LTM4612 Ver la hoja de datos (PDF) - Linear Technology

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Fabricante

LTM4612

Ultralow Noise 36VIN, 15VOUT, 5A, DC/DC µModule

Linear Technology 

LTM4612

APPLICATIONS INFORMATION

If lower output ripple is required, the operating frequency

f can be increased by adding a resistor RfSET between fSET

pin and SGND, as shown in Figure 19.

( ) f

=

1.5

•

VOUT

10−10 RfSET

||

93.1k

For output voltages more than 12V, the frequency can be

higher than 1MHz, thus reducing the efficiency significantly.

Additionally, the minimum off time 400ns normally limits

the operation when the input voltage is close to the output

voltage. Therefore, it is recommended to lower the fre-

quency in these conditions by connecting a resistor (RfSET)

from the fSET pin to VIN, as shown in Figure 20.

f=

VOUT

5

•

10

−11⎛⎝⎜

3 • RfSET

RfSET − 3

•

•

93.1k

93.1k

⎞

⎠⎟

The load current can affect the frequency due to its constant

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

PLLIN pin can be used to synchronize the frequency of

the LTM4612 to an external clock, as shown in Figures

21 to 23.

Input Capacitors

LTM4612 is designed to achieve the low input conducted

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

In the LTM4612, 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

capacitor C1 on the VD pins is used to handle most of

the RMS current into the converter, so careful attention

is needed for capacitor C1 selection.

For a buck converter, the switching duty cycle can be

estimated as:

D = VOUT

VIN

Without considering the inductor current ripple, the RMS

current of the input capacitor can be estimated as:

( ) ICIN(RMS)

=

IOUT(MAX)

η

•

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 5A output application, one very low ESR, X5R

or X7R, 10μF ceramic capacitor is recommended for C1.

This decoupling capacitor should be placed directly adja-

cent 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 to 3A of RMS ripple

current. Refer to your ceramics capacitor catalog for the

RMS current ratings.

To attenuate the high frequency noise, extra input capacitors

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

impedance is compromised by long inductive leads or

traces. Figure 4 shows the conducted EMI testing results

to meet the Level 5 of the CISPR 25 limit. For different

applications, input capacitance may be varied to meet

different conducted EMI limits.

80

70

CIS25QP

60

50

40

30

20

10

0

0.15

1

10

FREQUENCY (MHz)

30

4612 F04

Figure 4. Conducted Emission Scan with 24VIN to

12VOUT at 5A (3 × 10μF Ceramic Capacitors on VIN

Pads and 1 × 10μF Ceramic Capacitor on VD Pads).

4612f

11

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