RT8205A Datasheet, PDF (19/28 Pages) Richtek Technology Corporation – High Efficiency, Main Power Supply Controllers for Notebook Computers

Application Information
The RT8205A/B/C is a dual, Mach Response^TM DRV^TM
dual ramp valley mode synchronous buck controller. The
controller is designed for low-voltage power supplies for
notebook computers. Richtek's Mach Response^TM
technology is specifically designed for providing 100ns
“instant-on” response to load steps while maintaining a
relatively constant operating frequency and inductor
operating point over a wide range of input voltages. The
topology circumvents the poor load-transient timing
problems of fixed-frequency current-mode PWMs while
avoiding the problems caused by widely varying switching
frequencies in conventional constant-on-time and constant-
off-time PWM schemes. The DRV^TM mode PWM
modulator is specifically designed to have better noise
immunity for such a dual output application. The RT8205A/
B/C includes 5V (VREG5) and 3.3V (VREG3) linear
regulators. VREG5 linear regulator can step down the
battery voltage to supply both internal circuitry and gate
drivers. The synchronous-switch gate drivers are directly
powered from VREG5. When VOUT1 voltage is above
4.66V, an automatic circuit will switch the power of the
device from VREG5 linear regulator from VOUT1.
PWM Operation
The Mach Response^TM DRV^TM mode controller relies on
the output filter capacitor's effective series resistance
(ESR) to act as a current-sense resistor, so the output
ripple voltage provides the PWM ramp signal. Refer to the
RT8205A/B/C's function block diagram, the synchronous
high-side MOSFET will be 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 voltage and
the output voltage to keep the frequency fairly constant
over the input voltage range. Another one-shot sets a
minimum off-time (300ns 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
The Mach Response^TM control architecture runs with
pseudo-constant frequency by feed-forwarding the input
Copyright ©2012 Richtek Technology Corporation. All rights reserved.
DS8205A/B/C-06 July 2012
RT8205A/B/C
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 the V_IN, and proportional to
the output voltage. There are two benefits of a constant
switching frequency. The first is the frequency can be
selected to avoid noise-sensitive regions such as the
455kHz IF band. The second is the inductor ripple-current
operating point remains relatively constant, resulting in
easy design methodology and predictable output voltage
ripple. The frequency for 3V SMPS is set at 1.25 times
higher than the frequency for 5V SMPS. This is done to
prevent audio-frequency “beating” between the two sides,
which switch asynchronously for each side. The
frequencies are set by TONSEL pin connection as Table1.
The on-time is given by :
On-Time = K x (V_OUT / V_IN)
where “K” is set by the TONSEL pin connection (Table
1). The on-time guaranteed in the Electrical Characteristics
tables are influenced by switching delays in the external
high-side power MOSFET. Two external factors that
influence switching-frequency accuracy are resistive drops
in the two conduction loops (including inductor and PC
board resistance) and the dead-time effect. These effects
are the largest contributors to the change of frequency
with changing load current. The dead-time effect increases
the effective on-time, reducing the switching frequency
as one or both dead times. It occurs only in PWM mode
(SKIPSEL= GND) when the inductor current reverses at
light or negative load currents. With reversed inductor
current, the inductor's EMF causes PHASEx to go high
earlier than normal, extending the on-time by a period
equal to the low-to-high dead time. For loads above the
critical conduction point, the actual switching frequency
is :
f = (V_OUT + V_DROP1) / (t_ON x (V_IN + V_DROP1 -V_DROP2) )
where V_DROP1 is the sum of the parasitic voltage drops in
the inductor discharge path, including synchronous
rectifier, inductor, and PC board resistances; V_DROP2 is
the sum of the resistances in the charging path; and t_ON
is the on-time calculated by the RT8205A/B/C.
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