12V/15V or Adjustable, High-Efficiency,
Low I
Q
, Step-Up DC-DC Converters
MAX761/MAX762
LBO
V+
FB
LBI
N
LBI
100mV
REF
DUAL-MODE™
COMPARATOR
MAX761
MAX762
ERROR
COMPARATOR
V+
1.5V
REFERENCE
Q
TRIG
ONE-SHOT
N
S
R
Q
LOW INPUT
VOLTAGE
OSCILLATOR
UNDER VOLTAGE
COMPARATOR
2.5V
Q
TRIG
ONE-SHOT
LX
CURRENT
COMPARATOR
N
0.2V
CURRENT CONTROL
CIRCUITRY
0.1V
GND
Figure 1. Simple Block Diagram
________________Detailed Description
Operating Principle
The MAX761/MAX762 BiCMOS step-up switch-mode
power supplies provide fixed outputs of 12V and 15V,
respectively. They have a unique control scheme that
combines the advantages of pulse-frequency modulation
(low supply current) and pulse-width modulation (high
efficiency at high loads). The internal N-channel power
MOSFET allows 1A peak currents, increasing the output
current capability over previous pulse-frequency-modu-
lation (PFM) devices. Figure 1 shows the MAX761/
MAX762 block diagram.
The MAX761/MAX762 offer three main improvements
over prior solutions: (1) the converters operate with tiny
surface-mount inductors (less than 5mm diameter)
because of their 300kHz switching frequency, (2) the
current-limited PFM control scheme allows 86% efficien-
cies over a wide range of load currents, and (3) the max-
imum supply current is only 110µA.
Bootstrapped/Non-Bootstrapped Modes
Figures 2 and 3 show the standard application circuits
for bootstrapped and non-bootstrapped modes. In boot-
strapped mode, the IC is powered from the output
(V
OUT
). In other words, the current needed to power the
bootstrapped circuit is different from the V+ current the
chip consumes. The voltage applied to the gate of the
internal N-channel FET is switched from V
OUT
to ground,
providing more switch-gate drive and increasing the effi-
ciency of the DC-DC converter compared with non-boot-
strapped operation.
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