LTM4620AMPY データシートの表示（PDF） - Linear Technology

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LTM4620AMPY

Dual 13A or Single 26A DC/DC µModule Regulator

Linear Technology 

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 elec-

trolytic 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 = VOUT

VIN

Without considering the inductor current ripple, for each

output, the RMS current of the input capacitor can be

estimated as:

ICIN(RMS)

=

IOUT(M AX )

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 rip-

ple noise and good transient response. The bulk output

capacitors defined as COUT are chosen with low enough

effective series resistance (ESR) to meet the output volt-

age ripple and transient requirements. COUT 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 opera-

tion 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 care-

fully 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 VOUT to the VOUTS pin to allow for a bode plot ana-

lyzer to inject a signal into the control loop and validate

the regulator stability. The same resistor could be place

in series from VOUT to DIFFP and a bode plot analyzer

could inject a signal into the control loop and validate the

regulator stability.

For more information www.linear.com/LTM4620A

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