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LTM4648

4648f

For more information www.linear.com/LTM4648

applicaTions inForMaTion

Soft-Start

The TRACK/SS pin of the master can be controlled by a

capacitor placed on the master regulator TRACK/SS pin

to ground. A 1.2µA current source will charge the TRACK/

SS pin up to the reference voltage and then proceed up

to INTVCC. After the 0.6V ramp, the TRACK/SS pin will no

longer be in control, and the internal voltage reference

will control output regulation from the feedback divider.

Foldback current limit is disabled during this sequence of

turn-on during tracking or soft-starting. The TRACK/SS

pins are pulled low when the RUN pin is below 1.2V. The

total soft-start time can be calculated as:

tSS =

CSS

1.2µA

⎛

⎝⎜

⎞

⎠⎟

• 0.6

Regardless of the mode selected by the MODE pin, the

regulator channels will always start in pulse-skipping

mode up to TRACK/SS = 0.5V. Between TRACK/SS = 0.5V

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

revert to the selected mode once TRACK/SS > 0.54V. In

order to track with another channel once in steady state

operation, the LTM4648 is forced into continuous mode

operation as soon as VFB is below 0.54V regardless of the

setting on the MODE pin.

Output Voltage Tracking

Output voltage tracking can be programmed externally

using the TRACK/SS pins. The output can be tracked up

and down with another regulator. The master regulator’s

output is divided down with an external resistor divider

that is the same as the slave regulator’s feedback divider

to implement coincident tracking. The LTM4648 uses an

accurate 60.4k resistor internally for the top feedback

resistor for each channel. Figure 6 shows an example of

coincident tracking. Equations:

VSLAVE = 1+

10k

RTA

⎛

⎝⎜

⎞

⎠⎟

• VTRACK

VTRACK is the track ramp applied to the slave’s track pin.

VTRACK has a control range of 0V to 0.6V, or the internal

reference voltage. When the master’s output is divided

down with the same resistor values used to set the slave’s

output, then the slave will coincident track with the master

until it reaches its final value. The master will continue to

its final value from the slave’s regulation point. Voltage

tracking is disabled when VTRACK is more than 0.6V. RTA

in Figure 5 will be equal to the RFB for coincident tracking.

Figure 6 shows the coincident tracking waveforms.

Ratiometric tracking can be achieved by a few simple cal-

culations and the slew rate value applied to the master’s

TRACK/SS pin. As mentioned above, the TRACK/SS pin

has a control range from 0V to 0.6V. The master’s TRACK/

SS pin slew rate is directly equal to the master’s output

slew rate in Volts/Time. The equation:

• 10k = RTB

where MR is the master’s output slew rate and SR is the

slave’s output slew rate in Volts/Time. When coincident

tracking is desired, then MR and SR are equal, thus RTB

is equal the 10k. RTA is derived from equation:

RTA =

0.6V

VFB

10k

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 10k top

feedback resistor of the slave regulator in equal slew rate

or coincident tracking, then RTA is equal to RFB with VFB

= VTRACK. Therefore RTB = 10k, and RTA = 10k in Figure 4.

In ratiometric tracking, a different slew rate maybe desired

for the slave regulator. RTB can be solved for when SR

is slower than MR. Make sure that the slave supply slew

rate is chosen to be fast enough so that the slave output

voltage will reach it final value before the master output.

Each of the TRACK/SS pins will have the 1.2µA current

source on when a resistive divider is used to implement

trackingonthatspecificchannel. Thiswillimposeanoffset

on the TRACK/SS 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 10k

is used then a 1.0k can be used to reduce the TRACK/SS

pin offset to a negligible value.

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