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

8

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_______________Detailed Description

Operating Principle

The MAX764/MAX765/MAX766 are BiCMOS, inverting,

switch-mode power supplies that provide fixed outputs

of -5V, -12V, and -15V, respectively; they can also be

set to any desired output voltage using an external

resistor divider. Their unique control scheme combines

the advantages of pulse-frequency modulation (pulse

skipping) and pulse-width modulation (continuous puls-

ing). The internal P-channel power MOSFET allows

peak currents of 0.75A, increasing the output current

capability over previous pulse-frequency-modulation

(PFM) devices. Figure 1 shows the MAX764/MAX765/

MAX766 block diagram.

The MAX764/MAX765/MAX766 offer three main

improvements over prior solutions:

1) They can operate with miniature (less than 5mm

diameter) surface-mount inductors, because of their

300kHz switching frequency.

2) The current-limited PFM control scheme allows efficien-

cies exceeding 80% over a wide range of load currents.

3) Maximum quiescent supply current is only 120µA.

Figures 2 and 3 show the standard application circuits

for these devices. In these configurations, the IC is

powered from the total differential voltage between the

this arrangement is that it applies the largest available

signal to the gate of the internal P-channel power MOS-

FET. This increased gate drive lowers switch on-resis-

tance and increases DC-DC converter efficiency.

Since the voltage on the LX pin swings from V+ (when the

switch is ON) to

MAX764

MAX765

MAX766

P

TRIG

Q

ONE-SHOT

TRIG

Q

ONE-SHOT

S

R

Q

CURRENT

CONTROL CIRCUITS

1.5V

REFERENCE

N

FROM OUT

FROM V+

FROM V+

0.1V

(HALF

CURRENT)

0.2V

(FULL

CURRENT)

GND

LX

V+

OUT

V+

REF

SHDN

ERROR

COMPARATOR

COMPARATOR

CURRENT

COMPARATOR

FB

Figure 1. Block Diagram