Soft-Start Power Up

Track

V

I

GND

PTH08T255W

6

20

7

8

9

12

13

GND

GND

GND

C

I

+

UDG-08147

14

15

V

I

V

I

V

I

GND

V

I

V

I

(5 V/div)

t – Time – 10 ms/div

V

O

(2 V/div)

I

I

(5 A/div)

Differential Output Voltage Remote Sense

PTH08T255W

The Auto-Track feature allows the power-up of multiple PTH/PTV modules to be directly controlled from the

Track pin. However in a stand-alone configuration, or when the Auto-Track feature is not being used, the Track

Figure 10. Defeating the Auto-Track Function

Figure 11. Power-Up Waveform

When the Track pin is connected to the input voltage the Auto-Track function is permanently disengaged. This

allows the module to power up entirely under the control of its internal soft-start circuitry. When power up is

under soft-start control, the output voltage rises to the set-point at a quicker and more linear rate.

From the moment a valid input voltage is applied, the soft-start control introduces a short time delay (typically

10 ms–15 ms) before allowing the output voltage to rise.

The output then progressively rises to the module’s setpoint voltage. Figure 11 shows the soft-start power-up

characteristic of the PTH08T255W operating from a 12-V input bus and configured for a 5.0-V output. The

waveforms were measured with a 25-A constant current load and the Auto-Track feature disabled. The initial rise

in input current when the input voltage first starts to rise is the charge current drawn by the input capacitors.

Power-up is complete within 30 ms.

Differential remote sense improves the load regulation performance of the module by allowing it to compensate

for any IR voltage drop between its output and the load in either the positive or return path. An IR drop is caused

by the output current flowing through the small amount of pin and trace resistance. With the sense pins

measured at the Sense pins, is the amount of IR drop being compensated by the regulator. This should be

limited to a maximum of 0.3V.

(pin 16) to the module GND (pin 13).

The remote sense feature is not designed to compensate for the forward drop of nonlinear or frequency

dependent components that may be placed in series with the converter output. Examples include OR-ing

diodes, filter inductors, ferrite beads, and fuses. When these components are enclosed by the remote sense

connection they are effectively placed inside the regulation control loop, which can adversely affect the

stability of the regulator.

Copyright © 2008, Texas Instruments Incorporated

17

Product Folder Link(s): PTH08T255W