LTM4613 datasheet(10/28 Pages) LINER

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LTM4648

4648f

For more information www.linear.com/LTM4648

applicaTions inForMaTion

The typical LTM4648 application circuit is shown in

Figure 18. External component selection is primarily

determined by the maximum load current and output

voltage. Refer to Table 3 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 10A 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 10k 0.5%

internal feedback resistor connects the VOUT_LCL and

VFB pins together. When the remote sense amplifier is

used, then DIFFOUT 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 feedback resistor. Adding a resistor RFB from VFB

to ground programs the output voltage:

VOUT = 0.6V •

10k +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

6.65

4.99

3.09

2.21

1.37

For parallel operation of N LTM4648, the following equa-

tion can be used to solve for RFB:

RFB =

10k

VOUT

0.6

–1

In parallel operation the VFB pins have an IFB current of

20nA maximum each channel. To reduce output voltage

error due to this current, an additional VOUT_LCL pin can

be tied to VOUT, and an additional RFB resistor can be used

to lower the total Thevenin equivalent resistance seen by

this current.

Input Capacitors

The LTM4648 module should be connected to a low AC

impedance DC source. Additional input capacitors are

neededfortheRMSinputripplecurrentrating.TheICIN(RMS)

equation which follows can be used to calculate the input

capacitor requirement. Typically 22µF X7R ceramics are a

good choice with RMS ripple current ratings of ~2A each.

A47µFto100µFsurfacemountaluminumelectrolyticbulk

capacitor can be used for more input bulk capacitance.

This bulk input capacitor is only needed if the input source

impedanceiscompromisedbylonginductiveleads,traces

ornotenoughsourcecapacitance.Iflowimpedancepower

planes are used, then this bulk capacitor is not needed.

For a buck converter, the switching duty cycle can be

estimated as:

D =

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

(

)

In the previous equation, η% is the estimated efficiency of

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

rated electrolytic aluminum capacitor or a Polymer

capacitor.

Output Capacitors

The LTM4648 is designed for low output voltage ripple

noise. The bulk output capacitors defined as COUT are

chosen with low enough effective series resistance (ESR)

to meet the output voltage ripple and transient require-

ments. COUT can be a low ESR tantalum capacitor, low

ESR Polymer capacitor or ceramic capacitors. The typical

outputcapacitancerangeisfrom200µFto470µF.Additional

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