RT6263A/B
Once the negative current exceeds the NOC threshold,
the low-side MOSFET is turned off immediately, and
then the high-side MOSFET will be turned on to
discharge the energy of output inductor. This behavior
can keep the valley of negative current at NOC
threshold to protect low-side MOSFET. However, the
negative current can’t be limited at NOC threshold
anymore since minimum off-time is reached.
The Over-Current Protection
The RT6263A/B features cycle-by-cycle current-limit
protection on both the high-side and low-side
MOSFETs and prevents the device from the
catastrophic
damage
in
output
short-circuit,
over-current or inductor saturation conditions.
The high-side MOSFET over-current protection is
achieved by an internal current comparator that
monitors the current in the high-side MOSFET during
each on-time. The switch current is compared with the
high-side switch peak-current limit (ILIM_H) after a
certain amount of delay when the high-side switch
being turned on each cycle. If an over-current condition
occurs, the converter will immediately turns off the
high-side switch and turns on the low-side switch to
prevent the inductor current exceeding the high-side
current limit.
Thermal Shutdown
The RT6263A/B includes an over-temperature
protection (OTP) circuitry to prevent overheating due to
excessive power dissipation. The OTP will shut down
switching operation when junction temperature
exceeds a thermal shutdown threshold (TSD). Once the
junction temperature cools down by a thermal
shutdown hysteresis (TSD), the IC will resume normal
operation with a complete soft-start.
The low-side MOSFET over-current protection is
achieved by measuring the inductor current through the
synchronous rectifier (low-side switch) during the
low-side on-time. Once the current rises above the
low-side switch valley current limit (ILIM_L), the on-time
one-shot will be inhibited until the inductor current
ramps down to the current limit level (ILIM_L), that is,
another on-time can only be triggered when the
inductor current goes below the low-side current limit. If
the output load current exceeds the available inductor
current (clamped by the low-side current limit), the
output capacitor needs to supply the extra current such
that the output voltage will begin to drop. If it drops
below the output under-voltage protection trip threshold,
the IC will stop switching to avoid excessive heat.
Note that the over temperature protection is intended to
protect the device during momentary overload
conditions. The protection is activated outside of the
absolute maximum range of operation as a secondary
fail-safe and therefore should not be relied upon
operationally. Continuous operation above the
specified absolute maximum operating junction
temperature may impair the reliability of the device or
permanently damage the device.
Negative Over-Current Limit
The RT6263B is the part which is forced to PWM and
allows negative current operation.
In case of PWM operation, high negative current may
be generated as an external power source which is tied
to output terminal unexpectedly.
As the risk described above, the internal circuit
monitors negative current in each on-time interval of
low-side MOSFET and compares it with NOC
threshold.
Copyright © 2020 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS6263A/B-00 January 2020
www.richtek.com
7
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
