RT8238A
10
RT8238A-07 January 2014
www.richtek.com
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Copyright 2014 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.
Application Information
The RT8238A PWM controller provides high efficiency,
excellent transient response, and high DC output accuracy
needed for stepping down high voltage batteries to
generate low voltage CPU core, I/O, and chipset RAM
supplies in notebook computers. Richtek 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 single output application.
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. Referring to
the function block diagram, the synchronous UGATE driver
will be turned on at the beginning of each cycle. After the
internal one shot timer expires, the UGATE driver 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 (400ns
typ.).
On-Time Control
The on-time one-shot comparator has two inputs. One
input looks at the output voltage, while the other input
samples the input voltage and converts it to a current.
This input voltage proportional current is used to charge
an internal on-time capacitor. The on-time is the time
required for the voltage on this capacitor to charge from
zero volts to VOUT, thereby making the on-time of the
high side switch directly proportional to the output voltage
and inversely proportional to the input voltage. The
implementation results in a nearly constant switching
frequency without the need of a clock generator.
Mode Selection Operation
DEM (Diode Emulation Mode) and ASM (Audio Skipping
Mode) operati
on can be enabled by driving the tri-state
MODE pin to a logic high level. The RT8238A can switch
operation into DEM when the MODE pin is pulled up to
5V. If MODE is pulled to 2.5V, the controller will switch
operation into ASM. Finally, if the pin is pulled to GND,
the RT8238A will operate in CCM mode.
Diode Emulation Mode
In diode emulation mode, the RT8238A automatically
reduces switching frequency at light load conditions to
maintain high efficiency. This reduction of frequency is
achieved smoothly and without increasing VOUT ripple or
load regulation. As the output current decreases from heavy
load condition, the inductor current is also reduced, and
eventually comes to the point that its valley touches zero
current, which is the boundary between continuous
conduction and discontinuous conduction modes. By
emulating the behavior of diodes, the low side MOSFET
allows only partial of negative current when the inductor
freewheeling current reach negative. As the load current
is further decreased, it takes longer and longer to discharge
the output capacitor to the level than requires the next
“
ON
”
cycle. The on-time is kept the same as that in the
heavy load condition. In reverse, when the output current
increases from light load to heavy load, the switching
frequency increases to the preset value as the inductor
current reaches the continuous condition. The transition
load point to the light load operation can be calculated as
follows (Figure 1) :
where t
ON
is On-time.
(
)
IN
OUT
LOAD
ON
V
V
I
t
2L
−
≈
×
OUT
IN
ON
V
Frequency =
V
t
×
where R
TON
is the resistor connected from the input supply
(V
IN
) to the TON pin.
And then the switching frequency is :
TON
OUT
ON
IN
8.8p R
V
t
=
(V
0.5)
×
×
−
