In non-bootstrapped mode, the IC is powered from the

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).

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 (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 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 (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

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

8

_______________________________________________________________________________________

Figure 2. Bootstrapped Operating Circuit

Figure 3. Non-Bootstrapped Operating Circuit

+5V

LX

LBO

GND

MAX761

SHDN

V+

REF

L1

18µH

LBI

FB

D1

1N5817

C1

33µF

C3

0.1µF

C4

33µF

C2

0.1µF

R4

R3

100k

+12V at

150mA

LOW-BATTERY

OUTPUT

1

8

7

5

4

2

3

6

LX

LBO

GND

MAX761

MAX762

SHDN

V+

REF

L1

18µH

LBI

FB

D1

1N5817

C1

C2

C4

R4

R3

100k

ADJUSTABLE

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

C1 = 33µF

C2 = 0.1µF

C3 = 0.1µF

C4 = 33µF

R1

7