4

Mar28-08

SP4446 High Output Voltage Boost Regulator, LCD Bias Regulator

© 2008 Exar Corporation

inherently low ESR, which will help produce low

peak to peak output ripple, and reduce high fre-

quency spikes.

For the typical application, a 4.7

µF input capacitor

and a 2.2

µF output capacitor are sufficient. The

input and output ripple could be further reduced by

increasing the value of the input and output capaci-

tors. Place all the capacitors as close to the SP4446

as possible for layout. For use as a voltage source,

to reduce the output ripple, a small feedforward

capacitor (47pF) across the top feedback resistor

can be used to provide sufficient overdrive for the

error comparator, thus reduce the output ripple.

Refer to Table 2 for some suggested low ESR capacitors.

MANUF.

PART

CAP/

SIZE/

NUMBER

VOLTAGE

TYPE

MURATA

GRM32RR71E

2.2

µF/25V

1210/X5R

770-436-1300

225KC01B

MURATA

GRM31CR61A

4.7

µF/10V

1206/X5R

770-436-1300

475KA01B

TDK

C3225X7R1E

2.2

µF/25V

1206/X7R

847-803-6100

225M

TDK

C3216X5R1A

4.7

µF/10V

1206/X5R

847-803-6100

475K

APPLICATION INFORMATION

Inductor Selection

For SP4446, the internal switch will be turned off

only after the inductor current reaches the typical

typically a propagation delay of 200nS between

the time when the current limit is reached and

when the switch is actually turned off. During this

200nS delay, the peak inductor current will in-

crease,exceedingthecurrentlimitbyasmallamount.

The peak inductor current can be estimated by:

• 200nS

L

The larger the input voltage and the lower the

inductor value, the greater the peak current.

In selecting an inductor, the saturation current

specified for the inductor needs to be greater than

the SP4446 peak current to avoid saturating the

inductor, which would result in a loss of efficiency

and could damage the inductor. Choosing an in-

ductor with low DCR decreases power losses and

increase efficiency.

Refer to Table 1 for some suggested low ESR inductors.

MANUF.

PART NUMBER

DCR

Current

(

Rating

(mA)

MURATA

LQH32CN100K21

0.44

300

770-436-1300

(10

µH)

MURATA

LQH32CN220K21

0.71

250

770-436-1300

(22

µH)

TDK

NLFC453232T-100K

847-803-6100

(10

µH)

0.273

250

TDK

NLC453232T-100K

0.9

370

847-803-6100

(22

µH)

Table 1. Suggested Low ESR inductors

Diode Selection

A schottky diode with a low forward drop and fast

switching speed is ideally used here to achieve

high efficiency. In selecting a Schottky diode, the

current rating of the schottky diode should be

larger than the peak inductor current. Moreover,

the reverse breakdown voltage of the Schottky

diode should be larger than the output voltage.

Capacitor Selection

Ceramic capacitors are recommended for their

Table 2. Suggested Low ESR Capacitor

Output Voltage Program

In order for the SP4446 to be programmed as a

voltage source, the SP4446 requires 2 feedback

shown in Figure 1.

L1

1.22V

R2

D1

C1

C2

R1

VOUT

U1

SP4446

1

2

3

4

5

SW

G ND

FB

SHDN

V

I N

VIN

Figure 1. Using SP4446 as Voltage Source