LTM4641
APPENDICES
Appendix C. Switching Frequency Considerations and
(3) Inrail-trackingapplications,LTM4641’soutputvoltage
Usage of R
must track a reference voltage not only during V
fSET
OUT
ramp up but also during V
ramp down; fulfilling
OUT
There exist many scenarios in which a resistor, R
,
fSET
the latter requires LTM4641 to sink current from the
output capacitors. A value of R should be used
should be connected externally to LTM4641’s f pin—to
SET
fSET
decrease the on-time of M : most commonly, when the
TOP
that assures the output voltage can be ramped down
to one’s minimum desired output voltage of regula-
tion—not just the intended nominal output voltage.
Figure 3 provides this guidance.
output voltage setting is less than or equal to 3V, and in
rail-tracking applications; and less commonly, when V
INL
and V are operating from different source supplies. In
INH
the former cases, R
is usually applied from f to V
fSET
SET INL
(Figure 45 and front page application circuit); in the latter,
is usually applied from f to the voltage source
feeding LTM4641’s power stage—upstream of MSP, if a
power-interrupt input MOSFET is used (Figure 49). There
are several motivations and considerations behind this
guidance:
(4) In order to maintain a relatively constant switching
frequency for a given output voltage (across the full
R
fSET
SET
line voltage), the on-time of M
should be inversely
TOP
proportional to the voltage source feeding the V
INH
power stage—upstream of MSP, if a power-interrupt
MOSFET is used (Figure 46). When V and V are
INL
INH
operated from different rails, this goal can be accom-
(1) Inherent to LTM4641’s constant on-time architec-
ture, the switching frequency of LTM4641 decreases
as output voltage decreases. In order to maintain a
reasonable output capacitor value solution size and
output voltage ripple—even at lower output voltages
plished satisfactorily by placing R between f
fSET
SET
and the power V input source (see Figure 49: the
IN
connectionistoV andnotV ,andusuallynotV ,
IN
INL
INH
but see a counterexample in Figure 47 and explana-
tion in item number 5 of this list). A minor error term
to the on-time is introduced by the internal 1.3MΩ
(≤3V )—R
shouldbeapplied,sothatthecontrol-
OUT
fSET
ler’sI pincurrentandtheresultingnominalswitching
ON
V
-to-f -connected resistor in such scenarios, so
INL
SET
frequency is higher than the on-time dictated by the
calculation of I at all operating input voltage corner
ION
internal V -to-f -connected 1.3MΩ resistor.
INL
SET
cases (power, V and control bias, V extremes)
INH
INL
(2) ThePFMcontrolschemeemployedbyLTM4641yields
a switching frequency at zero load current (“no-load
operation”) that is typically 20% to 25% lower than
what it is at full load. As a result, inductor ripple cur-
rent is proportionally higher at no load than what it is
and the resulting switching frequency range of opera-
tion, given by Equation 6, should be considered.
(5) When MSP is used, and when V
and V
are
INL
INH
operated from different rails—here is the reason it
is recommended to connect R from f to the
fSET
SET
at heavy load. Recall that LTM4641 employs R
DS(ON)
drain of MSP rather than V : prior to start-up, MSP
INH
currentsensing;furthermore,realizethatitisessential
forthecontroller’scurrent-senseamplifiertobeableto
perceive and command sufficiently negative inductor
troughcurrent,enoughtomaintainamaximumaverage
inductor current of 0A, so that output voltage can be
is off, and V
INH
is discharged. Connecting R
to
INH
fSET
V
would set the on-time at the instant switching
activity commenced to be much lower than intended.
The on-time would not reach its final settling value
until V
INH
circuitry had turned on MSP enough for
ING
properly regulated down to no load. A value of R
fSET
V
to become pulled up to V potential. It should
IN
should be used to assure that switching frequency is
high enough (or on-time is small enough) at no load
sothatthecurrent-senseinformationrepresentingthe
trough of choke current is never too large in ampli-
tude. Figure 3 provides conservative guidance on the
become apparent that a mechanism may exist for
dynamic interaction between how rapidly the output
voltagerampsup(dependingonTRACK/SSpinusage)
versushowrapidlyMSPmightturnon. Weknowfrom
item number 2 of this list that on-time should not be
arbitrarily large. In general, to avoid any undesirable
maximumvalueofR
ON
(orequivalently,theminimum
fSET
I
current) that assures proper no-load operation.
4641f
57
Linear [ Linear ]
