Rainbow Electronics MAX749 - Manual (Page 5)

_______________Detailed Description
The MAX749 is a negative-output inverting power con-
troller that can drive an external PNP transistor or P-
channel MOSFET. An external resistor and an internal
DAC control the output voltage (Figure 1).
The MAX749 is designed to operate from 2V to 6V inputs,
ideal for operation from low-voltage batteries. In systems
with higher-voltage batteries, such as notebook comput-
ers, the MAX749 may also be operated from the regulat-
ed +5V supply. A high-efficiency +5V regulator, such as
the MAX782, is an ideal source for the MAX749. In this
example, the MAX749 efficiency (80%) is compounded
with the MAX782 efficiency (95%): 80% x 95% = 76%,
which is still high.
Operating Principle
The MAX749 and the external components shown in the
Typical Operating Circuit form a flyback converter.
When the external transistor is on, current flows through
the current-sense resistor, the transistor, and the coil.
Energy is stored in the core of the coil during this phase,
and the diode does not conduct. When the transistor
turns off, current flows from the output through the diode
and the coil, driving the output negative. Feedback con-
trol adjusts the external transistor’s timing to provide a
regulated negative output voltage.
The MAX749’s unique control scheme combines the
ultra-low supply current of pulse-skipping, pulse-fre-
quency modulation (PFM) converters with the high full-
load efficiency characteristic of pulse-width modulation
(PWM) converters. This control scheme allows the
device to achieve high efficiency over a wide range of
loads. The current-sense function and high operating
frequency allow the use of tiny external components.
Switching control is accomplished through the combi-
nation of a current limit in the switch plus on- and off-
time limits (Figure 2).
Once turned on, the transistor stays on until either:
- the maximum on-time one-shot turns it off
(8µs later), or
- the switch current reaches its limit (as determined
by the current-sense resistor and the current
comparator).
MAX749
Digitally Adjustable LCD Bias Supply
_______________________________________________________________________________________ 5
L1
47
µ H
DLOW
SWITCH-
MODE
POWER
SUPPLY
6-BIT
CURRENT-OUTPUT
DAC
REF
LOGIC
BIAS
POWER-ON
RESET
6-BIT
COUNTER
MAX749
V OUT
(NEGATIVE)
CTRL
ADJ
RESET
INCREMENT
ON/OFF
+2V TO +6V
INPUT 22 µ F
6.2V
6.66 µ A TO 20 µ A
CS
DHI
FB
GND
R SENSE
Q1
ZTX750
D1
1N5819
R BASE
470 Ω
22 µ F
30V
C COMP
R FB
V+
0.1 µ F
Figure 1. Block Diagram, Showing External Circuitry Using a PNP Transistor