Data Sheet

LTM4640

analog.com

Rev. 0

19 of 38

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 programs the ramp rate of the output voltage. An internal 6µA current source

charges up the external soft start capacitor towards INTVCC voltage. When the TRACK/SS voltage is below 0.6V, it

takes over the internal 0.6V reference voltage to control the output voltage. The total soft start time is calculated

with Equation 6.

������������������ = 0.6������ ×

������������������

6µ������

(6)

where CSS is the capacitance on the TRACK/SS pin. Current foldback and FCM are disabled during the soft

start process.

Output voltage tracking can also be externally programmed using the TRACK/SS pin. The output is tracked up and

down with another regulator. Figure 21 and Figure 22 show examples of a waveform and the schematic of ratio

metric tracking where the subordinate regulator’s output slew rate is proportional to the main device.

Figure 21. Output Ratio Metric Tracking Waveform

Figure 22. Example Schematic of Ratio Metric Output Voltage Tracking

TIME

SUBORDINATE OUTPUT

MAIN OUTPUT

RFB(MAIN)

6.65kΩ

VOUT(MAIN)

1.5V, 20A

RTR(BOT)

6.65kΩ

RTR(TOP)

10kΩ

RFB(SUB)

10kΩ

VOUT(SUB)

1.2V, 20A

VIN

RUN

INTVCC

MODE/CLKIN

COMPa

COMPb

LTM4640 VOUT

VOSNS

+

2 × 22µF

25V

2 × 22µF

25V

4 × 100µF

6.3V

VIN

3.1V TO 20V

FB

GND

VOSNS

–

TRACK/SS

VIN

RUN

INTVCC

MODE/CLKIN

COMPa

COMPb

LTM4640 VOUT

VOSNS

+

4 × 100µF

6.3V

FB

GND

VOSNS

–

TRACK/SS