Rainbow Electronics MAX749 - Manual (Page 6)

MAX749
Once turned off, a one-shot holds the switch off for a
minimum of 1µs, and the switch either stays off (if the
output is in regulation), or turns on again (if the output
is out of regulation).
With light loads, the transistor switches for one or more
cycles and then turns off, much like a traditional PFM
converter. With heavy loads, the transistor stays on until
the switch current reaches the current limit; it then
shuts off for 1µs, and immediately turns on again until
the next time the switch current reaches its limit. This
cycle repeats until the output is in regulation.
Output Voltage Control
The output voltage is set using a single external resistor
and the internal current-output DAC (Figure 1). The full-
scale output voltage is set by selecting the feedback
resistor, R
FB
. The output voltage is controlled from 33%
to 100% of the full-scale output by an internal 64-step
DAC/counter.
On power-up or after a reset, the counter sets the DAC
output to mid-range. Each rising edge of ADJ incre-
ments the DAC output. When incremented beyond full
scale, the counter rolls over and sets the DAC to the
minimum value. In this way, a single pulse applied to
ADJ increases the DAC set point by one step, and 63
pulses decrease the set point by one step.
Table 1 is the logic table for the CTRL and ADJ inputs,
which control the internal DAC and counter. Figures 3-7
show various timing specifications and different ways of
incrementing and resetting the DAC, and of placing it in
the low-power standby mode. As long as the timing
specifications for ADJ and CTRL are observed, any
sequence of operations can be implemented.
Table 1. Input Truth Table
Digitally Adjustable LCD Bias Supply
6 ______________________________________________________________________________________
L1
47
µ H
DLOW
6-BIT
CURRENT-OUTPUT
DAC
REF
MAXIMUM
ON-TIME
ONE-SHOT
MAX749
V OUT
(NEGATIVE)
22 µ F
0.1 µ F
V+
DHI
FB
GND
R SENSE
Q1
ZTX750
D1
1N5819
R BASE
470 Ω
22 µ F
30V
C COMP
R FB
TRIG
MINIMUM
OFF-TIME
ONE-SHOT
TRIG Q
Q
Q S
R
FLIP-FLOP
VOLTAGE
COMPARATOR
140mV
+2V TO +6V
INPUT
CURRENT
COMPARATOR
Figure 2. Switch-Mode Power-Supply Section Block Diagram
ADJ CTRL RESULT
Low Low Shut down
High Low
Reset counter to mid-range. The
device is not shut down.
X High On
High Increment the counter