RT8239A/B/C
16
DS8239A/B/C-06 October 2012
www.richtek.com
©
Copyright 2012 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.
Application Information
The RT8239A/B/C is a dual,
Mach Response
TM
DRV
TM
mode synchronous buck controller targeted for notebook
system power supply solutions. RICHTEK's Mach
Response
TM
technology provides fast response to load
steps. The topology circumvents the poor load transient
timing problems of fixed frequency current mode PWMs
while avoiding the problems caused by widely varying
switching frequency in conventional 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 RT8239A/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 V
OUT1
rises above 4.66V, an automatic circuit disconnects the
linear regulator and allows the device to be powered by
V
OUT1
via the BYP1 pin.
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 that the output
ripple voltage provides the PWM ramp signal. Referring to
the RT8239A/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 entire input voltage range. Another one-
shot sets a minimum off-time (400ns 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 V
IN
and proportional to the output voltage.
There are two benefits of a constant switching frequency.
First, the frequency can be selected to avoid noise
sensitive regions such as the 455kHz IF band. Second,
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 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 TON pin is connected to GND through the external
resistor, R
TON
, to set the switching frequency.
The RT8239A/B/C adaptively changes the operation
frequency according to the input voltage. Higher input
voltage usually comes from an external adapter, so the
RT8239A/B/C operates with higher frequency to have
better performance. Lower input voltage usually comes
from a battery, so the RT8239A/B/C operates with lower
switching frequency for lower switching losses. For a
specific input voltage range, the switching cycle period is
given by :
For 5.5V < V
IN
< 6.5V :
t
S1
= 61.28p x R
TON
t
S2
= 44.43p x R
TON
For 6.5V < V
IN
< 12V :
t
S1
= 51.85p x R
TON
t
S2
= 44.43p x R
TON
For 12V < V
IN
< 25V :
t
S1
= 45.75p x R
TON
t
S2
= 39.2p x R
TON
The on-time guaranteed in the Electrical Characteristics
table is 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 by reducing the switching frequency
