Datasheet

Tss(mS) Iss( A)
Css(nF) =
Vref(V)
´ m
( )
´
- + +
ENFALLING
STOP ENFALLING p h
R1 V
R2 =
V V R1 I I
1
æ ö
-
ç ÷
è ø
æ ö
- +
ç ÷
è ø
ENFALLING
START STOP
ENRISING
ENFALLING
p h
ENRISING
V
V V
V
R1 =
V
I I
V
EN
i
1
i
hys
VIN
+
–
TPS54618
R1
R2
1.9 mA
1.6 mA
TPS54618
www.ti.com SLVSAE9D –NOVEMBER 2010–REVISED DECEMBER 2013
ENABLE AND ADJUSTING UNDER-VOLTAGE LOCKOUT
The TPS54618 is disabled when the VIN pin voltage falls below 2.28 V. If an application requires a higher under-
voltage lockout (UVLO), use the EN pin as shown in Figure 26 to adjust the input voltage UVLO by using two
external resistors. It is recommended to use the EN resistors to set the UVLO falling threshold (V
STOP
) above
2.6V. 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 TPS54618
operating when the EN pin floats. Once the EN pin voltage exceeds 1.25 V, an additional 1.6 μA of hysteresis is
added. When the EN pin is pulled below 1.18 V, the 1.6 μA is removed. This additional current facilitates input
voltage hysteresis.
Figure 26. Adjustable Under Voltage Lock Out
(2)
vertical spacer
(3)
Where R1 and R2 are in ohms, I
h
= 1.6uA, I
p
= 1.9uA, V
ENRISING
= 1.25V, V
ENFALLING
= 1.18V
SLOW START / TRACKING PIN
The TPS54618 regulates to the lower of the SS/TR pin and the internal reference voltage. A capacitor on the
SS/TR pin to ground implements a slow start time. The TPS54618 has an internal pull-up current source of 2 μA
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 2 μA, and Vref is
the internal voltage reference of 0.799 V.
vertical spacer
(4)
If during normal operation, the VIN goes below the UVLO, EN pin pulled below 1.2 V, or a thermal shutdown
event occurs, the TPS54618 stops switching. When the VIN goes above UVLO, EN is released or pulled high, or
a thermal shutdown is exited, then SS/TR is discharged to below 40 mV before reinitiating a powering up
sequence. The VSENSE voltage will follow the SS/TR pin voltage with a 54mV offset up to 85% of the internal
voltage reference. When the SS/TR voltage is greater than 85% on the internal reference voltage the offset
increases as the effective system reference transitions from the SS/TR voltage to the internal voltage reference.
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