TPS54260
SLVSA86 –MARCH 2010
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DETAILED DESCRIPTION (continued)
Vout - 0.8V
æ
ö
R1 = R2 ´
ç
÷
0.8 V
è
ø
(1)
Enable and Adjusting Undervoltage Lockout
The TPS54260 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
0.9mA that provides the default condition of the TPS54260 operating when the EN pin floats. Once the EN pin
voltage exceeds 1.25V, an additional 2.9mA 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.
TPS54260
VIN
Ihys
I1
0.9 mA
R1
2.9 mA
+
R2
EN
-
1.25 V
Figure 27. Adjustable Undervoltage Lockout (UVLO)
V
- V
STOP
START
R1=
I
HYS
(2)
(3)
VENA
R2 =
VSTART - VENA
+ I1
R1
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.
TPS54260
VIN
Ihys
R1
R2
I1
0.9 mA
2.9 mA
+
EN
1.25 V
-
VOUT
R3
Figure 28. Adding Additional Hysteresis
14
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