RT8239A/B/C
20
DS8239A/B/C-06 October 2012www.richtek.com
©
Copyright 2012 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.
Output Over Voltage Protection (OVP)
The output voltage can be continuously monitored for over
voltage. If the output voltage exceeds 12% of its set voltage
threshold, the over voltage protection is triggered and the
LGATEx low side gate drivers are forced high. This
activates the low side MOSFET switch, which rapidly
discharges the output capacitor and pulls the input voltage
downward.
The RT8239A/B/C is latched once OVP is triggered and
can only be released by either toggling ENLDO, ENTRIPx
or cycling VIN. There is a 5μs delay built into the over
voltage protection circuit to prevent false transition.
Note that latching LGATEx high will cause the output
voltage to dip slightly negative due to previously stored
energy in the LC tank circuit. For loads that cannot tolerate
a negative voltage, place a power Schottky diode across
the output to act as a reverse polarity clamp.
If the over voltage condition is caused by a short in high
side switch, turning the low side MOSFET on 100% will
create an electrical short between the battery and GND,
hence blowing the fuse and disconnecting the battery from
the output.
Output Under Voltage Protection (UVP)
The output voltage can be continuously monitored for under
voltage. If the output is less than 58% of its set voltage
threshold, the under voltage protection will be triggered
and then both UGATEx and LGATEx gate drivers will be
forced low. The UVP is ignored for at least 5ms (typ.)
after a start up or a rising edge on ENTRIPx. Toggle
ENTRIPx or cycle VIN to reset the UVP fault latch and
restart the controller.
Thermal Protection
The RT8239A/B/C features thermal shutdown to prevent
damage from excessive heat dissipation. Thermal
shutdown occurs when the die temperature exceeds
150°C. All internal circuitry is inactive during thermal
shutdown. The RT8239A/B/C triggers thermal shutdown
if LDOx is not supplied from V
OUTx
, while input voltage on
VIN and drawing current from LDOx are too high.
Nevertheless, even if LDOx is supplied from V
OUTx
,
overloading LDOx can cause large power dissipation on
automatic switches, which may still result in thermal
shutdown.
Discharge Mode (Soft Discharge)
When ENTRIPx is low and a transition to standby or
shutdown mode occurs, or the output under voltage fault
latch is set, the output discharge mode will be triggered.
During discharge mode, an internal switch creates a path
for discharging the output capacitors' residual charge to
GND.
Shutdown Mode
SMPS1, SMPS2, LDO3 and LDO5 all have independent
enabling control. Drive ENLDO, ENTRIP1 and ENTRIP2
below the precise input falling edge trip level to place the
RT8239A/B/C in its low power shutdown state. The
RT8239A/B/C consumes only 20μA of input current while
in shutdown. When shutdown mode is activated, the
reference turns off. The accurate 0.95V falling edge
threshold on ENLDO can be used to detect a specific
analog voltage level and to shutdown the device. Once in
shutdown, the 1.6V rising edge threshold activates,
providing sufficient hysteresis for most applications.
Power Up Sequencing and On/Off Controls
(ENTRIPx, ENM)
ENTRIP1 and ENTRIP2 control SMPS power up
sequencing. When the RT8239A/B/C is applied in the
single channel mode, ENTRIPx disables the respective
output when ENTRIPx voltage rises above 4.5V.
Furthermore, when the RT8239A is applied in the dual
channel mode, the outputs are enabled when ENM voltage
rises above 2.3V.
