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RT8249 Fiches technique(PDF) 15 Page - Richtek Technology Corporation

No de pièce RT8249
Description  Dual-Channel Synchronous DC/DC Step-Down Controller with 5V/3.3V LDOs
PDF  23 Pages
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Fabricant  RICHTEK [Richtek Technology Corporation]
Site Internet  http://www.richtek.com
Logo RICHTEK - Richtek Technology Corporation

RT8249 Fiches technique(HTML) 15 Page - Richtek Technology Corporation

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RT8249A/B/C
15
DS8249A/B/C-02
June 2014
www.richtek.com
©
Copyright
2014 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
Application Information
The RT8249A/B/C is a dual-channel, low quiescent, Mach
ResponseTM DRVTM mode synchronous Buck controller
targeted for Ultrabook system power supply solutions.
Richtek's Mach ResponseTM technology provides fast
response to load steps. The topology solves the poor load
transient response timing problems of fixed frequency
current mode PWMs, and avoids the problems caused
by widely varying switching frequencies in CCR (constant
current ripple) constant on-time and constant off-time
PWM schemes. A special adaptive on-time control trades
off the performance and efficiency over wide input voltage
range. The RT8249A/B/C includes 5V (LDO5) and 3.3V
(LDO3) linear regulators. The LDO5 linear regulator steps
down the battery voltage to supply both internal circuitry
and gate drivers. The synchronous switch gate drivers are
directly powered by LDO5. When VOUT1 rises above 4.66V,
an automatic circuit disconnects the linear regulator and
allows the device to be powered by VOUT1 via the BYP1
pin.
PWM Operation
The Mach ResponseTM DRVTM mode controller relies on
the output filter capacitor's Effective Series Resistance
(ESR) to act as a current sense resistor, so that the output
ripple voltage provides the PWM ramp signal. Referring to
the RT8249A/B/C's Function Block Diagram, the
synchronous high-side MOSFET is turned on at the
beginning of each cycle. After the internal one-shot timer
expires, the MOSFET will be turned off. The pulse width
of this one-shot is determined by the converter's input
output voltages to keep the frequency fairly constant over
the entire input voltage range. Another one-shot sets a
minimum off-time (200ns typ.). The on-time one-shot will
be triggered if the error comparator is high, the low-side
switch current is below the current limit threshold, and
the minimum off-time one-shot has timed out.
PWM Frequency and On-time Control
For each specific input voltage range, the Mach
Response
TM control architecture runs with pseudo constant
frequency by feed forwarding the input and output voltage
into the on-time one-shot timer. The high-side switch
on-time is inversely proportional to the input voltage as
measured by VIN and proportional to the output voltage.
The inductor ripple current operating point remains
relatively constant, resulting in easy design methodology
and predictable output voltage ripple. The frequency of 3V
output controller is set higher than the frequency of 5V
output controller. This is done to prevent audio frequency
“beating” between the two sides, which switch
asynchronously for each side.
The RT8249A/B/C adaptively changes the operation
frequency according to the input voltage. Higher input
voltage usually comes from an external adapter, so the
RT8249A/B/C operates with higher frequency to have
better performance. Lower input voltage usually comes
from a battery, so the RT8249A/B/C operates with lower
switching frequency for lower switching losses. For a
specific input voltage range, the switching cycle period is
given by :
RT8249A/B :
For 5V VOUT,
IN
IN
V2.025
Period (usec.) =
V3.79
where the VIN is in volt.
The on-time guaranteed in the Electrical Characteristics
table is influenced by switching delays in the external
high-side power MOSFET.
Operation Mode Selection
The RT8249C supports two operation modes : diode
emulation mode (DEM) and ultrasonic mode (ASM). The
operation mode can be set via the SKIPSEL pin. When
For 3.3V VOUT,
IN
IN
V1.83
Period (usec.) =
V2.59
For 3.3V VOUT,
RT8249C :
For 5V VOUT,
IN
IN
V1.62
Period (usec.) =
V3.79
IN
IN
V1.45
Period (usec.) =
V2.59



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