RT6545A/B
Application Information
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 RT6545A/B 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. Aspecial adaptive on-time control trades
off the performance and efficiency over wide input voltage
range. The RT6545A/B 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.
The RT6545A/B adaptively changes the operation
frequency according to the input voltage. Higher input
voltage usually comes from an external adapter, so the
RT6545A/B operates with higher frequency to have better
performance. Lower input voltage usually comes from a
battery, so the RT6545A/B operates with lower switching
frequency for lower switching losses. For a specific input
voltage range, the switching cycle period is given by :
For 5V VOUT,
V
IN 2.025
IN 3.79
Period (usec.) =
V
PWM Operation
For 3.3V VOUT,
Period (usec.) =
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 RT6545A/B'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.
V
IN 1.83
VIN 2.59
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.
Diode Emulation Mode
In diode emulation mode, the RT6545A/B 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 condition, 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
PWM Frequency and On-time Control
For each specific input voltage range, the Mach
ResponseTM 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
Copyright 2016 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
DS6545A/B-00 May 2016
www.richtek.com
13
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