Rainbow Electronics MAX749 - Manual (Page 7)

Figure 4. Count-Up Operation
Figure 5. Reset Sequence without Shutdown. The device is not
shut down during reset.
Figure 6. Reset Sequence with 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.
Shutdown Mode
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.
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.
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.
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
MAX749
Digitally Adjustable LCD Bias Supply
_______________________________________________________________________________________ 7
Figure 3. Shutdown-Reset-On-Shutdown Sequence of Operation.
The device is not shut down during reset.
Figure 7. Control Sequence Example (see Output Voltage
Control section)
ADJ
CTRL
SHUTDOWN RESET
ON
SHUTDOWN
t R t SD
ADJ
CTRL
HIGH
t SH t SL
ADJ
CTRL
ON RESET ON
t RS t RH
t R
ADJ
CTRL
SHUTDOWN
RESET
ON SHUTDOWN
INCREMENT INCREMENT
ADJ
CTRL
SHUTDOWN RESET ON
t RH
t R