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). For applications where the ripple fre-

quency 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 generat-

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

Bootstrapped vs.

Nonbootstrapped Operation

Figures 2 and 3 are the standard application circuits

for bootstrapped mode, and Figure 4 is the circuit for

nonbootstrapped mode. Since EXT is powered by OUT,

using bootstrapped or nonbootstrapped mode will

directly affect the gate drive to the FET. EXT swings

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

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

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

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

MAX775, MAX776 = 120

µF, 20V