Datasheet

EN
Ihys
VIN
+
-
TPS54240
R1
R2
VOUT
R3
I1
0.9 Am
2.9 Am
1.25V
ENA
START ENA
1
V
R2
V V
I
R1
=
-
+
START STOP
HYS
V V
R1
I
-
=
EN
VIN
+
-
TPS54240
R1
R2
Ihys
I1
0.9 Am
1.25V
2.9 Am
Vout 0.8V
R1 = R2
0.8 V
-
æ ö
´
ç ÷
è ø
TPS54240
SLVSAA6A –APRIL 2010–REVISED DECEMBER 2012 www.ti.com
DETAILED DESCRIPTION (continued)
(1)
Enable and Adjusting Undervoltage Lockout
The TPS54240 is disabled when the VIN pin voltage falls below 2.5 V. If an application requires a higher
undervoltage lockout (UVLO), use the EN pin as shown in Figure 27 to adjust the input voltage UVLO by using
the two external resistors. Though it is not necessary to use the UVLO adjust registers, for operation it is highly
recommended to provide consistent power up behavior. The EN pin has an internal pull-up current source, I1, of
0.9μA that provides the default condition of the TPS54240 operating when the EN pin floats. Once the EN pin
voltage exceeds 1.25V, an additional 2.9μA of hysteresis, Ihys, is added. This additional current facilitates input
voltage hysteresis. Use Equation 2 to set the external hysteresis for the input voltage. Use Equation 3 to set the
input start voltage.
spacer
Figure 27. Adjustable Undervoltage Lockout (UVLO)
(2)
(3)
Another technique to add input voltage hysteresis is shown in Figure 28. This method may be used, if the
resistance values are high from the previous method and a wider voltage hysteresis is needed. The resistor R3
sources additional hysteresis current into the EN pin.
spacer
Figure 28. Adding Additional Hysteresis
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