LTM4657 datasheet(10/28 Pages) AD

10 / 28 page

LTM4657

Rev. A

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APPLICATIONS INFORMATION

The typical LTM4657 application circuit is shown in

Figure 23. External component selection is primarily deter-

mined by the input voltage, the output voltage and the

maximum load current. Refer to Table 7 for specific exter-

nal capacitor requirements for a particular application.

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 LTM4657, and fSW (Hz) 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, typically 25ns

for LTM4657. 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

The PWM controller has an internal 0.5V reference

voltage. 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 =

0.5V

VOUT –0.5V

•60.4k

Table 1. RFB Resistor Table vs Various Output Voltages

VOUT (V) 0.5

1.0

1.2

1.5

1.8

2.5

3.3

5.0

RFB (kΩ) OPEN 60.4 43.2 30.1 23.2

10.7

6.65

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+ to the output as

shown 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 LTM4657 module should be connected to a low AC

impedance DC source. For the regulator, a 10µF input

ceramic capacitor is required for RMS ripple current

decoupling. Bulk input capacitance 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 aluminum electrolytic capac-

itor 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 a single low ESR output ceramic capacitor is required

for the LTM4657 to achieve low output ripple voltage and

very good transient response. In extreme cold or hot tem-

perature or high output voltage case, additional ceramic

4657 F02

COMP

1.5V/16A

VOUT

VOSNS+

VOSNS–

VFB

LTM4657

TRACK/SS

COMP

VOUT

VOSNS+

VOSNS–

VFB

LTM4657

TRACK/SS

0.1µF

RFB

30.1k

Figure 2. 2-Phase Parallel Configurations

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