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

MP1531 Rev. 1.2

www.MonolithicPower.com

9

5/22/2006

MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.

© 2006 MPS. All Rights Reserved.

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

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:

SW

MAIN

LOAD

IN

MAIN

RIPPLE

f

2

C

V

I

)

V

V

(

V

×

×

×

−

≅

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

()

D

MAIN

MAIN

DROPOUT

GH

POS

V

2

V

V

V

V

N

×

−

−

+

≅

dropout margin for the linear regulator. The

positive charge-pump can also be configured

equation will be:

(

)

D

MAIN

IN

DROPOUT

GH

POS

V

2

V

V

V

V

N

×

−

−

+

≅

The number of negative charge-pump stages

D

MAIN

DROPOUT

GL

NEG

V

2

V

V

V

N

×

−

+

−

≅

Selecting the Flying Capacitor in Charge-

Pump Stages

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:

MAIN

CX

V

N

V

×

>

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

equated from the product of current control to

1

FB

EA

CSCONTROL

DC

A

Av

A

Av

×

×

=

LOAD

IN

CSCONTROL

I

V

4

A

×

=

MAIN

1

FB

1

FB

V

V

A

=

MAIN

LOAD

1

FB

IN

DC

V

I

V

V

1600

Av

×

×

×

=

The output filter pole is given in hertz by:

2

C

V

I

f

MAIN

LOAD

FILTERPOLE

×

×

π

=