LTM4606MPV datasheet(10/24 Pages) LINER

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LTM4606

4606f

The typical LTM4606 application circuit is shown in

Figure 20. External component selection is primarily

determined by the maximum load current and output

voltage. Refer to Table 2 for speciﬁc external capacitor

requirements for a particular application.

VIN to VOUT Step-Down Ratios

There are restrictions in the maximum VIN and VOUT step-

down ratio that can be achieved for a given input voltage.

These constraints are caused by the limitation of the

minimum on and off time in the internal switches. Refer

to the Frequency Adjustment section to change the switch-

ing frequency and get wider input and output ranges. See

the Thermal Considerations and Output Current Derating

section in this data sheet for the current restrictions.

Output Voltage Programming and Margining

The PWM controller has an internal 0.6V reference voltage.

As shown in the Block Diagram, a 60.4k internal feedback

resistor connects the VOUT and VFB pins together. Adding

a resistor RFB from the VFB pin to the SGND pin programs

the output voltage:

OUT

60 4

Table 1. RFB Standard 1% Resistor Values vs VOUT

RFB

(kΩ)

Open

60.4

40.2

30.1

25.5

19.1

13.3

8.25

VOUT

(V)

0.6

1.2

1.5

1.8

2.5

3.3

The MPGM pin programs a current that when multiplied

by an internal 10k resistor sets up the 0.6V reference ±

offset for margining. A 1.18V reference divided by the

RPGM resistor on the MPGM pin programs the current.

Calculate VOUT(MARGIN):

OUT MARGIN

OUT

()

•

100

where %VOUTisthepercentageofVOUTyouwanttomargin,

and VOUT(MARGIN) is the margin quantity in volts:

PGM

OUT

OUT MARGIN

118

•

()

APPLICATIONS INFORMATION

where RPGM is the resistor value to place on the MPGM

pin to ground.

The output margining will be ± margining of the value.

This is controlled by the MARG0 and MARG1 pins. See

the truth table below:

MARG1

MARG0

MODE

LOW

NO MARGIN

LOW

HIGH

MARGIN UP

HIGH

LOW

MARGIN DOWN

HIGH

NO MARGIN

Input Capacitors and Input EMI Noise Attenuation

The LTM4606 is designed to achieve the low input con-

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

and turn-off. In the LTM4606, a high frequency inductor

is integrated to the input line for noise attenuation. VD

and VIN pins are available for external input capacitors to

form a high frequency ∏ ﬁlter. As shown in Figure 19, 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:

OUT

Without considering the inductor ripple current, the RMS

current of the input capacitor can be estimated as:

CIN RMS

OUT MAX

()

••

–

()

In the above 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 capacitor, or

choose a capacitor rated at a higher temperature than

required. Always contact the capacitor manufacturer for

derating requirements.

In a typical 6A output application, one or two very low

ESR X5R or X7R, 10μF ceramic capacitors are recom-

mended for C1. This decoupling capacitor should be placed

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