LTM4643 datasheet(10/32 Pages) LINER

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LTM4643

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APPLICATIONS INFORMATION

The typical LTM4643 application circuit is shown in

Figure 29. External component selection is primarily

determined by the input voltage, the output voltage and

the maximum load current. Refer to Table 6 for specific

externalcapacitorrequirementsforaparticularapplication.

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

due to the minimum off-time and minimum on-time limits

of each regulator. The minimum off-time limit imposes a

maximum duty cycle which can be calculated as:

DMAX = 1 – tOFF(MIN) • fSW

where tOFF(MIN) is the minimum off-time, 70ns typical for

LTM4643, and fSW is the switching frequency. Conversely

the minimum on-time limit imposes a minimum duty cycle

of the converter which can be calculated as:

DMIN = tON(MIN) • fSW

where tON(MIN) is the minimum on-time, 40ns typical for

LTM4643. In the rare cases where the minimum duty

cycle is surpassed, the output voltage will still remain

in regulation, but the switching frequency will decrease

from its programmed value. Note that additional thermal

derating may be applied. See the Thermal Considerations

and Output Current Derating section in this data sheet.

Output Voltage Programming

ThePWMcontrollerhasaninternal0.6Vreferencevoltage.

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

resistor connects each regulator channel from VOUTpin to

FB pin. Adding a resistor RFBfromFBpintoGNDprograms

the output voltage:

RFB =

60.4k

VOUT

0.6

− 1

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

90.9

60.4

40.2

30.1

19.1

13.3

For parallel operation of N channels, use the following

equation can be used to solve for RFB. Tie the VOUT and

the FB and COMP pins together for each paralleled output

with a single resistor to GND as determined by:

RFB =

60.4k













VOUT

0.6

– 1













Input Decoupling Capacitors

The LTM4643 module should be connected to a low AC-

impedance DC source. For each regulator channel, a 10µF

input ceramic capacitor is recommended for RMS ripple

current decoupling. A bulk input capacitor is only needed

when the input source impedance is compromised by long

inductive leads, traces or not enough source capacitance.

The bulk capacitor can be an electrolytic aluminum capaci-

tor or polymer capacitor.

Without considering the inductor ripple current, the RMS

current of the input capacitor can be estimated as:

ICIN(RMS) =

IOUT(MAX)

η%

• D •(1− D)

where η% is the estimated efficiency of the power module.

Output Decoupling Capacitors

With an optimized high frequency, high bandwidth design,

only single piece of low ESR output ceramic capacitor is

required for each regulator channel to achieve low output

voltagerippleandverygoodtransientresponse.Additional

output filtering may be required by the system designer,

if further reduction of output ripples or dynamic transient

spikesisrequired.Table6providesareferencematrixshow-

ingtransientperformancefordifferentoutputcapacitorcon-

figurations.Multiphaseoperationwillreduceeffectiveout-

putrippleasafunctionofthenumberofphases.Application

Note77discussesthisnoisereductionversusoutputripple

current cancellation, but the output capacitance will be

more a function of stability and transient response. The

LTpowerCAD® Design Tool is available to download online

for output ripple, stability and transient response analysis

and calculating the output ripple reduction as the number

of phases implemented increases by N times.

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