RT6253A/B
not-fully-enhanced internal MOSFET switches can be
prevented. After the device is powered up, if the input
voltage VIN goes below the UVLO falling threshold
voltage (V
UVLO
V
UVLO
), this switching will be
inhibited; if VIN rises above the UVLO rising threshold
(V
UVLO
), the device will resume normal operation with a
complete soft-start.
Output Under-Voltage Protection and Hiccup Mode
The RT6253A/B includes output
protection (UVP) against over-load or
condition by constantly monitoring
voltage V
FB
. If V
FB
drops below the
protection trip threshold (typically 65%
under-voltage
short-circuited
the feedback
under-voltage
of the internal
The Over-Current Protection
The RT6253A/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 (I
LIM_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.
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 (I
LIM_L
), the on-time
one-shot will be inhibited until the inductor current
ramps down to the current limit level (I
LIM_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.
Negative Over-Current Limit
The RT6253B is the part which is forced to PWM and
allows negative current operation.
I
SW
, 4A/Div
V
SW
, 10/Div
feedback reference voltage), the UV comparator will go
high to turn off both the internal high-side and low-side
MOSFET switches.
If the output under-voltage condition continues for a
period of time, the RT6253A/B will enter output
under-voltage protection with hiccup mode. During
hiccup mode, the IC will shut down for t
HICCUP_OFF
(15ms), and then attempt to recover automatically for
t
HICCUP_ON
(1.8ms). Upon completion of the soft-start
sequence, if the fault condition is removed, the
converter will resume normal operation; otherwise,
such cycle for auto-recovery will be repeated until the
fault condition is cleared. The hiccup mode allows the
circuit to operate safely with low input current and
power dissipation, and then the converter resumes
normal operation as soon as the over-load or
short-circuit condition is removed.
Output short
V
OUT
, 2V/Div
Fault condition removed
Resume normal operation
10ms/Div
In case of PWM operation, high negative current may
be generated as an external power source 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.
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
Copyright © 2019 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS6253A/B-01
September
2019
www.richtek.com
5
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
