SP6690 datasheet(4/16 Pages) SIPEX

4 / 16 page

Date: 05/25/04

SP6690 Micro Power Boost Regualtor, Series White LED Driver

Shutdown

Logic

250ns

ONE-SHOT

CLEAR

SET

DRIVER

52.5mV

0.15

GND

POWER

TRANSISTOR

DISABLE

VIN

SHDN

FUNCTIONAL DIAGRAM

THEORY OF OPERATION

General Overview:

Operation can be best understood by referring to

the functional diagram above and the typical

application circuit on 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

350mA, 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 350mA 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 SP6690 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 1.2uS 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 SP6690, the external

inductor and diode.

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