LTM4622
10
Rev F
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APPLICATIONS INFORMATION
Output Decoupling Capacitors
With an optimized high frequency, high bandwidth design,
only single piece of 22µF low ESR output ceramic capaci-
tor is required for each LTM4622 output to achieve low
output voltage ripple and very good transient response.
Additional output filtering may be required by the sys-
tem designer, if further reduction of output ripples or
dynamic transient spikes is required. Table6 shows a
matrix of different output voltages and output capacitors
to minimize the voltage droop and overshoot during a
1.25A (50%) load step transient. 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 cancellation,
but the output capacitance will be more a function of sta-
bility and transient response. The Analog Devices, Inc.
LTpowerCAD
®
Design Tool is available to download online
for output ripple, stability and transient response analysis
and calculating the output ripple reduction as the number
of phases implemented increases by N times.
Burst Mode Operation
In applications where high efficiency at intermediate cur
-
rent are more important than output voltage ripple, Burst
Mode operation could be used by connecting SYNC/
MODE pin to INTV
CC
to improve light load efficiency. In
Burst Mode operation, a current reversal comparator
(I
REV
) detects the negative inductor current and shuts off
the bottom power MOSFET, resulting in discontinuous
operation and increased efficiency. Both power MOSFETs
will remain off and the output capacitor will supply the
load current until the COMP voltage rises above the zero
current level to initiate another cycle.
Force Continuous Current Mode (CCM) Operation
In applications where fixed frequency operation is more
critical than low current efficiency, and where the low-
est output ripple is desired, forced continuous opera-
tion should be used. Forced continuous operation can
be enabled by tying the SYNC/MODE pin to GND. 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
Output Voltage Programming
The PWM controller has an internal 0.6V reference volt-
age. As shown in the Block Diagram, a 60.4k 0.5% internal
feedback resistor connects V
OUT
and FB pins together.
Adding a resistor R
FB
from FB pin to GND programs the
output voltage:
R
FB
=
0.6V
V
– 0.6V
• 60.4k
Table1. V
FB
Resistor Table vs Various Output Voltages
V
OUT
(V) 0.6 1.0 1.2 1.5 1.8 2.5 3.3 5.0
R
FB
(k) OPEN 90.9 60.4 40.2 30.1 19.1 13.3 8.25
Pease note that for 2.5 to 5V output, a higher operating
frequency is required to optimize inductor current ripple.
See Operating Frequency section.
For parallel operation of N-channels LTM4622, the follow-
ing equation can be used to solve for R
FB
:
R
FB
=
0.6V
V
– 0.6V
•
60.4k
N
Input Decoupling Capacitors
The LTM4622 module should be connected to a low
AC-impedance DC source. For each regulator channel,
one piece 4.7µF input ceramic capacitor is required for
RMS ripple current decoupling. Bulk input capacitor is
only needed when the input source impedance is com-
promised by long inductive leads, traces or not enough
sour
ce capacitance. The bulk capacitor can be an electro-
lytic aluminum capacitor and polymer capacitor.
Without considering the inductor current ripple, for each
output, the RMS current of the input capacitor can be
estimated as:
I
CIN(RMS)
=
OUT(MAX)
η%
• D • 1– D
( )
where is the estimated efficiency of the power module.
