RT8204B
12
DS8204B-04 April 2011www.richtek.com
Application Information
The RT8204B PWM controller provides the 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'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 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. Refer to the
function diagrams of RT8204B, the synchronous high side
MOSFET is turned on at the beginning of each cycle.
After the internal one-shot timer expires, the MOSFET is
turned off. The pulse width of this one shot is determined
by the converter's input and output voltages to keep the
frequency fairly constant over the input voltage range.
Another one-shot sets a minimum off-time (400ns typ.).
On-Time Control (TON)
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 V
OUT
, thereby making the on-time of the high
side switch directly proportional to output voltage and
inversely proportional to input voltage. The implementation
results in a nearly constant switching frequency without
the need of a clock generator.
T
ON
= 3.85p x R
TON
x V
OUT
/ (V
IN
− 0.5)
And then the switching frequency is :
Frequency = V
OUT
/ (V
IN
x T
ON
)
R
TON
is a resistor connected from the input supply (V
IN
)
to the TON pin.
Mode Selection (EN/DEM) Operation
The EN/DEM pin enables the supply. When EN/DEM is
tied to VDD, the controller is enabled and operates in
diode-emulation mode. When the EN/DEM pin is floating,
the RT8204B will operate in forced-CCM mode.
Diode-Emulation Mode (EN/DEM = High)
In diode-emulation mode, RT8204B automatically reduces
switching frequency at light-load conditions to maintain
high efficiency. This reduction of frequency is achieved
smoothly without increasing the V
OUT
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) :
IN OUT
LOAD ON
(V V )
I T
2L
−
≈×
I
L
t
0
t
ON
Slope = (V
IN
-V
OUT
) / L
i
L, peak
i
Load
= i
L, peak
/ 2
Figure 1. Boundary Condition of CCM/DEM
