Diodes ZXSC310 - Manual (Page 5)

DEVICE DESCRIPTION
The ZXSC310 is PFM, controller IC which, when
combined with a high performance external transistor,
enables the production of a high efficiency boost
converter for LED driving applications. A block diagram
is shown for the ZXSC310 below.
The on chip comparator forces the driver circuit and
therefore the external switching transistor off if the
voltage at I
SENSE exceeds 19mV. An internal reference
circuit and divider set this threshold.
The voltage at I
SENSE
is taken from a current sense
resistor connected in series with the emitter of the
switching transistor. A monostable following the
output of the comparator forces the turn-off time of the
output stage to be typically 1.7us. This ensures that
there is sufficient time to discharge 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 external switching
transistor with a fixed drive current. To maximise
efficiency the external transistor switched quickly,
typically being forced off within 30ns.
ZXSC310
ISSUE 3 - SEPTEMBER 2007
5 S E M I C O N D U C T O R S
I
2
Drive
I
SENSE
R2
V
RE
R1
V
DRIVE
G
ND
I
V
CC
S
TDN
ZXSC310 Block Diagram