t-Time-4ms/div

V

(1V/div)

O

I

(2 A/div)

O

V

(1V/div)

IN

ASIC

+

+

V

o

=2.5V

3.3V

V

I

=5V

R

2.37kW

SET

VCORE

VCCIO

I

o

PTH08T220W

Track

V

I

V

O

GND

Inhibit

Vadj

+Sense

C

I

+

C

O

-Sense

PTH08T255W

The prebias start-up feature is not compatible with Auto-Track. When the module is under Auto-Track control, it

sinks current if the output voltage is below that of a back-feeding source. To ensure a pre-bias hold-off one of

two approaches must be followed when input power is applied to the module. The Auto-Track function must

ensures that the Track pin voltage is above the set-point voltage at start up.

1. Startup includes the short delay (approximately 10 ms) prior to the output voltage rising, followed by the rise

of the output voltage under the module’s internal soft-start control. Startup is complete when the output

voltage has risen to either the set-point voltage or the voltage at the Track pin, whichever is lowest.

2. To ensure that the regulator does not sink current when power is first applied (even with a ground signal

applied to the Inhibit control pin), the input voltage must always be greater than the output voltage throughout

the power-up and power-down sequence.

3. The Auto-Track function can be disabled at power up by immediately applying a voltage to the module’s

Track pin that is greater than its set-point voltage. This can be easily accomplished by connecting the Track

Figure 23. Prebias Startup Waveforms

Figure 24. Application Circuit Demonstrating Prebias Startup

28

Copyright © 2008, Texas Instruments Incorporated

Product Folder Link(s): PTH08T255W