Feed-Forward Capacitors

pensate the feedback loop and ensure stability (Figure

4). Use values up to 100pF for most applications.

Choose the lowest capacitor value that ensures stability;

high capacitance values may degrade line regulation.

Applications Information

Using a Potentiometer to Adjust the

Output Voltage

The output can be adjusted with a potentiometer

Ω

and connect it between REF and GND. Connect R3 to

the potentiometer’s wiper instead of to DACOUT. Use

the same design equations for adjusting the output volt-

age with the DAC.

Controlling the LCD Using

POK and

LLC

CD

DO

ON

N

When the voltage at POK is greater than 1.125V (typical),

the open-drain LCDON output pulls low. LCDON can

withstand up to 27.5V to control an external PNP transis-

tor to switch on the MAX686’s positive output (Figures 6

and 7). A PFET can also be used, but a resistor-divider

must be used in conjunction with it, so that the PFET does

this feature are as follows:

•

An off-switch driver to ensure that a positive boosted

output goes to 0V during shutdown. Connect POK to

SHDN. Without this switch, the positive output falls to

down. LCDON is not needed for negative outputs,

which already fall to 0V in shutdown.

•

An input-sensing cutoff for positive outputs. Connect

POK to a voltage divider to sense the input voltage.

The output switches on only when the input reaches

the set level (Figure 6).

•

An output-sensing cutoff for positive outputs. Connect

POK to the feedback voltage divider to sense the out-

put voltage. The output switches on only when it

reaches 90% of the set voltage (Figure 7).

For positive output voltage sensing, connect POK

directly to FB to monitor the output voltage (Figure 7).

The POK threshold is 10% less than the set voltage at

FB. Therefore, when the output voltage drops 10%

below its set value, the POK circuit turns off the external

PNP transistor, disconnecting the load.

For input voltage sensing, a resistor-divider (R4-R5,

Figure 6) from VIN to POK controls the open-drain out-

Choose R5 = 100k

Ω. For example, if the minimum bat-

tery voltage is 5.3V, then determine R4 as follows:

= 100k x [(5.3 / 1.125) -1] = 371k

Ω

LCDON typically drives a low-cost PNP transistor (such

as a 2N2907 or equivalent), switching a positive VOUT to

load current. R7 limits the base current in the PNP, and

DAC-Controlled Boost/Inverter

LCD Bias Supply with Internal Switch

______________________________________________________________________________________

13

MAX686

POSITIVE

OUTPUT

VOLTAGE

TO 27.5V

POK

GND

R4

R5

LX

DACOUT

LCDON

FB

R6

R2

22

µH

R1

R3

MBR0530L

R7

Figure 6. Using the POK for Input Voltage Monitoring

MAX686

POSITIVE

OUTPUT

VOLTAGE

TO 27.5V

GND

LX

R2

R1

R3

DACOUT

MBR0530L

LCDON

FB

POK

R6

R7

22

µH

Figure 7. Using the POK for Output Voltage Monitoring