Datasheet
UVLO
I
230
R (k )
V 4.6
= W
-
V
I
GND
PTH08T250W
6
7
8 9 12 13
GNDGNDGND
C
I
+
UDG-07103
14
15
V
I
V
I
V
I
GND
V
I
21 Inhibit/UVLO
R
UVLO
PTH08T250W
SLTS278G –NOVEMBER 2010–REVISED MARCH 2009
www.ti.com
ADJUSTING THE UNDERVOLTAGE LOCKOUT (UVLO)
The PTH08T250W 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.
The UVLO characteristic is defined by the ON threshold (V
THD
) voltage. Below the ON threshold, the Inhibit
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 PTH08T250W 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 27). When
pin 11 is left open circuit, the ON threshold voltage is internally set to its default value, which is 4.3 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.
Equation 4 determines the value of R
UVLO
required to adjust V
THD
to a new value. The default value is 4.3 V, and
it may only be adjusted to a higher value.
(4)
Table 6 lists the standard resistor values for R
UVLO
for different values of the on-threshold (V
THD
) voltage.
Table 6. Standard R
UVLO
values for Various V
THD
values
V
THD
(V) 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0 10.5 11.0
R
UVLO
(kΩ) 255 165 121 95.3 78.7 68.1 59.0 52.3 46.4 42.2 39.2 35.7
Figure 27. Undervoltage Lockout Adjustment Resistor Placement
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