Digitally Adjustable LCD Bias Supply
ADJ
CTRL
t
R
SHUTDOWN RESET
ON
t
SD
SHUTDOWN
In Figure 3, the MAX749 is reset when it is taken out of
shutdown, which sets the output at mid-scale. Figure 4
shows how to increment the counter. Figure 5 illustrates
a reset without shutting the device down.
Figure 7 provides an example of a sequence of opera-
tions: Starting from shutdown, the device is turned on,
incremented, reset to mid-scale without being shut
down, incremented again, and finally shut down.
MAX749
Shutdown Mode
Figure 3. Shutdown-Reset-On-Shutdown Sequence of Operation.
The device is not shut down during reset.
When CTRL and ADJ are both low, the MAX749 is shut
down (Table 1): The internal reference and biasing cir-
cuitry turn off, the output voltage drops to zero, and the
supply current drops to 15µA. The MAX749 retains its
DAC setting, simplifying software control.
ADJ
CTRL
t
SH
t
SL
HIGH
Reset Mode
If ADJ is high when CTRL is low, the DAC set point is
reset to mid-scale and the MAX749 is not shut down.
Mid-scale is 32 steps from the minimum, 31 steps from
the maximum.
Figure 4. Count-Up Operation
Design Procedure
_________and Component Selection
Setting the Output Voltage
The MAX749’s output voltage is set using an external
resistor and the internal current-output DAC. The full-
scale output voltage is set by selecting the feedback
resistor R
FB
according to the formula:
-V
OUT(MAX)
= R
FB
x 20µA (Figure 1).
The device is in regulation when V
FB
= 0V.
ON
ADJ
CTRL
t
RS
t
R
ON
RESET
t
RH
Figure 5. Reset Sequence without Shutdown. The device is not
shut down during reset.
DAC Adjustment
On power-up or after a reset, the counter sets the DAC
output to mid-range, and -V
OUT
= R
FB
x 13.33µA. Each
rising edge of ADJ increments the counter (and there-
fore the DAC output) in the direction of -V
OUT(MAX)
by
one count. When incremented beyond -V
OUT(MAX)
, the
INCREMENT
ADJ
CTRL
t
R
SHUTDOWN
RESET
t
RH
ON
SHUTDOWN
ADJ
CTRL
RESET
INCREMENT
ON
SHUTDOWN
Figure 6. Reset Sequence with Shutdown
Figure 7. Control Sequence Example (see Output Voltage
Control section)
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MAXIM [ MAXIM INTEGRATED PRODUCTS ]
