LTM4633_15 Datasheet, PDF (12/32 Pages) Linear Technology – Triple 10A Step-Down DC/DC Module Regulator

LTM4633
Applications Information
above formula. This will give the maximum RMS capacitor
current requirement. Increasing the output current drawn
from the other channels will actually decrease the input
RMS ripple current from its maximum value. The out-of-
phase technique typically reduces the input capacitor’s
RMS ripple current by a factor of 50% when compared to
a single phase power supply solution. If the three channels
are powered from independent input sources, then each
of the input RMS current ratings will need to be calculated
specific to that channel.
Output Capacitors
The LTM4633 is designed for low output voltage ripple
noise. The bulk output capacitors defined as C_OUT are
chosen with low enough effective series resistance (ESR)
to meet the output voltage ripple and transient require-
ments. C_OUT can be a low ESR tantalum capacitor, low ESR
Polymer capacitor or ceramic capacitor. The typical output
capacitance range is from 200µF to 470µF. Additional output
filtering may be required by the system designer if further
reduction of output ripple or dynamic transient spikes is
required. Table 5 shows a matrix of different output voltages
and output capacitors to minimize the voltage droop and
overshoot during a 5A/µs transient. The table optimizes
total equivalent ESR and total bulk capacitance to optimize
the transient performance. Stability criteria are considered
in the Table 5 matrix, and LTpowerCAD is available for free
to conduct stability analysis. LTpowerCAD can calculate
the output ripple reduction as the number of implemented
phases increases by N times.
Burst Mode Operation
The LTM4633 is capable of Burst Mode operation in
which the power MOSFETs operate intermittently based
on load demand, thus saving quiescent current. For ap-
plications where maximizing the efficiency at very light
loads is a high priority, Burst Mode operation should be
applied. To enable Burst Mode operation, simply float
the MODE/PLLIN pin. During Burst Mode operation, the
peak current of the inductor is set to approximately 30%
of the maximum peak current value in normal operation
even though the voltage at the COMP pin indicates a
lower value. The voltage at the COMP pin drops when
the inductor’s average current is greater than the load
requirement. As the COMP voltage drops below 0.5V, the
burst comparator trips, causing the internal sleep line to
go high and turn off both power MOSFETs.
In sleep mode, the internal circuitry is partially turned
off, reducing the quiescent current. The load current is
now being supplied from the output capacitors. When the
output voltage drops, causing COMP to rise, the internal
sleep line goes low, and the LTM4633 resumes normal
operation. The next oscillator cycle will turn on the top
power MOSFET and the switching cycle repeats.
Pulse-Skipping Mode Operation
In applications where low output ripple and high efficiency
at intermediate currents are desired, pulse-skipping
mode should be used. Pulse-skipping operation allows
the LTM4633 to skip cycles at low output loads, thus
increasing efficiency by reducing switching loss. Tying
the MODE/PLLIN pin to INTV_CC enables pulse-skipping
operation. With pulse-skipping mode at light load, the
internal current comparator may remain tripped for several
cycles, thus skipping operation cycles. This mode has
lower ripple than Burst Mode operation and maintains a
higher frequency operation than Burst Mode operation.
Forced Continuous Operation
In applications where fixed frequency operation is more
critical than low current efficiency, and where the lowest
output ripple is desired, forced continuous operation
should be used. Forced continuous operation can be
enabled by tying the MODE/PLLIN pin to ground. In this
mode, inductor current is allowed to reverse during low
output loads, the COMP voltage is in control of the current
comparator threshold throughout, and the top MOSFET
always turns on with each oscillator pulse. During start-up,
forced continuous mode is disabled and inductor current
is prevented from reversing until the LTM4633 output
voltage is in regulation.
4633f
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