RT6214A/B
Operation
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.
The RT6214A/B is a high-efficiency, synchronous
step-down DC-DC converter that can deliver up to 3A
output current from a 4.5V to 18V input supply. The
RT6214A/B adopts ACOTTM control mode, which can
reduce the output capacitance and provide ultrafast
transient responses, and allow minimal component
sizes without any additional external compensation
network.
High-Side Current Limit Protection
The RT6214A/B also includes
a
cycle-by-cycle
peak-type current limit protection against the condition
that the inductor current increasing abnormally, even
over the inductor saturation current rating. To ensure
the low-side current limit protection can still function
properly, the inductor current through the high-side
switch will only be measured after a certain amount of
delay when the high-side switch being turned on. If an
over-current condition occurs, the converter will
immediately turn off the high-side switch and turn on
the low-side switch to prevent the inductor current
exceeding the high-side switch peak-current limit.
Enable Control
The RT6214A/B provides an EN pin, as an external
chip enable control, to enable or disable the device. If
VEN is held below a logic-low threshold voltage
(VENHVEN) of the enable input (EN), the converter
will enter into shutdown mode, that is, the converter is
disabled and switching is inhibited even if the VIN
voltage is above VIN under-voltage lockout threshold
(VUVLO). During shutdown mode, the supply current
can be reduced to ISHDN (5A or below). If the EN
voltage rises above the logic-high threshold voltage
(VENH) while the VIN voltage is higher than UVLO
threshold (VUVLO), the device will be turned on, that is,
switching being enabled and soft-start sequence being
initiated.
Output Under-Voltage Protection and Hiccup Mode
The RT6214A/B includes output under-voltage
protection (UVP) against over-load or short-circuited
condition by constantly monitoring the feedback
voltage VFB. If VFB drops below the under-voltage
protection trip threshold (typically 50% 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.
Low-Side Current Limit Protection
The RT6214A/B features a cycle-by-cycle valley-type
current limit protection, measuring the inductor current
through the synchronous rectifier (low-side switch). The
inductor current level is determined by measuring the
low-side switch voltage between the LX pin and GND,
which is proportional to the switch current, during the
low-side on-time. For greater accuracy, temperature
compensation is added to the voltage sensing. Once
the current rises above the low-side switch valley
current limit (ILIM), the on-time one-shot will be inhibited
until the inductor current ramps down to the current
limit level (ILIM), that is, another on-time can only be
triggered when the inductor current goes below the
low-side current limit. This function can prevent the
average output current from greatly exceeding the
guaranteed low-side current limit value.
If the output under-voltage condition continues for a
period of time, the RT6214A/B will enter output
under-voltage protection with hiccup mode. During
hiccup mode, the IC will shut down for tHICCUP_OFF
(4.8ms), and then attempt to recover automatically for
tHICCUP_ON (1.6ms). 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.
If the output load current exceeds the available inductor
current (clamped by the above-mentioned low-side
Copyright © 2018 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS6214A/B-09 August 2018
www.richtek.com
5
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