LTM4622A
10
Rev B
For more information
APPLICATIONS INFORMATION
Output Decoupling Capacitors
With an optimized high frequency, high bandwidth design,
only single piece of 47µF low ESR output ceramic capaci-
tor is required for each LTM4622A output to achieve low
output voltage ripple and very good transient response.
Additional output filtering may be required by the system
designer, if further reduction of output ripples or dynamic
transient spikes is required. Table 7 shows a matrix of dif-
ferent output voltages and output capacitors to minimize the
voltage droop and overshoot during a 1A (50%) load step
transient. Multiphase operation will reduce effective output
ripple as a function of the number of phases.
discusses this noise reduction versus output
ripple current cancellation, but the output capacitance
will be more a function of stability and transient response.
The Linear Technology LTpowerCAD
®
Design Tool is avail-
able 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 current
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 lowest
output ripple is desired, forced continuous operation should
be used. Forced continuous operation can be enabled by
tying the SYNC/MODE pin to GND. In this mode, induc-
tor 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
Output Voltage Programming
The PWM controller has an internal 0.6V reference voltage.
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
OUT
– 0.6V
• 60.4k
Table 1. V
FB
Resistor Table vs Various Output Voltages
(1% Resistor)
V
OUT
(V)
1.5
1.8
2.5
3.3
5.0 8.0
10.0
12.0
R
FB
(k)
40.2
30.1
19.1
13.3
8.25 4.87
3.83
3.16
Pease note that for output above 3.3V, a higher operating
frequency is required to optimize inductor current ripple.
See Operating Frequency section.
For parallel operation of N-channels LTM4622A, the fol-
lowing equation can be used to solve for R
FB
:
R
FB
=
0.6V
V
OUT
– 0.6V
•
60.4k
N
Input Decoupling Capacitors
The LTM4622A 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 compromised by long
inductive leads, traces or not enough source capacitance.
The bulk capacitor can be an electrolytic aluminum capaci-
tor 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)
=
I
OUT(MAX)
η
%
• D • 1– D
(
)
where is the estimated efficiency of the power module.
