DEVICE 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. If the

REF terminal is used as a reference for external

devices, the maximum load should not exceed ±2 A.

Dynamic drive output

Depending on the input signal, the output is either

"LOW" or "HIGH". In the high state a 3.4mA 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

sensed by comparator, Comp2, at input SENSE. Once

the current sense voltage across the sensing resistor

exceeds 28mV, comparator, Comp2, through gate U1,

triggers a re-triggerable monostable and turns off the

output drive stage for 1.7 s. The inductor discharges

into the reservoir capacitor. After 1.7 s a new charge

cycle begins, thus ramping the output voltage. When

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, with

slight hysteresis on the feedback threshold.

READY detector

The READY circuit is a re-triggerable 195 s

monostable, which is re-triggered by every down

regulating action of comparator Comp1. As long as

regulation takes place, output READY is "HIGH" (high

voltage of less than 195 s are ignored. If the output

voltage falls below the nominal value for more than

195 s, output READY goes "LOW". This can be used to

signal to the camera controller that the flash unit has

charged fully and is ready to use.

ZXSC440

SEMICO NDUC TORS

ISSUE 1 - JANUARY 2005

4