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

MP1531 Rev. 1.2

www.MonolithicPower.com

8

5/22/2006

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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. However, the larger value inductor has

a larger physical size, higher series resistance,

lower saturation current for a given package

size.

Choose an inductor that does not saturate at

heavy load transients and start-up conditions. A

good rule for determining the inductance 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 the device current limit to

prevent loss of regulation. Calculate the

required inductance value by the equation:

()

I

f

V

V

V

V

1

L

SW

MAIN

IN

MAIN

IN

∆

×

×

−

=

switching frequency, and ∆I is the peak-to-peak

inductor ripple current.

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). The

input capacitor in Figure 3 is necessary in most

lab setups, but can be reduced in typical

application circuits if the source impedance is

lower.

MP1531

SW

FB1

IN

CT

PGND

GND

FB3

GH

IN3

REF

EN

COMP

RDY

RDY

IN2

GL

FB2

-10V

C9

56pF

C3

10nF

C4

10nF

D1

B0530W

2.7V to 4.2V

5V

15V

C7

56pF

D2

BAS40-04-7

D3

BAS40-04-7

D4

BAS40-04-7

SW

D5

BAS40-04-7

D7

BAS40-04-7

D6

BAS40-04-7

OFF ON

Figure 3—Triple Output Boost Application Circuit