Datasheet

LTM4620A
13
4620afb
For more information www.linear.com/LTM4620A
APPLICATIONS INFORMATION
Input Capacitors
The LTM4620A module should be connected to a low ac-
impedance DC source. For the regulator input four 22µF
input ceramic capacitors are used for RMS ripple current.
A 47µF to 100µF surface mount aluminum electrolytic bulk
capacitor can be used for more input bulk capacitance.
This bulk input capacitor is only needed if the input source
impedance is compromised by long inductive leads, traces
or not enough source capacitance. If low impedance power
planes are used, then this bulk capacitor is not needed.
For a buck converter, the switching duty-cycle can be
estimated as:
D =
V
OUT
V
IN
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
( )
In the above equation, η% is the estimated efficiency of
the power module. The bulk capacitor can be a switcher-
rated electrolytic aluminum capacitor, Polymer capacitor.
Output Capacitors
The LTM4620A is designed for low output voltage ripple
noise and good transient response. The bulk output
capacitors defined as C
OUT
are chosen with low enough
effective series resistance (ESR) to meet the output volt-
age ripple
and transient requirements. C
OUT
can be a low
ESR tantalum capacitor, the low ESR polymer capacitor
or ceramic capacitor. The typical output capacitance range
for each output is from 200µF to 470µF. Additional output
filtering may be required by the system designer, if further
reduction of output ripples 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 7A/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 the LTpowerCAD will
be provided 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
cancellation, but the output
capacitance should be considered carefully as a function
of stability and transient response. The LTpowerCAD can
calculate the output ripple reduction as the number of
implemented phases increases by N times. A small value
10Ω to 50Ω resistor can be place in series from V
OUT
to
the V
OUTS
pin to allow for a bode plot analyzer to inject
a signal into the control loop and validate the regulator
stability. The same resistor could be place in series from
V
OUT
to DIFFP and a bode plot analyzer could inject a signal
into the control loop and validate the regulator stability.