LTM4671 datasheet(15/36 Pages) AD

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LTM4671

Rev. B

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APPLICATIONS INFORMATION

The typical LTM4671 application circuit is shown in Figure

30. External component selection is primarily determined

by the input voltage, the output voltage and the maxi-

mum load current. Refer to Table 3 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

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:

D(MAX) = 1– tOFF(MIN) • fSW

where tOFF(MIN) is the minimum off-time, 80ns typical for

LTM4671, and fSWistheswitchingfrequency.Conversely,

the minimum on-time limit imposes a minimum duty cycle

of the converter which can be calculated as:

D(MIN) = tON(MIN) • fSW

where TON(MIN) is the minimum on-time, 25ns typical

for LTM4671. 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. These constraints are shown in

the Typical Performance Characteristic curve labeled “VIN

to VOUT Step-Down Ratio.” 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.

For the 12A channels (CH0, CH3), a 60.4k 0.5% internal

feedbackresistorconnectseachregulatorchannelVOSNS+

and FB pin together. Adding a resistor RFB from FB pin to

VOSNS– programs the output voltage.

For the 5A channels (CH1, CH2), a 60.4k 0.5% internal

feedbackresistorconnectseachregulatorchannelVOSNS

and FB pin together. Adding a resistor RFB from FB pin to

GND programs the output voltage:

VOUT = 0.6V •

60.4k + RFB

RFB

For parallel operation of N-channels, tie the VOUT, the FB

pins and VOSNS– pins together but only hooking up one

VOSNS+ (VOSNS) pin to the VOUT so that all the parallel-

ing channels can share the same error amplifier and same

top 60.4k feedback resistor. See PolyPhase Operation for

details.

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

INPUT DECOUPLING CAPACITORS

The LTM4671 module should be connected to a low AC-

impedance DC source. For each 12A regulator channel,

one piece 22µF input ceramic capacitor is required, for

each 5A regulator channel, one piece 10µF input ceramic

capacitor is required for RMS ripple current decoupling.

Bulk input capacitor is only needed when the input source

impedanceiscompromisedbylonginductiveleads,traces

ornotenoughsourcecapacitance.Thebulkcapacitorcanbe

an electrolytic aluminum capacitor and polymer capacitor.

Without considering the inductor current ripple, the RMS

current of the input capacitor can be estimated as:

ICIN(RMS) =

IOUT(MAX)

η%

• D • (1– D)

where η%istheestimatedefficiencyofthepowermodule.

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.Table3showsamatrixofdifferentoutput

voltages and output capacitors to minimize the voltage

droop and overshoot during a 25% load step transient.

Multiphase operation will reduce effective output ripple as

a function of the number of phases. Application Note 77

discusses this noise reduction versus output ripple cur-

rent cancellation, but the output capacitance will be more

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