LTM4627EVPBF datasheet(10/28 Pages) LINER

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LTM4627

4627f

APPLICATIONS INFORMATION

The typical LTM4627 application circuit is shown in Fig-

ure 18. External component selection is primarily deter-

mined by the maximum load current and output voltage.

Refer to Table 4 for speciﬁc external capacitor requirements

for particular applications.

VIN to VOUT Step-Down Ratios

There are restrictions in the VIN to VOUT step-down ratio

that can be achieved for a given input voltage. The VIN to

VOUT minimum dropout is a function of load current and

at very low input voltage and high duty cycle applications

output power may be limited as the internal top power

MOSFET is not rated for 15A operation at higher ambient

temperatures. At very low duty cycles the minimum 90ns

on-time must be maintained. See the Frequency Adjust-

ment section and temperature derating curves.

Output Voltage Programming

The PWM controller has an internal 0.6V ±1% reference

voltage. As shown in the Block Diagram, a 60.4k internal

feedback resistor connects the VOUT_LCL and VFB pins

together. When the remote sense ampliﬁer is used, then

DIFF_OUT is connected to the VOUT_LCL pin. If the remote

sense ampliﬁer is not used, then VOUT_LCL connects to

VOUT. The output voltage will default to 0.6V with no feed-

back resistor. Adding a resistor RFB from VFB to ground

programs the output voltage:

VOUT = 0.6V •

60.4k +RFB

RFB

Table 1. VFB Resistor Table vs Various Output Voltages

VOUT (V)

0.6

1.0

1.2

1.5

1.8

2.5

3.3

5.0

RFB (k)

Open

90.9

60.4

40.2

30.1

19.1

13.3

8.25

For parallel operation of N LTM4627s, the following equa-

tion can be used to solve for RFB:

RFB=

60.4 / N

VOUT

0.6

–1

Tie the VFB pins together for each parallel output. The

COMP pins must be tied together also.

Input Capacitors

The LTM4627 module should be connected to a low

AC-impedance DC source. Additional input capacitors

are needed for the RMS input ripple current rating. The

ICIN(RMS) equation which follows can be used to calculate

the input capacitor requirement. Typically 22μF X7R ce-

ramics are a good choice with RMS ripple current ratings

of ~ 2A each. A 47μF to 100μF surface mount aluminum

electrolytic bulk capacitor can be used for more input bulk

capacitance. This bulk input capacitor is only needed if

the input source impedance is compromised by long in-

ductive leads, traces or not enough source capacitance.

If low impedance power planes are used, then this bulk

capacitor is not needed.

For a buck converter, the switching duty cycle can be

estimated as:

VOUT

VIN

Without considering the inductor ripple current, for each

output, the RMS current of the input capacitor can be

estimated as:

ICIN(RMS)=

IOUT(MAX)

η%

•D •(1–D)

Inthepreviousequation,

η%istheestimatedefﬁciencyofthe

power module. The bulk capacitor can be a switcher-rated

electrolytic aluminum capacitor or a Polymer capacitor.

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