MP1530 – TRIPLE OUTPUT STEP-UP PLUS LINEAR REGULATORS FOR TFT BIAS

MP1530 Rev. 1.4

www.MonolithicPower.com

10

5/19/2006

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© 2006 MPS. All Rights Reserved.

APPLICATION INFORMATION

COMPONENT SELECTION

Setting the Output Voltages

Set the output voltage on each output by

selecting the resistive voltage divider ratio. The

voltage divider drops the output voltage to the

feedback threshold voltage. Use 10kΩ to 50kΩ

divider.

For the step-up converter, determine the high-

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

=

L

1

FB

1

FB

MAIN

H

R

V

V

V

R

converter.

For the positive charge-pump, determine the

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

=

L

3

FB

3

FB

GH

H

R

V

V

V

R

For the negative charge-pump, determine the

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

=

L

REF

GL

H

R

V

V

R

Selecting the Inductor

The inductor is required to force the higher

output voltage while being driven by the input

voltage. A larger value inductor results in less

ripple current that results in lower peak inductor

current,

reducing

stress

on

the

internal

inductor has a larger physical size, higher

series resistance, and/or lower saturation

current.

A 4.7µH inductor is recommended for most

applications. A good rule of thumb is to allow

the

peak-to-peak

ripple

current

to

be

approximately 30-50% of the maximum input

current. Make sure that the peak inductor

current is below 75% of the current limit to

prevent loss of regulation due to the current

limit. Also make sure that the inductor does not

saturate under the worst-case load transient

and startup conditions.

Calculate the required inductance value by the

equation:

I

f

V

)

V

-

(V

V

L

SW

OUT

IN

OUT

IN

∆

×

×

=

×

η

×

×

=

IN

)

MAX

(

LOAD

OUT

)

MAX

(

IN

V

I

V

I

(

)

)

MAX

(

IN

I

%

50

%

30

I

−

=

∆

is the peak-to-peak inductor ripple current, and η

is efficiency.

Selecting the Input Capacitor

An input capacitor is required to supply the AC

ripple current to the inductor, while limiting

noise at the input source. A low ESR capacitor

is required to keep the noise at the IC to a

minimum. Since it absorbs the input switching

current it requires an adequate ripple current

rating. Use a capacitor with RMS current rating

greater than the inductor ripple current (see

Selecting

The

Inductor

to determine the

inductor ripple current). One 10µF ceramic

capacitor is used in the application circuit of

Figure 3 because of the high source impedance

seen in typical lab setups. Actual applications

usually have much lower source impedance

since the step-up converter typically runs

directly from the output of another regulated

supply. Typically, the input capacitance can be

reduced below the value used in the typical

application circuit.

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.