Compact, High-Efficiency, Dual-Output

Step-Up and LCD Bias DC-DC Converter

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13

in the 10k

Ω to 200kΩ range and calculate R1 using one

of the following two equations (for positive or negative

output).

For a positive LCD output, connect LCDPOL to OUT as

shown in Figure 2. This sets the threshold at LCDFB to

1.25V. Select R2 and the desired output voltage

Figure 3 shows the standard circuit for generating a

output voltage is required, D2’s cathode can be con-

nection permits output voltages from 0 to -

For a negative LCD output voltage, connect LCDPOL to

GND. The feedback threshold voltage of LCDFB is set

and calculate R1:

For negative LCD output: R1 = R2

×

To minimize ripple in the LCD output and prevent sub-

harmonic noise caused by switching pulse grouping, it

may be necessary in some PC board layouts to con-

nect a small capacitor in parallel with R1. For R1 values

in 500k

Ω to 2MΩ range, 22pF is usually adequate.

Many LCD bias applications require an adjustable out-

put voltage. In Figure 9, an external control voltage

(generated by a potentiometer, DAC, filtered PWM con-

trol signal, or other source) is coupled to LCDFB

cuit, for both positive and negative outputs, is given by:

added adjust voltage, as calculated in one of the pre-

configuration, and 0 for the negative configuration.

1.25V

es the magnitude of the negative output.

Higher LCD Output Voltages

If the application requires LCD output voltages greater

than +28V, use the connection in Figure 10. This circuit

adds one capacitor-diode charge pump stage to

increase the output voltage without increasing the volt-

age stress on the LCDLX pin. The maximum output

voltage of the circuit is +55V and output current is

slightly less than half that available from the standard

circuit in Figure 2. In Figure 10, diodes D1, D2, and D3

should be at least 30V-rated Schottky diodes such as

1N5818 or MBR0530L or equivalent. Capacitors C1

and C2 should also be rated for 30V, while C3 must be

rated for the maximum set output voltage.

Applications Information

Inductor Selection

The MAX1677’s high switching frequency allows the

use of small surface-mount inductors. The 10µH values

shown in Figures 2 and 3 are recommended for most

applications, although values between 4.7µH and 47µH

are suitable. Smaller inductance values typically offer a

smaller physical size for a given series resistance,

allowing the smallest overall circuit dimensions. Larger

inductance values exhibit higher output current capa-

bility, but larger physical dimensions.

MAX1677

FB

R2

R1

GND

(REF)

Figure 9. Adjusting LCD Output Voltage

MAX1677

LCDLX

OUT

LCDPOL

L2

10

µH

D1

D2

D3

C1

1

µF

30V

C2

2.2

µF

30V

C3

2.2

µF

+40V/5mA

(SET TO

NO MORE

THAN 55V)

D1, D2, D3 = 30V RATED SCHOTTKY DIODES:

MBR0530L OR EQUIVALENT.

R2

65k

R1

2M

1

7

12

10

LCDFB

Figure 10. Higher LCD Output Voltage