LTM4644 datasheet(11/24 Pages) AD

11 / 24 page

LTM4705

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

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

a graph is displayed representing the RMS ripple cur-

rent reduction as a function of the number of interleaved

phases. Figure 3 shows this graph.

Soft-Start and Output Voltage Tracking

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

the regulator 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 1.4µA current 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 by Equation 8.

tSS = 0.6 •

CSS

1.4µA

(8)

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

Current foldback and force continuous mode are disabled

during the soft-start process.

Output voltage tracking can also be programmed

externally using the TRACK/SS pin. 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 output

slew rate is proportional to the master’s.

Since the slave regulator’s TRACK/SS is connected to

the master’s output through a RTR(TOP) /RTR(BOT) resistor

divider and its voltage used to regulate the slave output

voltage when TRACK/SS voltage is below 0.6V, the slave

output voltage and the master output voltage should sat-

isfy Equation 9 during the start-up.

VOUT(SL) •

RFB(SL)

RFB(SL) +60.4k

VOUT(MA) •

RTR(TOP)

RTR(TOP) +RTR(BOT)

(9)

The RFB(SL) is the feedback resistor and the RTR(TOP)/

RTR(BOT) is the resistor divider on the TRACK/SS pin of

the slave regulator, as shown in Figure 5.

0.75 0.8

4705 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

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