RT6220
Application Information
the actual output voltage, potentially saving one pin
connection. The ACOTTM uses this method, measuring
the actual switching frequency and modifying the on-time
with a feedback loop to keep the average switching
frequency in the desired range.
The RT6220 is high-performance 500kHz 6A step-down
regulators with internal power switches and synchronous
rectifiers. It features an Advanced Constant On-Time
(ACOTTM) control architecture that provides stable
operation for ceramic output capacitors without
complicated external compensation, among other benefits.
The input voltage range is from 4.5V to 23V, and the output
voltage is adjustable from 0.6V to 5V.
ACOTTM One-shot Operation
The RT6220 control algorithm is simple to understand.
The feedback voltage, with the virtual inductor current ramp
added, is compared to the reference voltage. When the
combined signal is less than the reference, the on-time
one-shot is triggered, as long as the minimum off-time
one-shot is clear and the measured inductor current
(through the synchronous rectifier) is below the current
limit. The on-time one-shot turns on the high-side switch
and the inductor current ramps up linearly. After the on-
time, the high-side switch is turned off and the synchronous
rectifier is turned on and the inductor current ramps down
linearly. At the same time, the minimum off-time one-shot
is triggered to prevent another immediate on-time during
the noisy switching time and allow the feedback voltage
and current sense signals to settle. The minimum off-time
is kept short (200ns typical) so that rapidly-repeated on-
times can raise the inductor current quickly when needed.
The proprietary ACOTTM control scheme improves
conventional constant on-time architectures, achieving
nearly constant switching frequency over line, load, and
output voltage ranges. Since there is no internal clock,
response to transients is nearly instantaneous and inductor
current can ramp quickly to maintain output regulation
without large bulk output capacitance.
ACOTTM Control Architecture
In order to achieve good stability with low-ESR ceramic
capacitors, ACOTTM uses a virtual inductor current ramp
generated inside the IC. This internal ramp signal replaces
the ESR ramp normally provided by the output capacitor's
ESR. The ramp signal and other internal compensations
are optimized for low-ESR ceramic output capacitors.
Making the on-time proportional to VOUT and inversely
proportional to VIN is not sufficient to achieve good
constant-frequency behavior for several reasons. First,
voltage drops across the MOSFET switches and inductor
cause the effective input voltage to be less than the
measured input voltage and the effective output voltage to
be greater than the measured output voltage as sensing
input and output voltage. When the load changes, the
switch voltage drops change causing a switching
frequency variation with load current. Also, at light loads
if the inductor current goes negative, the switch dead-
time between the synchronous rectifier turn-off and the
high-side switch turn-on allows the switching node to rise
to the input voltage. This increases the effective on-time
and causes the switching frequency to drop noticeably.
Diode Emulation Mode (DEM)
In diode emulation mode, the RT6220 automatically
reduces switching frequency at light load conditions to
maintain high efficiency. This reduction of frequency is
achieved smoothly. As the output current decreases from
heavy load conditions, the inductor current is also reduced,
and eventually comes to the point that its current valley
touches zero, which is the boundary between continuous
conduction and discontinuous conduction modes. To
emulate the behavior of diodes, the low-side MOSFET
allows only partial negative current to flow when the
inductor free wheeling current becomes negative. As the
load current is further decreased, it takes longer and longer
time to discharge the output capacitor to the level that
requires the next “ON” cycle. In reverse, when the output
current increases from light load to heavy load, the
switching frequency increases to the preset value as the
One way to reduce these effects is to measure the actual
switching frequency and compare it to the desired range.
This has the added benefit eliminating the need to sense
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12
DS6220-10 December 2019
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
