LTM4616
APPLICATIONS INFORMATION
peak current value in normal operation even though the
voltage is in control of the current comparator threshold
throughout,andthetopMOSFETalwaysturnsonwitheach
oscillatorpulse.Duringstart-up,forcedcontinuousmodeis
disabled and inductor current is prevented from reversing
until the LTM4616’s output voltage is in regulation. Each
regulator can be configured for forced continuous mode.
voltage at the I pin indicates a lower value. The voltage
TH
at the I pin drops when the inductor’s average current
TH
is greater than the load requirement. As the I voltage
TH
drops below 0.2V, the BURST comparator trips, causing
the internal sleep line to go high and turn off both power
MOSFETs.
Multiphase Operation
In Burst Mode operation, the internal circuitry is partially
turned off, reducing the quiescent current to about 450µA
for each output. The load current is now being supplied
fromtheoutputcapacitors.Whentheoutputvoltagedrops,
For output loads that demand more than 8A of current,
two outputs in LTM4616 or even multiple LTM4616s can
be cascaded to run out-of-phase to provide more output
currentwithoutincreasinginputandoutputvoltageripple.
The CLKIN pin allows the LTC®4616 to synchronize to an
external clock (between 0.75MHz and 2.25MHz) and the
internal phase-locked loop allows the LTM4616 to lock
onto CLKIN’s phase as well. The CLKOUT signal can be
connected to the CLKIN pin of the following LTM4616
stage to line up both the frequency and the phase of the
causingI toriseabove0.25V,theinternalsleeplinegoes
TH
low,andtheLTM4616resumesnormaloperation.Thenext
oscillator cycle will turn on the top power MOSFET and the
switching cycle repeats. Each regulator can be configured
for Burst Mode operation.
Pulse-Skipping Mode Operation
entire system. Tying the PHMODE pin to SV , SGND or
IN
Inapplicationswherelowoutputrippleandhighefficiency
atintermediatecurrentsaredesired, pulse-skippingmode
should be used. Pulse-skipping operation allows the
LTM4616toskipcyclesatlowoutputloads,thusincreasing
efficiency by reducing switching loss. Floating the MODE
SV /2 (floating) generates a phase difference (between
IN
CLKIN and CLKOUT) of 180°, 120° or 90° respectively,
which corresponds to a 2-phase, 3-phase or 4-phase
operation. For a 6-phase example in Figure 2, the 2nd
stage that is 120° out-of-phase from the 1st stage can
generate a 240° (PHMODE = 0) CLKOUT signal for the 3rd
stage, which then can generate a CLKOUT signal that’s
pin or tying it to V /2 enables pulse-skipping operation.
IN
Thisallowsdiscontinuousconductionmode(DCM)opera-
tion down to near the limit defined by the chip’s minimum
on-time (about 100ns). Below this output current level,
the converter will begin to skip cycles in order to main-
tain output regulation. Increasing the output load current
slightly, above the minimum required for discontinuous
conduction mode, allows constant frequency PWM. Each
regulator can be configured for pulse-skipping mode.
420°, or 60° (PHMODE = SV ) for the 4th stage. With
IN
the 60° CLKIN input, the next two stages can shift 120°
(PHMODE = 0) for each to generate a 300° signal for the
6th stage. Finally, the signal with a 60° phase shift on the
6thstage(PHMODEisfloating)goesbacktothe1ststage.
Figure 3 shows the configuration for 12-phase operation.
A multiphase power supply significantly reduces the
amount of ripple current in both the input and output
capacitors. The RMS input ripple current is reduced by,
and the effective ripple frequency is multiplied by, the
number of phases used (assuming that the input voltage
isgreaterthanthenumberofphasesusedtimestheoutput
voltage). The output ripple amplitude is also reduced by
the number of phases used.
Forced Continuous Operation
In applications where fixed frequency operation is more
critical than low current efficiency, and where the lowest
outputrippleisdesired,forcedcontinuousoperationshould
be used. Forced continuous operation can be enabled by
tying the MODE pin to GND. In this mode, inductor cur-
rent is allowed to reverse during low output loads, the I
TH
4616fe
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LINEAR_DIMENSIONS [ Linear Dimensions ]
