With light loads, the MOSFET switches on for one or

more cycles and then switches off, much like in tradi-

tional PFM converters. To increase light-load efficiency,

the current limit for the first two pulses is set to one-half

the peak current limit. If those pulses bring the output

voltage into regulation, the voltage comparator keeps

the MOSFET off, and the current limit remains at one-half

the peak current limit. If the output voltage is out of

regulation after two consecutive pulses, the current limit

for the next pulse will equal the full current limit.

With heavy loads, the MOSFET first switches twice at

one-half the peak current value. Subsequently, it stays

on until the switch current reaches the full current limit,

and then turns off. After it is off for 2.3µs, the MOSFET

switches on once more, and remains on until the switch

current again reaches its limit. This cycle repeats until

the output is in regulation.

A benefit of this control scheme is that it is highly effi-

cient over a wide range of input/output ratios and load

currents. Additionally, PFM converters do not operate

with constant-frequency switching, and have relaxed

stability criterion (unlike PWM converters). As a result,

their external components require smaller values.

With PFM converters, the output voltage ripple is not

concentrated at the oscillator frequency (as it is with

PWM converters). So for applications where the ripple

frequency is important, the PWM control scheme must

be used. However, for many other applications, the

smaller capacitors and lower supply current of the PFM

control scheme make it the better choice. The output

voltage ripple with the MAX774/MAX775/MAX776 can

be held quite low. For example, using the circuit of

Figure 2, only 100mV of output ripple is produced when

generating a -5V at 1A output from a +5V input.

Bootstrapped vs.

Non-Bootstrapped Operation

Figures 2 and 3 are the standard application circuits for

bootstrapped mode, and Figure 4 is the circuit for non-

bootstrapped mode. Since EXT is powered by OUT,

-5V/-12V/-15V or Adjustable,

10

______________________________________________________________________________________

MAX774

MAX775

MAX776

OUT

V+

SHDN

CS

FB

EXT

GND

P

7

8

C2

0.1

µF

R1

0.07

Ω

C1

150

µF

6

5

2

3

1

REF

4

Q1

Si9435

1N5822/

MBR340

L1

22

µH

C4

*

C3

0.1

µF

µF, 10V

MAX775, MAX776 = 120

µF, 20V

PRODUCT

MAX774

MAX775

MAX776

OUTPUT

VOLTAGE (V)

-5

-12

-15

INPUT

VOLTAGE (V)

3 to 15

3 to 8

3 to 5

OUTPUT

CURRENT (A)

1

0.5

0.4

MAX774

MAX775

MAX776

OUT

V+

SHDN

CS

FB

EXT

GND

P

7

8

R2

R3

0.07

Ω

C1

150

µF

6

5

2

3

1

REF

4

Q1

Si9435

1N5822/

MBR340

L1

22

µH

C4

*

C3

0.1

µF

C2

0.1

µF

R1

µF, 10V

MAX775, MAX776 = 120

µF, 20V

Figure 2. Bootstrapped Connection Using Fixed Output

Voltages

Figure 3. Bootstrapped Connection Using External Feedback

Resistors

MAX774

MAX775

MAX776

OUT

V+

SHDN

CS

FB

EXT

GND

P

7

8

R2

R3

0.07

Ω

C1

150

µF

6

5

2

3

1

REF

4

Q1

Si9435

1N5822/

MBR340

L1

22

µH

C4

*

C3

0.1

µF

C2

0.1

µF

R1

µF, 10V

MAX775, MAX776 = 120

µF, 20V