ZXSC400
DESIGN INFORMATION
IC operation description
Block diagram
Bandgap Reference
All threshold voltages and internal currents are
derived from a tem perature com pensated bandgap
reference circuit with a reference voltage of 1.22V
nom inal.
Dynam ic Drive Output
Depending on the input signal, the output is either
“LOW” or “HIGH”. In the high state a 2.5m A current
source (m ax drive voltage = VCC-0.4V) drives the base
or gate of the external transistor. In order to operate
th e e xte rn a l s w itch in g tra n s is to r a t o p tim u m
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.
2µs
Sw itching Circuit
The switching circuit consists of two com parators,
Com p1 and Com p2, a gate U1, a m onostable and the
drive output. Norm ally the DRIVE output is “HIGH”; the
external switching transistor is turned on. Current
ram ps up in the inductor, the switching transistor and
external current sensing resistor. This voltage is
sensed by com parator, Com p2, at input ISENSE. Once
the current sense voltage across the sensing resistor
exceeds 30m V, com parator Com p2 through gate U1
triggers a re-triggerable m onostable and turns off the
output drive stage for 2s. The inductor discharges to
the load of the application. After 2s a new charge
cycle begins, thus ram ping the output voltage. When
the output voltage reaches the nom inal value and FB
gets an input voltage of m ore than 300m V, the
m onostable is forced “on” from Com p1 through gate
U1, until the feedback voltage falls below 300m V. The
above action continues to m aintain regulation.
Pin functions
Pin No.
Na m e
De s crip t io n
1
2
3
V
S u p p ly vo lta g e , 1.8V to 8V.
Gro u n d
CC
GND
S TDN
S h u td o w n
In d u cto r cu rre n t s e n s e in p u t. In te rn a l th re s h o ld vo lta g e s e t to 30m V. Co n n e ct
e xte rn a l s e n s e re s is to r
4
5
6
S ENS E
Re fe re n ce vo lta g e . In te rn a l th re s h o ld s e t to 300m V. Co n n e ct e xte rn a l re s is to r
to p ro g ra m LED cu rre n t
V
FB
Drive o u tp u t fo r e xte rn a l s w itch in g tra n s is to r. Co n n e ct to b a s e o r g a te o f
e xte rn a l s w itch in g tra n s is to r.
DRIVE
ISSUE 1 - J ANUARY 2003
4
DIODES [ DIODES INCORPORATED ]
