RT5779A/B
supply current can be reduced to I
SHDN
(1μA or below). If
the EN voltage
rises above the logic-high threshold voltage
(V
ENH
) while the V
IN
voltage is higher than UVLO threshold
(V
UVLO
), the device will be turned on, that is, switching
being enabled and soft-start sequence being initiated.
Input Under-Voltage Lockout
In addition to the EN pin, the RT5779A/B also provides
enable control through the VIN pin. It features an under-
voltage lockout (UVLO) function that monitors the internal
linear regulator (VCC). If V
EN
rises above V
ENH
first,
switching will still be inhibited until the VIN voltage rises
above V
UVLO
. It is to ensure that the internal regulator is
ready so that operation with not-fully-enhanced internal
MOSFET switches can be prevented. After the device is
powered up, if the input voltage V
IN
goes below the UVLO
falling threshold voltage (V
UVLO
− ΔV
UVLO
), this switching
will be inhibited; if V
IN
rises above the UVLO-rising
threshold (V
UVLO
), the device will resume switching.
Over-Current Protection
The RT5779A/B is protected from over current conditions
by cycle-by-cycle current limiting on both the high-side
MOSFET and the low-side MOSFET. The robust over
current protection mechanism prevents the converter to
be damaged from a catastrophic condition, i.e. the inductor
is shorted or saturated.
High-Side MOSFET Over-Current Protection
The device senses high-side current after a deglitch time
when the high-side MOSFET is turned-on. Each cycle
when the sensed current is higher than the high-side switch
peak current limit threshold, I
LIM_H
, then the device enters
high side over current protection. At the same time, the
high-side MOSFET is turned off and the low-side MOSFET
is turned on.
Low-Side MOSFET Over-Current Protection
The RT5779A/B detects low side current after a minimum-
off time while the low-side MOSFET is turned on. If the
detected low-side current is higher than the low-side switch
valley current limit threshold, I
LIM_L
, the on-time one-shot
will be inhibited until the inductor current ramps down to
the current limit level (I
LIM
). That is, another on-time can
only be triggered when the inductor current goes below
Copyright
©
2017 Richtek Technology Corporation. All rights reserved.
the low-side current limit.
If the output load current exceeds the available inductor
current (clamped by the above-mentioned 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.
Output Under-Voltage Protection
The RT5779A/B 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 50% of the internal reference voltage), the UV
comparator will go high to turn off both the internal high-
side and low-side MOSFET switches.
Hiccup Mode
If the output under-voltage condition continues for a period
of time, the RT5779A/B will enter output under-voltage
protection with hiccup mode. During hiccup mode, the
device remains shut down. After a period of time, a soft-
start sequence for auto-recovery will be initiated. 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.
Soft-Start (SS)
The soft-start function is used to prevent large inrush
currents while the converter is being powered up. The
RT5779A/B provides an internal soft-start feature for inrush
control. During the start-up sequence, the internal capacitor
is charged by an internal current source I
SS
to generate a
soft-start ramp voltage as a reference voltage to an error
amplifier. 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. The typical soft-start time is 1.5ms.
is a registered trademark of Richtek Technology Corporation.
DS5779A/B-00 October 2017
www.richtek.com
5
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
