DESIGN INFORMATION

IC operation description

Bandgap Reference

All threshold voltages and internal currents are

derived from a temperature compensated bandgap

reference circuit with a reference voltage of 1.22V

nominal.

Dynamic Drive Output

Depending on the input signal, the output is either

“LOW” or “HIGH”. In the high state a 2.5mA current

or gate of the external transistor. In order to operate

the external switching transistor at optimum

efficiency, both output states are initiated with a short

transient current in order to quickly discharge the base

or the gate of the switching transistor.

Switching Circuit

The switching circuit consists of two comparators,

Comp1 and Comp2, a gate U1, a monostable and the

drive output. Normally the DRIVE output is “HIGH”; the

external switching transistor is turned on. Current

ramps up in the inductor, the switching transistor and

external current sensing resistor. This voltage is

the current sense voltage across the sensing resistor

exceeds 30mV, comparator Comp2 through gate U1

triggers a re-triggerable monostable and turns off the

output drive stage for 2 s. The inductor discharges to

the load of the application. After 2 s a new charge

cycle begins, thus ramping the output voltage. When

the output voltage reaches the nominal value and FB

gets an input voltage of more than 300mV, the

monostable is forced “on” from Comp1 through gate

U1, until the feedback voltage falls below 300mV. The

above action continues to maintain regulation.

ZXSC400

ISSUE 1 - JANUARY 2003

4

Pin No.

Name

Description

Supply voltage, 1.8V to 8V.

2

GND

Ground

3

STDN

Shutdown

4

SENSE

Inductor current sense input. Internal threshold voltage set to 30mV. Connect

external sense resistor

5

Reference voltage. Internal threshold set to 300mV. Connect external resistor

to program LED current

6

DRIVE

Drive output for external switching transistor. Connect to base or gate of

external switching transistor.

Pin functions

2µs

Block diagram