RT8208A/B
10
DS8208A/B-04 May 2011www.richtek.com
Application Information
The RT8208A/B is a constant-on-time PWM controller
which provides four resistor-programmable DC output
voltages by controlling the G0 and G1 digital input. The
output voltage is programmable from 0.75V to 3.3V. The
constant on-time PWM control scheme handles wide input/
output rations with ease and 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 the RT8208A/B, 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 (t
ON
)
The on-time one-shot comparator has two inputs. One
input monitors 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 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
= 9.6p x R
TON
x (V
OUT
+ 0.1) / (V
IN
− 0.3) + 50ns
Although this equation provides a good approximation to
start with, the accuracy depends on each design and
selection of the high side MOSFET.
And then the switching frequency (f) is:
R
TON
is a resistor connected from the PHASE to TON pin.
Mode Selection (EN) Operation
The EN pin enables the supply. When EN/DEM is tied to
VDD, the controller is enabled and operates in diode-
emulation mode. When the EN pin is floating, the
RT8208A/B will operate in forced-CCM mode.
Diode-Emulation Mode (EN = High)
In diode-emulation mode, the RT8208A/B 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) :
−
≈×
IN OUT
LOAD ON
(V V )
I t
2L
where t
ON
is On-time.
×
OUT
IN
ON
V
f =
Vt
