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

DAC Adjustment

On power-up or after a reset, the counter sets the DAC

rising edge of ADJ increments the counter (and there-

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

ADJ

CTRL

HIGH

ADJ

CTRL

ON

RESET

ON

ADJ

CTRL

SHUTDOWN

RESET

ON

SHUTDOWN

INCREMENT

INCREMENT

ADJ

CTRL

SHUTDOWN

RESET

ON