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.

Dynamic Drive Output

Depending on the input signal, the output is either

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

source (max drive voltage = VCC-0.4V) drives the base

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 20mV, comparator Comp2 through gate U1

triggers a re-triggerable monostable and turns off the

output drive stage for 2µs. The inductor discharges to

the load of the application. After 2µs a new charge cycle

begins, thus ramping the output voltage. When the

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.

EOR, End of Regulation Detector

The EOR circuit is a retriggerable 120µs monostable,

which is re-triggered by every down regulating action

of comparator Comp1. As long as regulation takes

place, output EOR is “HIGH“ (high impedance, 100K to

120µs are ignored. If the output voltage falls below the

nominal value for more than 120µs, output EOR goes

”LOW“. The reason for this to happen is usually a

slowly progressing drop of input voltage from the

discharging battery. Therefore the output voltage will

also start to drop slowly. With the EOR detector,

batteries can be used to the ultimate end of discharge,

with enough time left for a safe shutdown.

Block Diagrams

ZXSC410

ZXSC420

SEMICO NDUC TORS

ISSUE 2 - May 2003

4

+

_

+

_

R2

R3

R1

STDN

VCC

DRIVE

SENSE

Shutdown

Bandgap

Reference

Bias Generator

Comp 1

Comp 2

U1

Dynamic

Drive

Monostable

2µs

Fig. 1 ZXSC410

Fig. 1 ZXSC420