LTM4650-2 datasheet(17/36 Pages) AD

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LTM4650-2

Rev. 0

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Figure 9. Output Coincident Tracking Waveform

TIME

MAIN OUTPUT

SUBORDINATE OUTPUT

46502 F09

APPLICATIONS INFORMATION

track with the main until it reaches its final value. The

main will continue to its final value from the subordinate’s

regulation point. Voltage tracking is disabled when VTRACK

is more than 0.6V. RTA in Figure 8 will be equal to the RFB

for coincident tracking. Figure 9 shows the coincident

tracking waveforms.

the LTM4650-2 is forced into continuous mode operation

as soon as VFB is below 0.54V, regardless of the setting

on the MODE_PLLIN pin.

Ratiometric tracking can be achieved by a few simple

calculations and the slew rate value applied to the main’s

TRACK pin. As mentioned above, the TRACK pin has a

control range from 0 to 0.6V. The main’s TRACK pin slew

rate is directly equal to the main’s output slew rate in

Volts/Time. The equation:

• 60.4k = RTB

where MR is the main’s output slew rate, and SR is the

subordinate’s output slew rate in Volts/Time. When coin-

cident tracking is desired, then MR and SR are equal. Thus

RTB is equal to the 60.4k. RTA is derived from the equation:

RTA =

0.6V

VFB

60.4k

VFB

RFB

−

VTRACK

RTB

where VFB is the feedback voltage reference of the regula-

tor, and VTRACK is 0.6V. Since RTB is equal to the 60.4k

top feedback resistor of the subordinate regulator in equal

slew rate or coincident tracking, then RTA is equal to RFB

with VFB = VTRACK. Therefore, RTB = 60.4k, and RTA =

60.4k in Figure 8.

In ratiometric tracking, a different slew rate may be

desired for the subordinate regulator. RTB can be solved

when SR is slower than MR. Make sure that the subordi-

nate supply slew rate is chosen to be fast enough so that

the subordinate output voltage will reach its final value

before the main output.

For example, MR = 1.5V/1ms, and SR = 1.2V/1ms. Then

RTB = 76.8k. Solve for RTA to equal to 49.9k.

Each of the TRACK pins will have the 1.3µA 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 TRACK pin of the main can be controlled by a capaci-

tor placed on the main regulator TRACK pin to ground. A

1.3µA current source will charge the TRACK pin up to the

reference voltage and then proceed up to INTVCC. After

the 0.6V ramp, the TRACK pin will no longer be in con-

trol, and the internal voltage reference will control output

regulation from the feedback divider. The foldback current

limit is disabled during this sequence of turn-on during

tracking or soft-starting. The TRACK pins are pulled low

when the RUN pin is below 1.2V. The total soft-start time

can be calculated as:

tSOFT-START =

CSS

1.3µA

• 0.6

Regardless of the mode selected by the MODE_PLLIN

pin, the regulator channels will always start in pulse-

skipping mode up to TRACK = 0.5V. Between TRACK =

0.5V and 0.54V, it will operate in forced continuous mode

and revert to the selected mode once TRACK > 0.54V. To

track with another channel once in steady state operation,

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