LTM4613 datasheet(12/30 Pages) LINER

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LTM4613

4613fd

For more information www.linear.com/LTM4613

APPLICATIONS INFORMATION

For output voltages more than 12V, the frequency can

be higher than 600kHz, thus reducing the efficiency sig-

nificantly. Additionally, the minimum off-time of 400ns

normally limits the operation when the input voltage is

close to the output voltage. Therefore, it is recommended

to lower the frequency in these conditions by connecting

a resistor (RfSET) from the fSET pin to VIN as shown in

Figure 20, where:

f =

VOUT

5•10−11

3•RfSET •133k

RfSET −2•133k

⎛

⎝

⎜

⎜⎜

⎞

⎠

⎟

⎟⎟

[Hz]

The load current can affect the frequency due to its con-

stant on-time control. If constant frequency is a necessity,

the PLLIN pin can be used to synchronize the frequency

of the LTM4613 to an external clock subject to minimum

on-time and off-time limits, as shown in Figures 21 to 23.

Input Capacitors

LTM4613 is designed to achieve low input conducted

EMI noise due to the fast switching of turn-on and turn-

off. Additionally, a high-frequency inductor is integrated

into the input line for noise attenuation. VD and VIN pins

are available for external input capacitors to form a high

frequency π filter. As shown in Figure 18, the ceramic

capacitors, C1-C3, on the VD pins are used to handle most

of the RMS current into the converter, so careful attention

is needed for capacitors C1-C3 selection.

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 this equation, η is the estimated efficiency of the

power module. 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 a typical 8A output application, three very low ESR,

X5R or X7R, 10µF ceramic capacitors are recommended

for C1-C3. This decoupling capacitance should be placed

directly adjacent to the module VD pins in the PCB layout

to minimize the trace inductance and high frequency AC

noise. Each 10µF ceramic is typically good for 2A of RMS

ripple current. Refer to your ceramics capacitor catalog

for the RMS current ratings.

Toattenuatethehighfrequencynoise,extrainputcapacitors

should be connected to the VIN pads and placed before the

high frequency inductor to form the π filter. One of these

low ESR ceramic input capacitors is recommended to be

close to the connection into the system board. A large bulk

100µF capacitor is only needed if the input source imped-

ance is compromised by long inductive leads or traces.

Output Capacitors

The LTM4613 is designed for low output voltage ripple.

ThebulkoutputcapacitorsdefinedasCOUTarechosenwith

low enough effective series resistance (ESR) to meet the

outputvoltagerippleandtransientrequirements.COUTcan

be low ESR tantalum capacitor, low ESR polymer capaci-

tor or ceramic capacitor. The typical capacitance is 4 ×

47µF 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

spikes is required. Table 2 shows a matrix of different

output voltages and output capacitors to minimize the

voltage droop and overshoot during a 4A load transient.

The table optimizes total equivalent ESR and total bulk

capacitance to maximize transient performance.

Multiphase operation with multiple LTM4613 devices in

parallel will also lower the effective output ripple current

due to the phase interleaving operation. Refer to Figure 4

for the normalized output ripple current versus the duty

cycle. Figure 4 provides a ratio of peak-to-peak output

ripple current to the inductor ripple current as functions of

duty cycle and the number of paralleled phases. Pick the

corresponding duty cycle and the number of phases to get

the correct output ripple current value. For example, each

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