ADJUSTING THE UNDERVOLTAGE LOCKOUT (UVLO)

UVLO

THD

250

R

(k

)

V

7.3

=

W

-

(4)

V

I

GND

PTH08T255W

6

7

8

9

12

13

GND

GND

GND

C

I

+

UDG-08151

14

15

V

I

V

I

V

I

GND

V

I

21

Inhibit/UVLO

R

UVLO

PTH08T255W

The PTH08T255W power modules incorporate an input undervoltage lockout (UVLO). The UVLO feature

prevents the operation of the module until there is sufficient input voltage to produce a valid output voltage. This

enables the module to provide a clean, monotonic powerup for the load circuit, and also limits the magnitude of

current drawn from the regulator’s input source during the power-up sequence.

control is overridden, and the module does not produce an output. The hysteresis voltage, which is the difference

between the ON and OFF threshold voltages, is set at 500 mV. The hysteresis prevents start-up oscillations,

which can occur if the input voltage droops slightly when the module begins drawing current from the input

source.

The UVLO feature of the PTH08T255W module allows for limited adjustment of the ON threshold voltage. The

adjustment is made via the Inhbit/UVLO Prog control pin (pin 11) using a single resistor (see Figure 18). When

pin 11 is left open circuit, the ON threshold voltage is internally set to its default value, which is 7.2 volts. The ON

threshold might need to be raised if the module is powered from a tightly regulated 12-V bus. Adjusting the

threshold prevents the module from operating if the input bus fails to completely rise to its specified regulation

voltage.

it may only be adjusted to a higher value.

8.0

8.5

9.0

9.5

10.0

10.5

11.0

357

210

147

113

93.1

78.7

68.1

Figure 18. Undervoltage Lockout Adjustment Resistor Placement

Copyright © 2008, Texas Instruments Incorporated

23

Product Folder Link(s): PTH08T255W