RT5715
Output Under-Voltage Protection and Hiccup Mode
The RT5715 includes output under-voltage protection
(UVP) against over-load or short-circuited condition by
constantly monitoring the feedback voltage V
FB
. If V
FB
drops below the under-voltage protection trip threshold
(typically 66% of the internal feedback reference voltage),
the UV comparator will go high to turn off both the internal
high-side and low-side MOSFET switches. The RT5715
will enter output under-voltage protection with hiccup
mode. 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. Hiccup mode
allows the circuit to operate safely with low input current
and power dissipation, and then resume normal operation
as soon as the over-load or short-circuit condition is
removed.
The Over-Current Protection
The RT5715 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.
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
Copyright
©
2019 Richtek Technology Corporation. All rights reserved.
V
IN
= 5V
V
IN
100µs
150µs
90%V
OUT
V
OUT
10%V
OUT
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.
Soft-Start (SS)
The RT5715 provides an internal soft-start feature for inrush
control. At power up, the internal capacitor is charged by
an internal current source to generate a soft-start ramp
voltage as a reference voltage to the PWM comparator.
The device will initiate switching and the output voltage
will smoothly ramp up to its targeted regulation voltage
only after this ramp voltage is greater than the feedback
voltage V
FB
to ensure the converters have a smooth start-
up from pre-biased output. The output voltage starts to
rise in 100μs from EN rising, and the soft-start ramp-up
time (10%V
OUT
to 90%V
OUT
) is 150μs.
EN
Thermal Shutdown
The RT5715 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 (T
SD
). Once the junction temperature cools down
by a thermal shutdown hysteresis (ΔT
SD
), the IC will
resume normal operation with a complete soft-start.
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
device reliability or permanently damage the device.
is a registered trademark of Richtek Technology Corporation.
DS5715-02
November 2019
www.richtek.com
5
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
