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LTM4649

4649fc

For more information www.linear.com/LTM4649

APPLICATIONS INFORMATION

The typical LTM4649 application circuit is shown in

Figure 17. 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 a minimum 110ns

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

ment section and temperature derating curves.

Output Voltage Programming

ThePWMcontrollerhasaninternal0.6Vreferencevoltage.

As shown in the Block Diagram, a 10k 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_LCLconnects to VOUT. The

output voltage will default to 0.6V with no feedback resis-

tor. 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

RFB(k)

OPEN

6.65

4.99

3.16

2.21

For parallel operation of N LTM4649, 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

25nA 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 LTM4649 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:

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

thepowermodule.Thebulkcapacitorcanbeaswitcher-rated

aluminum electrolytic capacitor or a polymer capacitor.

Output Capacitors

The LTM4649 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

output filtering may be required by the system designer

if further reduction of output ripple or dynamic transient

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