Output voltage control – Rainbow Electronics MAX1621 User Manual

Page 11

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MAX1620/MAX1621

Digitally Adjustable LCD Bias Supplies

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maximum switching frequency. The following equation
represents the output current for the ideal case (no
power losses) of Figure 5:

This means that a higher peak current is required to
achieve the same output current in the negative output
circuit as in the positive output circuit.

The output current for Figure 6 uses the same current
equation as the positive boost.

Output Voltage Control

The output voltage is set with a voltage divider to the
feedback pin (FB). For a positive output, the divider is
referred to GND; for a negative output, the divider is
referred to REF.

Output voltage can be adjusted with an internal DAC
summing current into FB through an external resistor.
The 5-bit DAC is controlled with a user-programmable
up/down counter. On power-up or after a reset, the
counter sets the DAC output to 10000 binary, or half-
scale.

x (k - factor / L) x V

/ (V

)

OUT

BATT

OUT

15V

BATT
POK

N1
MMFT3055VL

POL
SHDN (SUS)
DN (SDA)

UP (SCL)
REF
AGND

3
5

7
4
1
2
6

( ) ARE FOR MAX1621.

NOTE: CONNECTIONS TO DIGITAL INPUTS NOT SHOWN.

14
16
15
13

300k

R4
300k

1.2M

100pF

10
9

DHI

DLO

PGND

DOUT

LCDON

5.5V

C1
0.1

C2
0.1

C3
22

C5
22

-6V
TO
-12V OUT

L1
100

D1
MBRS0540

MBRS0540

MAX1620
MAX1621

R8
10k

TO REF

D3 1N6263 (ANY SCHOTTKY)

12V