MAX188 datasheet(21/32 Pages) MAXIM

21 / 32 page

Triple-Output TFT LCD Power Supply

with Fault Protection

______________________________________________________________________________________

The number of negative charge-pump stages is given by:

where NNEG is the number of negative charge-pump

stages, VNL is the negative linear-regulator output,

VMAIN is the main step-up regulator output, VD is the

forward voltage drop of the charge-pump diode, and

VDROPOUT is the dropout margin for the linear regulator.

Use VDROPOUT = 2V.

The above equations are derived based on the

assumption that the first stage of the positive charge

pump is connected to VMAIN and the first stage of the

negative charge pump is connected to ground.

Sometimes fractional stages are more desirable for bet-

ter efficiency. This can be done by connecting the first

stage to VIN or another available supply.

If the first charge-pump stage is powered from VIN,

then the above equations become:

Flying Capacitor

Increasing the flying capacitor (CX) value increases the

output current capability. Increasing the capacitance

indefinitely has a negligible effect on output current

capability because the internal switch resistance and

the diode impedance limit the source impedance. A

0.1µF ceramic capacitor works well in most low-current

applications. The flying capacitor’s voltage rating must

exceed the following:

where N is the stage number in which the flying capaci-

tor appears, and VMAIN is the main output voltage. For

example, the two-stage positive charge pump in the

typical application circuit (Figure 1) where VMAIN = 9V

contains two flying capacitors. The flying capacitor in

the first stage (C14) requires a voltage rating over 9V.

The flying capacitor in the second stage (C13) requires

a voltage rating over 18V.

Charge-Pump Output Capacitor

Increasing the output capacitance or decreasing the

ESR reduces the output ripple voltage and the peak-to-

peak transient voltage. With ceramic capacitors, the

output voltage ripple is dominated by the capacitance

value. Use the following equation to approximate the

required capacitor value:

where VRIPPLE is the peak-to-peak value of the output

ripple.

Charge-Pump Rectifier Diodes

Use Schottky diodes with a current rating equal to or

greater than two times the average charge-pump input

current.

Linear-Regulator Controllers

Output Voltage Selection

Adjust the positive linear-regulator output voltage by

connecting a resistive voltage-divider from VPL to GND

with the center tap connected to FBP (Figure 1). Select

R13 in the range of 10k

Ω to 30kΩ.

Calculate R12 with the following equation:

R12 = R13 [(VPL / VFBP) - 1]

where VFBP = 1.25V.

Adjust the negative linear-regulator output voltage by

connecting a resistive voltage-divider from VNL to REF

with the center tap connected to FBN (Figure 1). Select

R10 in the range of 10k

Ω to 30kΩ. Calculate R9 with the

following equation:

R9 = R10 [(VFBN - VNL) / (VREF - VFBN)]

where VFBN = 125mV, VREF = 1.25V. Note that REF is

only guaranteed to source 50µA. Using a resistor less

than 20k

Ω for R10 results in higher bias current than

REF can supply. Connecting another resistor (R14)

from VMAIN to REF (Figure 1) can solve this problem

because the main output can supply part of the resis-

tor’s (R10) bias current. Use the following equation to

determine the value of R14:

Drawing only 40µA from REF leaves the remaining

10µA for other purposes.

Pass Transistor Selection

The pass transistor must meet specifications for current

gain (

β), input capacitance, collector-emitter saturation

voltage, and power dissipation.

MAIN

REF

FBN









OUT

LOAD

OSC RIPPLE

≥

VN V

MAIN

>×

POS

DROPOUT

MAIN

NEG

DROPOUT

MAIN

NEG

DOPOUT

MAIN

-2

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