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MP1531

Low Power, Triple Output Step-Up Plus Charge Pump for TFT Bias

Monolithic Power Systems 

MP1531 – LOW POWER, TRIPLE OUTPUT STEP-UP PLUS CHARGE PUMP FOR TFT BIAS

To insure stable operation place the input

capacitor as close to the IC as possible.

Alternately a smaller high quality 0.1µF ceramic

capacitor may be placed closer to the IC if the

larger capacitor is placed further away.

Selecting the Rectifier Diodes

Schottky diodes are recommended for most

applications because of their fast recovery time

and low forward voltage. Use Schottky diodes

with a current rating equal to or greater than 4

times the average output current, and a voltage

rating at least 1.5 times VGH for the positive

charge-pump and VGL for the negative charge-

pump. 100mA Schottky diodes such as Central

Semiconductor CMPSH-3 are recommended

for low current charge-pump circuits.

Selecting the Output Capacitor of the Step-

Up Converter

The output capacitor is required to maintain the

DC output voltage. Low ESR capacitors are

preferred to keep the output voltage ripple to a

minimum. The characteristics of the output

capacitor also affect the stability of the

regulation control system. A 10-22µF ceramic

capacitor works well in most applications. In the

case of ceramic capacitors, the impedance of

the capacitor at the switching frequency is

dominated by the capacitance, and so the

output voltage ripple is mostly independent of

the ESR. The output voltage ripple is estimated

to be:

VRIPPLE

≅

(VMAIN − VIN ) × ILOAD

VMAIN × C2 × fSW

Where VRIPPLE is the output ripple voltage, ILOAD

is the load current, and C2 is the capacitance of

the output capacitor of the step-up converter.

Selecting the Number of Charge-Pump

Stages

For highest efficiency, always choose the

lowest number of charge-pump stages that

meets the output requirement.

The number of positive charge-pump stages

NPOS is approximately given by:

( ) NPOS ≅

VGH + VDROPOUT − VMAIN

VMAIN − 2 × VD

Where VD is the forward voltage drop of the

charge-pump diode, and VDROPOUT is the

dropout margin for the linear regulator. The

positive charge-pump can also be configured

based on VIN for better efficiency. Then the

equation will be:

( ) NPOS ≅

VGH + VDROPOUT − VIN

VMAIN − 2 × VD

The number of negative charge-pump stages

NNEG is approximately given by:

NNEG

≅

−VGL + VDROPOUT

VMAIN − 2 × VD

Use VDROPOUT = 0.5V for positive charge-pump

and VDROPOUT = 0.15V for negative charge-pump.

Selecting the Flying Capacitor in Charge-

Pump Stages

Increasing the flying capacitor CX values

increases the output current capability. A

0.33µF ceramic capacitor works well in most

low current applications. The flying capacitor’s

voltage rating must exceed the following:

VCX > N × VMAIN

Where N is the stage number in which the flying

capacitor appears.

Step-Up Converter Compensation

The MP1531 uses current-mode control which

unlike voltage mode has only a single pole roll

off due to the output filter. The DC gain (AVDC) is

equated from the product of current control to

output gain (AVCSCONTROL), error amplifier gain

(AVEA), and the feedback divider.

Av DC = A CSCONTROL × Av EA × A FB1

A CSCONTROL

= 4 × VIN

ILOAD

A FB1

=

VFB1

VMAIN

Av DC

=

1600 × VIN × VFB1

ILOAD × VMAIN

The output filter pole is given in hertz by:

fFILTERPOLE

=

ILOAD

π × VMAIN × C2

MP1531 Rev. 1.2

www.MonolithicPower.com

9

5/22/2006

MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.

© 2006 MPS. All Rights Reserved.

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