Applications information, Using a potentiometer to adjust the output voltage, Controlling the lcd using pok and llc cd do on n – Rainbow Electronics MAX686 User Manual

Page 13

Feed-Forward Capacitors

Parallel a feed-forward capacitor (C

) across R2 to com-

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
instead of the DAC (Figure 5). Choose R

POT

= 100k

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

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
not exceed its V

rating. Three useful applications of

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

one diode drop below the input voltage (V

) in shut-

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-
put LCDON, which pulls low when V

POK

> 1.125V.

Choose R5 = 100k

Ω

. For example, if the minimum bat-

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

R4 = R5 x [(V

/ V

POK

) - 1]

= 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
the LCD. Choose a PNP with low V

CESAT

at the required

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

MAX686

DAC-Controlled Boost/Inverter

LCD Bias Supply with Internal Switch

______________________________________________________________________________________

MAX686

OUTSW

OUT

LCD

POSITIVE
OUTPUT
VOLTAGE

= 0.8V

TO 27.5V

POK

GND

DACOUT

LCDON

MBR0530L

Figure 6. Using the POK for Input Voltage Monitoring