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

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

µH

DLOW

6-BIT

CURRENT-OUTPUT

DAC

REF

MAXIMUM

ON-TIME

ONE-SHOT

MAX749

(NEGATIVE)

22

µF

0.1

µF

V+

DHI

FB

GND

Q1

ZTX750

D1

1N5819

470

Ω

22

µF

30V

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