Diodes ZXSC380 - Manual (Page 5)

ZXSC380
Issue 3 - September 2007 5 www.zetex.com
© Zetex Semiconductors plc 2007
Device description
The ZXSC380 is non-synchronous PFM, DC-DC controller IC which, with a high performance
internal transistor, for a high efficiency boost converter for use in single cell applications. A block
diagram is shown in Figure 1.
Figure 1 ZXSC380 Block diagram
The on chip comparator forces the driver circuit and therefore the internal switching transistor to
switch off when the voltage at I
SENSE
exceeds 20mV. This threshold is set by an internal reference
circuit and divider. The voltage at I
SENSE
is taken from a current sense resistor connected in series
with the emitter of the switching transistor. This resistor is chosen to give 20mV at I
SENSE
for an
emitter current of 80mA.
A monostable following the output of the comparator forces the turn-off time of the output stage
to be typically 2.2
␮s. This ensures that there is sufficient time to discharge a significant proportion
of the energy stored in the inductor coil before the next On period.
With every On pulse the switching transistor is kept on until the voltage across the current-sense
resistor exceeds the threshold of the I
SENSE
input. The On-pulse length, and therefore the
switching frequency, is determined by the programmed peak current, the input voltage and the
input to output voltage differential. See applications section for details.
The Driver circuit supplies the internal switching transistor with a fixed drive current. To maximize
efficiency the internal transistor is switched quickly, typically being switched off within 30ns.
R1
R2
R
SENSE
+
-
+
V
REF
GND
I
SENSE
R1
R2
R
SENSE
+
-
+
V
REF
GND
I
SENSE
V
CC
1
3
2
V
OUT