LTM4641
APPLICATIONS INFORMATION—POWER SUPPLY FEATURES
to implement a custom UVLO falling setting above the
dropout curve in Figure 4 (see also Figure 11).
scheme. During a load transient step-up, the control
loop will command a higher inductor trough current to
compensateforadeficiencyinoutputvoltage;theeffective
switching frequency will increase until the output voltage
returnstonormal(anovercurrentevent,notwithstanding).
During a load transient step-down, the control loop will
command a lower inductor trough current to compensate
for an excess of output voltage; the effective switching
frequency will decrease until the output voltage returns to
normal.Thecontrolloopperceivesinductorcurrent-sense
information via the voltage signal that appears across the
LT3010-5 is shown in Figure 47 to provide bias for V
,
INL
to enable ride-through of 80V transients on V . UVLO
IN
detection of V is realized in this example by D2 creating
IN
a discharge path for V in the event of loss of V .
INL
IN
V
and V have no specific power-down sequencing
INL
INH
requirement, only that V should stay above 3.5V when-
INL
ever V is above 3.5V.
INH
V
and V sequencing is inherently addressed by the
INH
INL
synchronous power MOSFET, M , when M
is on
BOT
BOT
LTM4641 in the Figure 45 and Figure 46 circuits.
(this is commonly referred to in the industry as R
DS(ON)
The V and V start-up and shutdown waveforms of
IN
INL
current sensing).
the Figure 47 circuit—but with 1Ω output load and TMR
The on-time of the one-shot timer—and hence the power
tied to INTV —are shown in Figure 2. The effect of the
CC
control MOSFET, M ,—is given, in units of seconds, by:
TOP
timingcapacitor,C ,thatnormallygeneratesapower-on
TMR
reset (POR) delay at start-up is negated by tying TMR to
0.7V •10pF
tON
=
(1)
INTV . The ~3ms V -to-V start-up delay time seen in
CC
IN
OUT
IION
Figure 2 is due to POR of the LTM4641’s fault-monitoring
circuitry and soft-start ramp (C ).
SS
where I
is in units of amperes. For output voltages
ION
greater than 3V, and for non-rail-tracking applications,
no external R resistor is needed, and the I current
fSET
ION
V
IN
(units: amperes) is set solely by the V voltage (units:
5V/DIV
INL
volts) and the internal 1.3MΩ V -to-f resistor:
INL
SET
V
INL
5V/DIV
V
INL
I
=
(2)
ION
V
1.3MΩ
OUT
500mV/DIV
The switching frequency of operation of the LTM4641’s
buck converter power stage at full load in this scenario
is given, in Hz, by:
4641 F02
2ms/DIV
Figure 2. Start-Up and Shutdown Waveforꢃs of Figure
47 Circuit. TꢁR Tied to INTVCC to Highlight VIN and VINL
Sequencing without POR Delay. 1Ω Load
VOUT
fSW
=
(3)
0.7V •1.3MΩ•10pF
is the desired nominal output voltage, in units
OUT
Switching Frequency (On Tiꢃe) Selection and Voltage
Dropout Criteria (Achievable V -to-V
Step-Down
where V
of volts.
IN
OUT
Ratios)
TheLTM4641controlleremploysacurrentmodeconstant
on-time architecture, in which the COMP voltage corre-
spondstothetroughinductorcurrentatwhichtheinternal
AnexternalR
resistorcanbeappliedwhensettingV
fSET OUT
greater than 3V, if desired, to obtain increased switching
frequency.Usually,increasingswitchingfrequencycomes
fromadesiretoreduceoutputvoltagerippleand/oroutput
capacitance requirement—but at a moderate penalty to
DC/DC conversion efficiency. There are some limitations
high side power MOSFET (M ) is commanded on by
TOP
the control loop—for a duration of time proportional to
controller’s I pin current (Refer to Figure 1). Regulation
ON
is maintained by a pulsed frequency modulation (PFM)
to how low an R
value can be applied in practice due
fSET
4641f
18
Linear [ Linear ]
