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RT6203B Datasheet(PDF) 4 Page - Richtek Technology Corporation

Part No. RT6203B
Description  5A, 18V, 700kHz ACOTTM Synchronous Step-Down Converter with VID Control
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Maker  RICHTEK [Richtek Technology Corporation]
Homepage  http://www.richtek.com
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 4 page
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RT6203B
4
DS6203B-01 July 2017
www.richtek.com
©
Copyright 2017 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
and turn on the synchronous rectifier. Weighing these small
signals in a switching environment is difficult to do just
after switching large currents, making those architectures
problematic at low duty cycles and in less than ideal board
layouts. Because no switching decisions are made during
noisy time periods, COT architectures are preferable in
low duty cycle and noisy applications. However, traditional
COT control schemes suffer from some disadvantages
that preclude their use in many cases. Many applications
require a known switching frequency range to avoid
interference with other sensitive circuitry. True constant
on-time control, where the on-time is actually fixed,
exhibits variable switching frequency. In a step-down
converter, the duty factor is proportional to the output
voltage and inversely proportional to the input voltage.
Therefore, if the on-time is fixed, the off-time (and therefore
the frequency) must change in response to changes in
input or output voltage. Modern pseudo-fixed frequency
COT architectures greatly improve COT by making the
one-shot on-time proportional to VOUT and inversely
proportional to VIN. In this way, an on-time is chosen as
approximately what it would be for an ideal fixed-frequency
PWM in similar input/output voltage conditions. The result
is a big improvement but the switching frequency still varies
considerably over line and load due to losses in the
switches and inductor and other parasitic effects. Another
problem with many COT architectures is their dependence
on adequate ESR in the output capacitor, making it difficult
to use highly-desirable, small, low-cost, but low-ESR
ceramic capacitors. Most COT architectures use AC
current information from the output capacitor, generated
by the inductor current passing through the ESR, to
function in a way like a current mode control system.
With ceramic capacitors the inductor current information
is too small to keep the control loop stable, like a current
mode system with no current information.
ACOTTM Control Architecture
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 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. 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. 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 ESR. The ramp signal and other
internal compensations are optimized for low-ESR ceramic
output capacitors.
ACOTTM One-Shot Operation
The RT6203B 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 and 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 (230ns typical) so that rapidly-repeated on-
times can raise the inductor current quickly when needed.




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