3

Rev. 7/14/03

SP4446 High Output Voltage Boost Regualtor, LCD Bias Regulator

© Copyright 2003 Sipex Corporation

FUNCTIONAL DIAGRAM

+

-

VI

(EXTERNAL)

(EXTERNAL)

R1

R2

C1

SHDN

4

Q1

Q2

R3

R4

R6

R5

X1

Shutdown

Logic

300ns

ONE-SHOT

CLEAR

X2

+

-

SET

DRIVER

52.5mV

0.35

GND

2

1

5

3

L1

SW

C2

D1

POWER

TRANSISTOR

DISABLE

FB

General Overview:

Operation can be best understood by referring to

the above block diagram. Q1 and Q2 along with

R3 and R4 form a band gap reference. The input

to this circuit completes a feedback path from

the high voltage output through a voltage di-

vider, and is used as the regulation control input.

When the voltage at the FB pin is slightly above

1.22V, comparator X1 disables most of the

internal circuitry. Current is then provided by

capacitor C2, which slowly discharges until the

voltage at the FB pin drops below the lower

hysteresis point of X1, about 6mV. X1 then

enables the internal circuitry, turns on chip

power, and the current in the inductor begins to

ramp up. When the current through the driver

transistor reaches about 150mA, comparator X2

clears the latch, which turns off the driver tran-

sistor for a preset 0.3

µs. At the instant of shutoff,

inductor current is diverted to the output through

diode D1. During this 0.3

µs time limit, inductor

current decreases while its energy charges C2.

THEORY OF OPERATION

At the end of the 0.3

µs time period, driver

transistor is again allowed to turn on which

ramps the current back up to the 150mA level.

Comparator X2 clears the latch, it’s output turns

off the driver transistor, and this allows delivery

of L1’s stored kinetic energy to C2. This switch-

ing action continues until the output capacitor

voltage is charged to the point where FB is at

band gap (1.22V). When this condition is

reached, X1 turns off the internal circuitry and

the cycle repeats. The SP4446 contains circuitry

to provide protection during start-up and while

in short-circuit conditions. When FB pin volt-

age is less than approximately 300mV, the switch

off time is increased to about 1.2

µs and the

current limit is reduced to about 70% of its

normal value. While in this mode, the average

inductor current is reduced and helps minimize

power dissipation in the SP4446, the external

inductor and diode.