ZXSC100 Zetex Semiconductors plc., ZXSC100 Datasheet - Page 5

• Manufacturer Part Number: ZXSC100
• Description: Single Cell Dc-dc Converter Solution
• Manufacturer: Zetex Semiconductors plc.

ISSUE 3 - JANUARY 2004
DEVICE DESCRIPTION
The ZXSC100 is non-synchronous PFM, DC-DC
controller IC which, when combined with a high
performance external transistor, enables the
production of a high efficiency boost converter for use
in single cell applications. A block diagram is shown
for the ZXSC100 in Figure 1.
Figure 1
ZXSC100 Block Diagram
A shutdown circuit turns the device on or off at V
with a hysteresis of typically 80mV. At start up,
comparator Comp1 turns the driver circuit and
therefore the external switching transistor on. This
circuit will remain active until the feedback voltage at
the pin FB rises above V
external resistive divider on the FB pin sets the output
voltage level.
Comparator Comp2 forces the driver circuit and the
external switching transistor off, if the voltage at
I
from a current sense resistor connected in series with
the emitter of the switching transistor.
A monostable following the output of Comp2 extends
the turn-off time of the output stage by a minimum of
2us. This ensures that there is sufficient time to
discharge the inductor coil before the next on period.
The AND gate between the monostable and Comp1
output ensures that the switching transistor always
remains on until the I
that the minimum discharge period is always
asserted. The pulse width is constant, the pulse
frequency varies with the output load.
SENSE
V
REF
G
exceeds 25mV. The voltage at I
V
ND
CC
Shutdown
R1
R2
Comp1
Comp2
SENSE
REF
, which is set to 730mV. An
threshold is reached and
Drive
I
SENSE
is taken
CC
RE
EM
BAS
V
I
FB
SENSE
DRIVE
=1V
5
The driver circuit supplies the external switching
transistor with a defined
programmed by an external resistor connected
between the RE pin and V
voltage for the circuit is 25mV below V
efficiency the external transistor is switched quickly,
typically being forced off within 20ns.
In higher power applications more current can be
supplied to the switching transistor by using a further
external component. The driver transistor in the IC can
be bypassed with the addition of a discrete PNP. More
information on this circuit configuration can be found
in the applications section.
CC
. The internal reference
S E M I C O N D U C T O R S
current, which is
ZXSC100
CC
. To maximise