Datasheet

Tss(mS) Iss( A)
Css(nF) =
Vref(V)
´ m
-
×
- + × ´
6
STOP
1.18 R1
R2 =
V 1.18 R1 3.2 10
-
× -
´
START STOP
6
0.944 V V
R1 =
2.59 10
EN
i
1
i
hys
VIN
+
-
TPS54418
R1
R2
0.6 mA
2.55 mA
VSENSE
V
O
+
-
TPS54418
R1
R2
0.8V
TPS54418
SLVS946C –MAY 2009–REVISED JULY 2013 www.ti.com
Figure 23. Voltage Divider Circuit
ENABLE AND ADJUSTING UNDER-VOLTAGE LOCKOUT
The TPS54418 is disabled when the VIN pin voltage falls below 2.6 V. If an application requires a higher under-
voltage lockout (UVLO), use the EN pin as shown in Figure 24 to adjust the input voltage UVLO by using two
external resistors. It is recommended to use the enable resistors to set the UVLO falling threshold (V
STOP
) above
2.7 V. The rising threshold (V
START
) should be set to provide enough hysteresis to allow for any input supply
variations. The EN pin has an internal pull-up current source that provides the default condition of the TPS54418
operating when the EN pin floats. Once the EN pin voltage exceeds 1.25 V, an additional 2.55 μA of hysteresis is
added. When the EN pin is pulled below 1.18 V, the 2.55 μA is removed. This additional current facilitates input
voltage hysteresis.
Figure 24. Adjustable Under Voltage Lock Out
(2)
(3)
SLOW START PIN
The TPS54418 regulates to the lower of the SS pin and the internal reference voltage. A capacitor on the SS pin
to ground implements a slow start-time. The TPS54418 has an internal pull-up current source of 1.8μA which
charges the external slow start capacitor. Equation 4 calculates the required slow start capacitor value where Tss
is the desired slow start time in ms, Iss is the internal slow start charging current of 1.8 μA, and Vref is the
internal voltage reference of 0.8 V. TI recommends keeping the soft-start time in the range of 1 to 10 ms.
(4)
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