Diodes ZXCL SERIES - Manual (Page 8)

ZXCL SERIES
Issue 8 - October 2007 8 www.zetex.com
© Zetex Semiconductors plc 2007
Scheme to boost output current to 2A
Input to Output Diode
In common with many other LDO regulators, the
ZXCL device has an inherent diode associated
with the output series pass transistor. This diode
has its anode connected to the output and its
cathode to the input. The internal diode is
normally reverse biased, but will conduct if the
output is forced above the input by more than a
VBE (approximately 0.6V). Current will then flow
from V
out
to V
in
. For safe operation, the
maximum current in this diode should be
limited to 5mA continuous and 30mA peak. An
external schottky diode may be used to provide
protection when this condition cannot be
satisfied.
Increased Output current
Any ZXCL series device may be used in
conjunction with an external PNP transistor to
boost the output current capability. In the
application circuit shown below, a FMMT717
device is employed as the external pass
element. This SOT23 device can supply up to
2.5A maximum current subject to the thermal
dissipation limits of the package (625mW).
Alternative devices may be used to supply
higher levels of current. Note that with this
arrangement, the dropout voltage will be
increased by the V
BE
drop of the external device.
Also, care should be taken to protect the pass
transistor in the event of excessive output
current.