Jun26-07 Rev D

Micro Power Boost Regulator Series White LED Driver

© 2007 Sipex Corporation

+

-

4

Q1

Q2

R3

R4

R2

R1

X1

Shutdown

Logic

250ns

ONE-SHOT

CLEAR

X2

+

-

SET

DRIVER

GND

2

1

5

3

SW

POWER

TRANSISTOR

DISABLE

VIN

SHDN

FB

THEORY OF OPERATION

Operation can be best understood by referring to

the functional diagram above and the typical

application circuit in the front page. Q1 and Q2

along with R3 and R4 form a band gap refer-

ence. The input to this circuit completes a feed-

back path from the high voltage output through

a voltage divider, 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 dis-

charges 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

450mA, comparator X2 clears the latch, which

turns off the driver transistor for a preset 250nS.

At the instant of shutoff, inductor current is

diverted to the output through diode D1. During

this 250nS time limit, inductor current decreases

while its energy charges C2.

At the end of the 250ns time period, driver

transistor is again allowed to turn on which

ramps the current back up to the 450mA 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.

FUNCTIONAL DIAGRAM

PIN DESCRIPTION

PIN NUMBER

PIN NAME

DESCRIPTION

1

SW

Switch input to the internal power switch.

2

GND

Ground

3

FB

Feedback

4

SHDN

Shutdown. Pull high (on) to enable. Pull low (off) for shutdown.

5

Input Voltage. Bypass this pin with a capacitor as close to the device

as possible.