Zxsc310 – Diodes ZXSC310 User Manual

Page 5

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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

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