LTC6902 datasheet(13/28 Pages) LINER

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LTM4632

4632fc

For more information www.linear.com/LTM4632

APPLICATIONS INFORMATION

Application Note 77 provides a detailed explanation of

multiphase operation. The input RMS ripple current can-

cellation mathematical derivations are presented, and

a graph is displayed representing the RMS ripple cur-

rent reduction as a function of the number of interleaved

phases. Figure 4 shows this graph.

Channel 1 Output Voltage Tracking and Soft-Start

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

the Channel 1 regulator or track it to a different power

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

the ramp rate of the channel 1 output voltage. An internal

1.2µA 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 as:

tSS = 0.6 •

CSS

1.2µA

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

Forced continuous mode are disabled during the soft-

start process.

Channel 1 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 5 and

Figure 6 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 the following equation during the start-up.

VOUT(SL) •

RFB(SL)

RFB(SL) +60.4k

VOUT(MA) •

RTR(BOT)

RTR(TOP) +RTR(BOT)

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 6.

Following the upper equation, the master’s output slew

rate (MR) and the slave’s output slew rate (SR) in Volts/

Time is determined by:

RFB(SL)

RFB(SL) +60.4k

RTR(BOT)

RTR(TOP) +RTR(BOT)

For example, VOUT(MA) = 1.5V, MR = 1.5V/1ms and

VOUT(SL) = 1.2V, SR = 1.2V/1ms. From the equation, we

could solve out that RTR(TOP) = 60.4k and RTR(BOT) = 40.2k

is a good combination for the Ratiometric tracking.

TIME

SLAVE OUTPUT

MASTER OUTPUT

4632 F05

Figure 5. Output Ratiometric Tracking Waveform

The TRACK pins will have the 1.2µA current source on

when a resistive divider is used to implement tracking on

that specific channel. This will impose an offset on the

TRACK pin input. Smaller values resistors with the same

ratios as the resistor values calculated from the above

equation can be used. For example, where the 60.4k is

used then a 6.04k can be used to reduce the TRACK pin

offset to a negligible value.

The Coincident output tracking can be recognized as a

special Ratiometric output tracking which the master’s

output slew rate (MR) is the same as the slave’s output

slew rate (SR), as waveform shown in Figure 7.

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