LTM4601A datasheet(11/28 Pages) LINER

11 / 28 page

LTM4601A/LTM4601A-1

4601afb

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

estimated as:

VOUT

VIN

Without considering the inductor current ripple, the RMS

current of the input capacitor can be estimated as:

ICIN(RMS) =

IOUT(MAX)

•D • 1– D

( )

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 rat-

ings 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 Figure 18, the 10μF ceramic capacitors are together

used as a high frequency input decoupling capacitor. In a

typical 12A output application, three very low ESR, X5R or

X7R, 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 LTM4601A 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 LTM4601A 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 current to a

DC load current of 24A equals ~6A of input RMS ripple

current for the external input capacitors.

Output Capacitors

The LTM4601A 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)

APPLICATIONS INFORMATION

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

4601A F02

6-PHASE

4-PHASE

12-PHASE

3-PHASE

2-PHASE

1-PHASE

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