LTM4650A datasheet(18/36 Pages) AD

18 / 36 page

LTM4671

Rev. B

For more information www.analog.com

APPLICATIONS INFORMATION

Figure 3. Input RMS Current Ratios to DC Load Current as a Function of Duty Cycle

0.75 0.8

4671 F03

0.7

0.65

0.6

0.55

0.5

0.45

0.4

0.35

0.3

0.25

0.2

0.15

0.1

0.85 0.9

DUTY FACTOR (VOUT/VIN)

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

1-PHASE

2-PHASE

3-PHASE

4-PHASE

6-PHASE

4671 F04

TIME

VOUT0 = 1.0V

VOUT3 = 1.2V

VOUT2 = 2.5V

VOUT1 = 3.3V

Figure 4. Output Ratiometric Tracking Waveform

SOFT-START AND OUTPUT VOLTAGE TRACKING

The TRACK/SS pin provides a means to either soft-start

of each regulator channel or track it to a different power

supply. A capacitor on the TRACK/SS pin will program the

ramp rate of the output voltage. An internal soft-start cur-

rent source will charge up the external soft-start capacitor

towards INTVCC voltage. When the TRACK/SS voltage is

below 0.6V, it will take over the internal 0.6V reference

voltage to control the output voltage. The total soft-start

time can be calculated as:

tSS = 0.6 •

CSS

ISS

where CSS is the capacitance on the TRACK/SS pin and

the ISS is the soft-start current which equals 6µA for the

12A output channels (CH0, CH3) and 1.4µA for the 5A

output channels (CH1, CH2).

Outputvoltagetrackingcanalsobeprogrammedexternally

using the TRACK/SS pin of each regulator channel. The

output can be tracked up and down with another regulator.

Figure 4 and Figure 5 show an example waveform and

schematic of a ratiometric tracking where the slave

regulator’s (VOUT2, VOUT3 and VOUT0) output slew rate is

proportional to the master’s (VOUT1).

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