RT6248B
Application Information
ACOTTM One-Shot Operation
The RT6248B are high-performance 8A step-down
regulators with internal power switches and synchronous
rectifiers. They feature 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 18V. The output
voltage is adjustable from 0.6V to 6V.
The RT6248B 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 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.
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 (400ns typical) so that rapidly-repeated on-
times can raise the inductor current quickly when needed.
ACOTTM Control Architecture
The conventional CFCOT (constant frequency constant
on-time) control which making the on-time proportional to
VOUT and inversely proportional to VINis not sufficient to
achieve good constant-frequency behavior. Because
voltage drops across the MOSFET switches and inductor
cause sensing mismatch as sensing input and output
voltage from LX pin. When the load change, the voltage
drops across the MOSFET switches and inductor cause
a switching frequency variation with load current. 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 the actual
output voltage, potentially saving one pin connection.
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.
Linear Regulators (VCC)
The RT6248B also includes a 5V linear regulator (VCC).
The VCC regulator steps down input voltage to supply
both internal circuitry and gate drivers. Do not connect
the VCC pin to external loads.
Bypass Function for VCC
When PGOODis pulled high and BYP pin of the RT6248B
voltage is above 4.7V, an internal 3Ω P-MOSFET switch
connects VCC to the BYP pin while the VCC linear
regulator is simultaneously turned off. Because the VCC
is power source for internal logic device and gate driver. If
bypass function was enabled, it's recommended to put a
RC filter to BYP pin to enhance power quality for IC internal
power. But when BYP pin is not used, it should be
connected to ground.
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.
Current Limit
The RT6248B current limit is adjustable (8A,10A,12A) by
ILMT pin and it is a cycle-by-cycle “valley” type,
measuring the inductor current through the synchronous
rectifier during the off-time while the inductor current ramps
down. The current is determined by measuring the voltage
Copyright 2018 Richtek Technology Corporation. All rights reserved.
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is a registered trademark of Richtek Technology Corporation.
www.richtek.com
10
DS6248B-01 May 2018
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
