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DS8241-03 January 2014
www.richtek.com
RT8241
©
Copyright 2014 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.
Application Information
The RT8241 is of a constant on-time PWM controller which
provides four DC feedback voltages by controlling the G0
and G1 digital input. The constant on-time PWM control
scheme handles wide input/output ratios with ease and
provides 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 diagrams of the RT8241, the synchronous
high side MOSFET is turned on at the beginning of each
cycle. After the internal one-shot timer expires, the high
side 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 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 V
OUT
, 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.
Diode-Emulation Mode
RT8241 automatically reduces switching frequency at light-
load conditions to maintain high efficiency. This reduction
of frequency is achieved smoothly and without increasing
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 negative current
when the inductor freewheeling current becomes negative.
As the load current is further decreased, it takes longer
and longer to discharge the output capacitor to the level
that is required for 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 the on-time.
Figure 1. Boundary Condition of CCM/DCM
The switching waveforms may appear noisy and
asynchronous when light loading causes diode-emulation
operation, but this is a normal operating condition that
results in high light-load efficiency. Trade-offs in DEM noise
vs. light-load efficiency is made by varying the inductor
value. Generally, low inductor values produce a broader
efficiency vs. load curve, while higher values result in higher
full-load efficiency (assuming that the coil resistance
remains fixed) and less output voltage ripple. The
disadvantages for using higher inductor values include
0
I
L
t
I
L_Peak
I
LOAD
= I
L_Peak
/2
t
ON
Slope = (V
IN
-V
OUT
) / L
