Datasheet
INH/UVLO
VIN
R
UVLO1
R
UVLO2
AGND
V
IN
( )
( )
-
= W
æ ö
-
+ ´
ç ÷
ç ÷
è ø
UVLO2
ON
3
UVLO1
1.25
R k
V 1.25
0.9 10
R
( )
( )
-
-
= W
´
ON OFF
UVLO1
3
V V
R k
2.9 10
LMZ35003
SNVS988 –JULY 2013
www.ti.com
Undervoltage Lockout (UVLO) Threshold
At turn-on, the V
ON
UVLO threshold determines the input voltage level where the device begins power
conversion. During the power-down sequence, the V
OFF
UVLO threshold determines the input voltage where
power conversion ceases. The turn-on and turn-off thresholds are set by two resistors, R
UVLO1
and R
UVLO2
as
shown in Figure 26.
The V
ON
UVLO threshold must be set to at least (VOUT + 3 V) or 7 V whichever is greater to insure proper start-
up and reduce current surges on the host input supply as the voltage rises. If possible, it is recommended to set
the UVLO threshold to appproximantely 80 to 85% of the minimum expected input voltage.
Use Equation 2 and Equation 3 to calculate the values of R
UVLO1
and R
UVLO2
. V
ON
is the voltage threshold during
power-up when the input voltage is rising. V
OFF
is the voltage threshold during power-down when the input
voltage is decreasing. V
OFF
should be selected to be at least 500mV less than V
ON
. Table 4 lists standard resistor
values for R
UVLO1
and R
UVLO2
for adjusting the V
ON
UVLO threshold for several input voltages.
(2)
(3)
Figure 26. Adjustable VIN UVLO
Table 4. Standard Resistor Values to set V
ON
UVLO Threshold
V
ON
THRESHOLD (V) 6.5 10.0 15.0 20.0 25.0 30.0 35.0 40.0 45.0
R
UVLO1
(kΩ) 174 174 174 174 174 174 174 174 174
R
UVLO2
(kΩ) 40.2 24.3 15.8 11.5 9.09 7.50 6.34 5.62 4.99
Power Good (PWRGD)
The PWRGD pin is an open drain output. Once the output voltage is between 94% and 106% of the set voltage,
the PWRGD pin pull-down is released and the pin floats. The recommended pull-up resistor value is between 10
kΩ and 100 kΩ to a voltage source that is 5.5 V or less. The PWRGD pin is in a defined state once VIN is
greater than 1.0 V, but with reduced current sinking capability. The PWRGD pin achieves full current sinking
capability once the VIN pin is above 4.5V. The PWRGD pin is pulled low when the output voltage is lower than
91% or greater than 109% of the nominal set voltage. Also, the PWRGD pin is pulled low if the input UVLO or
thermal shutdown is asserted, the INH pin is pulled low, or the SS/TR pin is below 1.4 V.
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