LTM4656 datasheet(13/28 Pages) AD

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LTM4656/LTM4656-1

Rev. 0

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each cycle, the top MOSFET is turned on until either the

inductor current starts to reverse, as indicated by the cur-

rent comparator or the beginning of the next clock cycle.

In a step-up boost converter, the duty cycle can be cal-

culated at:

D = 1 –

VIN

VOUT

Note that at low input voltages, small voltage drops due

to series resistance become critical and greatly limit the

power delivery capability of the converter.

Theboostcontrollerhasaninternal1.2Vreferencevoltage,

and a 221k 1% internal feedback resistor connects VOUT

and VFB pins together. Adding a resistor RFB from VFB pin

to GND programs the output voltage:

RFB =

1.2V

VOUT – 1.2V

• 221k

Table 1. VFB Resistor Value vs Various Output Voltages

VOUT (V)

10V

12V

20V

24V

30V

36V

RFB

54.9k 39.2k 30.9k 24.3k 14k 11.5k 9.31k

7.5k

For parallel operation of N-piece of LTM4656 modules, the

following equation can be used to solve for RFB:

RFB =

1.2V

VOUT – 1.2V

•

221k

The VFB, COMP, SS, RUN and SDHN pins should be con-

nected together.

The UV pins can be tied together taking into account the

internal 100k resistor to VIN will reduce N times when

selecting a single resistor to set an UVLO operating point

for the paralleled modules.

INPUT DECOUPLING CAPACITORS

The LTM4656 module should be connected to a low

AC-impedance DC source. The input ripple current in a

boostconverterisrelativelylow(comparedwiththeoutput

ripplecurrent)becausethiscurrentiscontinuous.Theinput

capacitor, CIN, voltage rating should comfortably exceed

the maximum input voltage, and placed on the BVIN pins.

Although ceramic capacitors can be relatively tolerant of

overvoltage conditions, aluminum electrolytic capacitors

are not. Be sure to characterize the input voltage for any

possible overvoltage transients that could apply excess

stress to the input capacitors.

The value of the CIN is a function of the source impedance,

andingeneral,thehigherthesourceimpedance,thehigher

the required input capacitance. The required amount of

input capacitance is also greatly affected by the duty cycle.

High output current applications that also experience high

duty cycles can place great demands on the input supply,

both in terms of DC current and ripple current.

The input IRMS current in boost is fairly low since the

input current is continuous. Primary input capacitance is

to maintain low input ripple voltage.

The input IRMS current equation:

IRMS =

IO2 +(∆I)2

Where IO is output current, and

∆I =

VIN • D

4.7µH • FREQ

The output capacitor will see discontinuous current, thus

the peak current can be high, and the COUT IRMS is signifi-

cant. The equation for the COUT IRMS current is:

IRMS = IOUT

VOUT – VIN

VIN

The output ripple has two components, one that is related

to output capacitance minimum:

COUT =

IOUT • D

FREQ • ∆VOUT

Where ∆VOUT is the output ripple based on total output

capacitance.Thesecondisbasedontotalequivalentoutput

capacitance ESR:

∆VOUTESR = ESR •

IOUT

1– D

∆I

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