (star ground con­figuration). 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.

CAPACITORS

Layout Considerations

Nihon
EC10 series

Motorola
1N5817,
MUR105

MAX761/MAX762

Coiltronics (USA) [phone] FAX [phone]
Matsuo (USA) [phone] FAX [phone]
Matsuo (Japan) 81-6-337-6450 FAX [phone]
Nichicon (USA) [phone] FAX [phone]
Nihon (USA) [phone] FAX [phone]
Renco (USA) [phone] FAX [phone]
Sanyo (USA) [phone] FAX [phone]
Sanyo (Japan) (0720) 70-1005 FAX [phone]
Sumida (USA) [phone]
Sumida (Japan) 81-3-607-5111 FAX [phone]
United Chem-Con (USA) [phone] FAX [phone]

______________________________________________________________________________________ 11

12V/15V or Adjustable, High-Efficiency,
Low IQ, Step-Up DC-DC Converters

___________________Chip Topography

LBI

LBO

FB

V+

MAX761/MAX762

SHDN

TRANSISTOR COUNT: 492;
SUBSTRATE CONNECTED TO V+.

0.080"

(2.030mm)

LX

0.142"

(3.607mm)

GND

REF

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

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