THD

R

=

12.9

V

− 2.63

THD

k

Ω

= 2.191

1

+

0.283

V

V

HYS

THD

R

PTH04040W

2

4

6

1

3 5

VI

VI

7

8

UVLOProg

Inhibit

GND

GND

RTHD

C

1,000 F

I

m

PTH04040W

www.ti.com

SLTS238C – SEPTEMBER 2005 – REVISED JUNE 2010

ADJUSTING THE UNDERVOLTAGE LOCKOUT (UVLO) OF THE PTH04040W

The PTH04040W 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 monotonic powerup for the load circuit, and also limits the magnitude of current drawn

from the module’s input source during the power-up sequence.

The UVLO characteristic is defined by the on-threshold (VTHD) and hysterisis (VHYS) voltages. Below the on

threshold, the Inhibit control is overriden, and the module does not produce an output. Hysterisis voltage is the

difference between the on and off threshold voltages. It ensures a clean power-up, even when the input voltage

is rising slowly. The hysterisis prevents start-up oscillations, which can occur if the input voltage droops slightly

when the module begins drawing current from the input source.

UVLO ADJUSTMENT

The UVLO feature of the PTH04040W gives the user the option of adjusting the on-threshold voltage higher than

the default value. This might be desirable if the module is powered from a 5-V input bus. This prevents the

module from operating until the input bus has risen closer to its regulation voltage.

The adjustment method uses the UVLO Prog control (pin 8). If the UVLO Prog pin is left open circuit, the

onthreshold voltage remains at its nominal value of 2.63 V (see electrical specification table). This ensures that

the unadjusted module produces a regulated output when the minimum input voltage is applied. The hysterisis

voltage is approximately 0.62 V, which correlates to an off-threshold voltage of about 2 V. The magnitude of the

hysterisis is automatically set to about 22% of the onthreshold. So if the on-threshold voltage is increased, then

the Hysterisis also increases.

UVLO ADJUSTMENT METHOD

Figure 10 shows the placement of the resistor, RTHD, for adjusting the UVLO on-threshold voltage. It connects

from the UVLO Prog control pin to GND. Equation 2 determines the value of RTHD required to adjust VTHD to a

new value. The default value is 2.63 V, and it can only be adjusted higher. Once the value of RTHD has been set,

Equation 3 is used to determine the new hysterisis voltage.

(2)

(3)

Figure 10. UVLO Program Resistor Placement

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