RT6218A/B
Operation
The RT6218A/B is a high-efficiency, synchronous
step-down DC-DC converter that can deliver up to 2A
output current from a 4.5V to 18V input supply. The
RT6218A/B adopts ACOT
TM
control mode, which can
reduce the output capacitance and provide ultrafast
transient responses, and allow minimal component
sizes without any additional external compensation
network.
Enable Control
The RT6218A/B provides an EN pin, as an external
chip enable control, to enable or disable the device. If
V
EN
is held below a logic-low threshold voltage
(V
ENH
V
EN
) 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
(V
UVLO
). During shutdown mode, the supply current
can be reduced to I
SHDN
(5A or below). If the EN
voltage rises above the logic-high threshold voltage
(V
ENH
) while the VIN 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.
Low-Side Current Limit Protection
The RT6218A/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 (I
LIM
), 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 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 load current exceeds the available inductor
current (clamped by the above-mentioned low-side
Copyright © 2018 Richtek Technology Corporation. All rights reserved.
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.
High-Side Current Limit Protection
The RT6218A/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.
Output Under-Voltage Protection and Hiccup Mode
The RT6218A/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 62.5% 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.
If the output under-voltage condition continues for a
period of time, the RT6218A/B will enter output
under-voltage protection with hiccup mode. During
hiccup mode, the IC will shut down for t
HICCUP_OFF
(C
SS
x 6.3 / I
SS
), and then attempt to recover
automatically for t
HICCUP_ON
(C
SS
x 0.85 / I
SS
). 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.
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
4
DS6218A/B-01
October
2018
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
