-5V/-12V/-15V or Adjustable,
High-Efficiency, Low I
Q
DC-DC Inverters
MAX764/MAX765/MAX766
FB
COMPARATOR
REF
SHDN
ERROR
COMPARATOR
MAX764
MAX765
MAX766
OUT
V+
N
1.5V
REFERENCE
Q
TRIG
ONE-SHOT
FROM V+
S
TRIG
ONE-SHOT
Q
Q
CURRENT
COMPARATOR
FROM OUT
LX
P
V+
R
CURRENT
CONTROL CIRCUITS
0.2V
(FULL
CURRENT)
0.1V
(HALF
CURRENT)
FROM V+
GND
Figure 1. Block Diagram
_______________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:
8
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
input (V+) and output (V
OUT
). The principal benefit of
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
I
V
OUT
I
plus a diode drop (when the
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