LTM4680 datasheet(11/36 Pages) AD

11 / 36 page

LTM4650-2

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

either VFB1 or VFB2. Adding a resistor RFB from the VFB

pin to GND programs the output voltage:

VOUT = 0.6V •

60.4k + RFB

RFB

Table 1. VFB Resistor vs Various Output Voltages

VOUT

0.6V

0.8V

0.9V

1.0V

1.2V

1.5V

1.8V

RFB

Open

182k

121k

90.9k

60.4k

40.2k

30.2k

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 4, 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 4.

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 4, to

VOUT, and another 60.4k resistor is connected from VFB2to

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

voltage error is acceptable, then no additional connections

are necessary. The onboard 60.4k resistor is 0.5% accu-

rate, 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

must have the soft-start current parameter increased by

the number of paralleled channels. See the Output Voltage

Tracking section.

Input Capacitors

The LTM4650-2 module should be connected to a low

AC-impedance DC source. For the regulator input, two

22µF input ceramic capacitors are required for each

channel for RMS ripple current. A 47µF to 100µF surface

mount aluminum electrolytic bulk capacitor can be used

for more input bulk capacitance. This bulk input capacitor

is only needed if the input source impedance is com-

promised 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:

D =

VOUT

VIN

Without considering the inductor current ripple, for each

output, the RMS current of the input capacitor can be

estimated as:

ICIN(RMS) =

IOUT(MAX)

η%

• D • 1− D

(

)

In the above equation, η% is the estimated efficiency of

the power module. The bulk capacitor can be a switcher-

rated electrolytic aluminum capacitor, Polymer capacitor.

Figure 4. 4-Phase Parallel Configurations

46502 F04

60.4k

TRACK1

TRACK2

VOUT1

VOUTS1

VFB1

VFB2

COMP1

4 PARALLELED OUTPUTS

FOR 1.2V, 100A

OPTIONAL CONNECTION

COMP2

VOUTS2

VOUT2

60.4k

TRACK1

TRACK2

0.1µF

VOUT1

VOUTS1

VFB1

VFB2

COMP1

COMP2

VOUTS2

VOUT2

60.4k

LTM4650-2

CTH

RFB

60.4k

OPTIONAL

RFB

60.4k

USE TO LOWER

TOTAL EQUIVALENT

RESISTANCE TO LOWER

IFB VOLTAGE ERROR

RTH

In parallel operation, the VFB pins have an IFB current

of 20nA maximum for each channel. To reduce output

voltage error due to this current, an additional VOUTS

pin can be tied to VOUT, and an additional RFB resis-

tor can be used to lower the total Thevenin equivalent

resistance seen by this current. For example in Figure 4,

the total Thevenin equivalent resistance of the VFB pin

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