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LTM4634

4634f

For more information www.linear.com/LTM4634

applicaTions inForMaTion

The typical LTM4634 application circuit is shown in Fig-

ure 22. External component selection is primarily deter-

mined by the maximum load current and output voltage.

RefertoTable4forspecificexternalcapacitorrequirements

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 5A operation at higher ambient

temperatures. At very low duty cycles the minimum 100ns

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

ment section and temperature derating curves.

Output Voltage Programming

The PWM controller has an internal 0.8V ±1% reference

voltage. As shown in the Block Diagram, a 60.4k preci-

sion internal feedback resistor connects the VOUT and VFB

pins together.

The output voltage will default to 0.8V with no feedback

resistor. Adding a resistor RFB from VFB to ground pro-

grams the output voltage:

VOUT = 0.8V •

60.4k

+RFB

RFB









or RFB =

48.32k

VOUT –0.8

Table 1. VFB Resistor Table vs Various Output Voltages

VOUT(V) 0.8

1.0

1.2

1.5

1.8

2.5

3.3

5.0 12.0

RFB (kΩ) Open 243 121 69.8 48.7 28.7 19.1 11.5 4.32

In the parallel operation the following pins should be tied

together, VFB1 and VFB2 pins, COMP1 and COMP2 pins,

TK/SS1 and TK/SS2, and RUN1 and RUN2.

For parallel operation of VOUT1 and VOUT2, connect VFB1

and VFB2 together with a single resistor to ground whose

value is determined by:

RFB =

60.4k

VOUT

0.8

–1

Input Capacitors

The LTM4634 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 for each channel. Typically 4.7µF

to 10µF X7R ceramics are a good choice with RMS ripple

currentratingsof~2Aeach.A47µFto100µFsurfacemount

aluminumelectrolyticcapacitorcanbeusedformoreinput

bulk capacitance. This bulk input capacitor is only needed

if the input source impedance is compromised by long

inductive leads, traces or not enough source capacitance.

If low impedance power 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

( )

(1)

In the previous equation,

η% is the estimated efficiency

of the power module in decimal form (0.nn) for a given

VOUT-to-VIN ratio.

The selection of CIN is simplified by the 3-phase architec-

ture and its impact on the worst-case RMS current draw

occurs when only one channel is operating. This is true

when the three channels are powered from a common

VIN. The channel with the highest duty cycle D peaking at

0.5 and maximum load current needs to be used in the

above formula. This will give the maximum RMS capacitor

current requirement. Increasing the output current drawn

from the other channels will actually decrease the input

RMS ripple current from its maximum value. The out-of-

phase technique typically reduces the input capacitor’s

RMS ripple current by a factor of 50% when compared to

a single phase power supply solution. If the three channels

are powered from independent input sources, then each

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