LTM4601AHVEV-PBF datasheet(11/28 Pages) LINER

Part #	LTM4601AHVEV-PBF

Description	12A, 28VIN DC/DC 關Module with PLL, Output Tracking and Margining

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Manufacturer	LINER [Linear Technology]

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LTM4601AHV

11

4601ahvf

For a buck converter, the switching duty-cycle can be

estimated as:

D

V

OUT

IN

=

Without considering the inductor current ripple, the RMS

current of the input capacitor can be estimated as:

I

DD

CIN RMS

OUT MAX

()

%

••

–

=

()

η

1

In the above equation,

η% is the estimated efﬁciency of

the power module. CIN can be a switcher-rated electrolytic

aluminum capacitor, OS-CON capacitor or high volume

ceramic capacitor. Note the capacitor ripple current ratings

are often based on temperature and hours of life. This

makes it advisable to properly derate the input capacitor,

or choose a capacitor rated at a higher temperature than

required. Always contact the capacitor manufacturer for

derating requirements.

In Figures 19 and 20, the 10μF ceramic capacitors are to-

gether used as a high frequency input decoupling capacitor.

In a typical 12A output application, three very low ESR,

X5R or X7R (extended temperature range), 10μF ceramic

capacitors are recommended. These decoupling capacitors

should be placed directly adjacent to the module input pins

in the PCB layout to minimize the trace inductance and high

frequency AC noise. Each 10μF ceramic is typically good

for 2A to 3A of RMS ripple current. Refer to your ceramics

capacitor catalog for the RMS current ratings.

Multiphase operation with multiple LTM4601AHV devices in

parallel will lower the effective input RMS ripple current due

to the interleaving operation of the regulators. Application

Note 77 provides a detailed explanation. Refer to Figure 2

for the input capacitor ripple current requirement as a

function of the number of phases. The ﬁgure provides a

ratio of RMS ripple current to DC load current as function

of duty cycle and the number of paralleled phases. Pick

the corresponding duty cycle and the number of phases

to arrive at the correct ripple current value. For example,

the 2-phase parallel LTM4601AHV design provides 24A

at 2.5V output from a 12V input. The duty cycle is DC =

2.5V/12V = 0.21. The 2-phase curve has a ratio of ~0.25

for a duty cycle of 0.21. This 0.25 ratio of RMS ripple cur-

rent to a DC load current of 24A equals ~6A of input RMS

ripple current for the external input capacitors.

Output Capacitors

The LTM4601AHV is designed for low output voltage ripple.

The bulk output capacitors deﬁned as COUT are chosen

with low enough effective series resistance (ESR) to meet

the output voltage ripple and transient requirements. COUT

can be a low ESR tantalum capacitor, a low ESR polymer

capacitor or a ceramic capacitor. The typical capacitance is

200μF if all ceramic output capacitors are used. Additional

output ﬁltering may be required by the system designer,

if further reduction of output ripple or dynamic transient

spike is required. Table 2 shows a matrix of different output

voltages and output capacitors to minimize the voltage

droop and overshoot during a 5A/μs transient. The table

optimizes total equivalent ESR and total bulk capacitance

to maximize transient performance.

Figure 2. Normalized Input RMS Ripple Current

vs Duty Factor for One to Six Modules (Phases)

DUTY FACTOR (VOUT/VIN)

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0.6

0.5

0.4

0.3

0.2

0.1

0

4601AHV F02

6-PHASE

4-PHASE

12-PHASE

3-PHASE

2-PHASE

1-PHASE

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