LTM4643 datasheet(11/30 Pages) AD

11 / 30 page

LTM4638

Rev. C

For more information www.analog.com

APPLICATIONS INFORMATION

The typical LTM4638 application circuit is shown in

Figure 23. External component selection is primarily

determined by the input voltage, the output voltage and

the maximum load current. Refer to Table 7 for specific

externalcapacitorrequirementsforaparticularapplication.

VIN to VOUT Step-Down Ratios

There are restrictions in the maximum VIN and VOUT step-

down ratios that can be achieved for a given input voltage

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

of the 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, typically 50ns

for LTM4638, and fSW (Hz) is the switching frequency.

Converselytheminimumon-timelimitimposesaminimum

duty cycle of the converter which can be calculated as:

DMIN = tON(MIN) • fSW

where tON(MIN) is the minimum on-time, typically 25ns

for LTM4638. 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 the VOUT and FB pins together. Adding a

resistor, RFB, from FB pin to VOSNS– programs the output

voltage:

RFB =

0.6V

VOUT – 0.6V

• 60.4k

Table 1. RFB 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 (kΩ) OPEN 90.9 60.4 40.2 30.1 19.1 13.3 8.25

For parallel operation of multiple channels the same feed-

back setting resistor can be used for the parallel design.

This is done by connecting the VOSNS+totheoutputshown

in Figure 2, thus tying one of the internal 60.4k resistors

to the output. All of the VFB pins tie together with one

programming resistor as shown in Figure 2.

See Figure 25 for an example of parallel operation.

Input Decoupling Capacitors

The LTM4638 module should be connected to a low AC

impedance DC source. For the regulator, a 22µF input

ceramic capacitor is required for RMS ripple current de-

coupling. Bulk input capacitance is only needed when the

inputsourceimpedanceiscompromisedbylonginductive

leads, traces or not enough source capacitance. The bulk

capacitor can be an aluminum electrolytic capacitor 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%istheestimatedefficiencyofthepowermodule.

Output Decoupling Capacitors

With an optimized high frequency, high bandwidth de-

sign, only a single low ESR output ceramic capacitor is

required for the LTM4638 to achieve low output ripple

voltage and very good transient response. In extreme

4638 F02

COMPa

COMPb

1.2V/30A

VOUT

VOSNS+

VOSNS–

VFB

LTM4638

TRACK/SS

COMPa

VOUT

VOSNS+

VOSNS–

VFB

LTM4638

TRACK/SS

0.1µF

RFB

60.4k

COMPb

Figure 2. 2-Phase Parallel Configurations

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