LTM4677 datasheet(13/38 Pages) LINER

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LTM4650A

4650afb

For more information www.linear.com/LTM4650A

The typical LTM4650A application circuit is shown in

Figure 32. External component selection is primarily

determined by the maximum load current and output

voltage. Refer to Table 6 for specific external capacitor

requirements for 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.

Each output of the LTM4650A is capable of 98% duty

cycle, but the VIN to VOUT minimum dropout is still shown

as a function of its load current and will limit output cur-

rent capability related to high duty cycle on the top side

switch.Minimumon-timetON(MIN)isanotherconsideration

in operating at a specified duty cycle while operating at

a certain frequency due to the fact that tON(MIN) < D/fSW,

where D is duty cycle and fSW is the switching frequency.

tON(MIN) is specified in the electrical parameters as 90ns.

Output Voltage Programming

ThePWMcontrollerhasaninternal0.6Vreferencevoltage.

AsshownintheBlockDiagram,a60.4kΩinternalfeedback

resistor connects between the VOUTS1 to VFB1 and VOUTS2

to VFB2. It is very important that these pins be connected

to their respective outputs for proper feedback regulation.

Overvoltage can occur if these VOUTS1andVOUTS2pinsare

left floating when used as individual regulators, or at least

one of them is used in paralleled regulators. The output

voltage will default to 0.6V with no feedback resistor on

either VFB1 or VFB2. Adding a resistor RFB from VFB pin to

GND programs the output voltage:

VOUT = 0.6V •

60.4k + RFB

RFB

Table 1. VFB Resistor Table vs Various Output Voltages

VOUT 0.6V

1.0V

1.2V

1.5V

1.8V

2.5V

3.3V

RFB

Open 90.9k 60.4k 40.2k 30.2k 19.1k 13.3k 8.25k

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 VOUTS1 to the output as

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

APPLICATIONS INFORMATION

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

one programming resistor as shown in Figure 2.

Inparalleloperation,theVFBpinshaveanIFBcurrentof20nA

maximumeachchannel.Toreduceoutputvoltageerrordue

to this current, an additional VOUTS pin can be tied to VOUT,

andanadditionalRFBresistorcanbeusedtolowerthetotal

Thevenin equivalent resistance seen by this current. For

exampleinFigure2,thetotalTheveninequivalentresistance

of the VFB pin is (60.4k//RFB), which is 30.2k where RFB is

equal to 60.4k for a 1.2V output. Four phases connected

in parallel equates to a worse case feedback current of

4 • IFB = 80nA maximum. The voltage error is 80nA • 30.2k

= 2.4mV. If VOUTS2 is connected, as shown in Figure 2, to

VOUT,andanother60.4kresistorisconnectedfromVFB2to

ground, then the voltage error is reduced to 1.2mV. If the

voltage error is acceptable then no additional connections

arenecessary.Theonboard60.4kresistoris0.5%accurate

and the VFB resistor can be chosen by the user to be as

accurate as needed. All COMP pins are tied together for

current sharing between the phases. The TRACK/SS pins

can be tied together and a single soft-start capacitor can

be used to soft-start the regulator. The soft-start equation

willneedtohavethesoft-startcurrentparameterincreased

by the number of paralleled channels. See Output Voltage

Tracking section.

Figure 2. 4-Phase Parallel Configurations

4650A F02

60.4k

TRACK1

TRACK2

VOUT1

VOUTS1

VFB1

VFB2

COMP1

4 PARALLELED OUTPUTS

FOR 1.2V AT 100A

OPTIONAL CONNECTION

COMP2

VOUTS2

VOUT2

60.4k

TRACK1

TRACK2

0.1µF

VOUT1

VOUTS1

VFB1

VFB2

COMP1

COMP2

VOUTS2

VOUT2

60.4k

LTM4650A

RFB

60.4k

OPTIONAL

RFB

60.4k

USE TO LOWER

TOTAL EQUIVALENT

RESISTANCE TO LOWER

IFB VOLTAGE ERROR

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