LTM4636
12
4636f
applicaTions inForMaTion
Output Capacitors
The LTM4636 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 capacitors. The typi-
cal output capacitance range is from 400µF to 1000µF.
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 dif-
ferent output voltages and output capacitors to minimize
the voltage droop and overshoot during a 15A/µs tran-
sient. 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 stability analysis. Multiphase
operation will reduce effective output ripple as a function
of the number of phases. Application Note 77 discusses
this noise reduction versus output ripple current cancel-
lation, but the output capacitance should be considered
carefully as a function of stability and transient response.
LTpowerCAD can be used to calculate the output ripple
reduction as the number of implemented phases increases
by N times. External loop compensation can be used for
transient response optimization.
Burst Mode Operation
The LTM4636 is capable of Burst Mode operation in which
the power MOSFETs operate intermittently based on load
demand, thus saving quiescent current. For applications
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 COMPA pin indicates a lower value. The
voltage at the COMPA pin drops when the inductor’s aver-
age current is greater than the load requirement. As the
COMPA 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 COMPA to rise, the internal
sleep line goes low, and the LTM4636 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 LTM4636 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 COMPA 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 LTM4636’s output
voltage is in regulation.
