ZXSC300
DEVICE DESCRIPTION
The ZXSC300 is PFM, controller IC which, when
combinedwith a highperformanceexternal transistor,
enables the production of a high efficiency boost
converter for use in single cell LED driving
applications. A block diagram is shown for the
ZXSC300 in Figure 1.
With every on pulse the switching transistor is kept on
until the voltage across the current-sense resistor
exceeds the threshold of the ISENSE 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 external switching
transistor with a fixed drive current. To maximise
efficiency the external transistor switched quickly,
typically being forced off within 30ns.
Pinout Diagram
Figure 1 ZXSC300 Block Diagram
The on chip comparator forces the driver circuit and
therefore the external switching transistor off if the
voltage at ISENSE exceeds 19mV. This threshold is set
by an internal reference circuit and divider.
The Voltage at ISENSE is taken from a current sense
resistor connected in series with the emitter of the
switching transistor. A monostable following the
outputof the comparator forces the turn-offtime of the
output stage to be typically 1.7µs. This ensures that
there is sufficient time to discharge the inductor coil
before the next on period.
Pin Descriptions
Pin No.
Name
Description
1
2
3
4
V
Supply voltage, generally Alkaline, NiMH or NiCd single cell
CC
Gnd
N/C
Ground
Not connected
I
Inductor current sense input. Internal threshold voltage set to 19mV.
Connect external sense resistor
SENSE
5
V
Drive output for external switching transistor. Connect to base of external
switching transistor
DRIVE
ISSUE 3 - NOVEMBER 2006
5
ZETEX [ ZETEX SEMICONDUCTORS ]
