10 DS8241-03 January 2014www.richtek.com
RT8241
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Copyright 2014 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.
Application Information
TheRT8241isof aconstanton-timePWMcontroller which
providesfourDC feedbackvoltages by controlling theG0
and G1 digital input. The constant on-time PWM control
scheme handles wide input/output ratios with ease and
provides100ns “instant-on”responseto loadstepswhile
maintaininga relatively constantoperatingfrequencyand
inductoroperatingpointoverawiderangeofinputvoltages.
The topology circumvents the poor load transient timing
problems of fixed-frequency current mode PWMs, while
avoidingthe problemscausedby widely varyingswitching
frequenciesinconventionalconstanton-timeandconstant
off-time PWM schemes. The DRVTM mode PWM
modulator is specifically designed to have better noise
immunity for such a single output application.
PWM Operation
TheMachResponseTM,DRVTM modecontroller relies on
the output filter capacitor's Effective Series Resistance
(ESR) to act as a current sense resistor, so the output
ripplevoltageprovides the PWMramp signal.Referringto
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
zerovoltstoVOUT, thereby makingthe on-timeof the high
sideswitchdirectlyproportional 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
RT8241automaticallyreducesswitchingfrequencyatlight-
loadconditionstomaintainhighefficiency. This reduction
offrequencyisachievedsmoothly andwithout increasing
VOUT ripple or load regulation. As the output current
decreasesfromheavyloadcondition, theinductorcurrent
is also reduced, and eventually comes to the point that
its valley touches zero current, which is the boundary
between continuous conduction and discontinuous
conductionmodes. By emulating thebehavior of diodes,
thelow side MOSFET allows only partial negativecurrent
whentheinductor freewheelingcurrent becomesnegative.
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 increasesfromlight load
to heavy load, the switching frequency increases to the
presetvalueastheinductorcurrentreachesthecontinuous
condition. The transition load point to the light-load
operation can be calculated as follows (Figure 1) :
IN OUT
LOAD ON
(V V )
It
2L
−
≈×
where tON is the on-time.
Figure1. Boundary Condition of CCM/DCM
The switching waveforms may appear noisy and
asynchronouswhenlightloadingcausesdiode-emulation
operation, but this is a normal operating condition that
resultsinhighlight-loadefficiency.Trade-offsinDEMnoise
vs. light-load efficiency is made by varying the inductor
value.Generally, low inductor values produce a broader
efficiencyvs.loadcurve,whilehighervaluesresultinhigher
full-load efficiency (assuming that the coil resistance
remains fixed) and less output voltage ripple. The
disadvantages for using higher inductor values include
0
IL
t
IL_Peak
ILOAD = IL_Peak/2
tON
Slope = (VIN-VOUT) / L
