Datasheet

EN
VIN
+
–
TPS5401
R1
R2
I
HYS
I
0.9 A
1
m
1.25 V
2.9 Am
START STOP
HYS
V V
R1
I
-
=
ENA
START ENA
1
V
R2
V V
I
R1
=
-
+
ss ss
ss
ref
t (ms) I ( A)
C (nF) =
V (V) 0.8
m´
´
TPS5401
www.ti.com SLVSAB0 –DECEMBER 2010
DETAILED DESCRIPTION (continued)
Enabling and Adjusting Undervoltage Lockout
The TPS5401 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 28 to adjust the input voltage UVLO by using
the two external resistors. Though it is not necessary to use the UVLO adjust resistors, for operation it is highly
recommended to provide consistent power-up behavior. The EN pin has an internal pullup current source, I
1
, of
0.9 mA that provides the default condition of the TPS5401 operating when the EN pin floats. Once the EN pin
voltage exceeds the enable threshold voltage (V
ENA
) of 1.25 V, an additional 2.9 mA of hysteresis, I
HYS
, 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.
Figure 28. Adjustable Undervoltage Lockout (UVLO)
(2)
(3)
Slow-Start/Tracking Pin (SS/TR)
The TPS5401 effectively uses the lower voltage of the internal voltage reference or the SS/TR pin voltage as the
power-supply reference voltage and regulates the output accordingly. A capacitor on the SS/TR pin to ground
implements a slow-start time. The TPS5401 has an internal pullup current source of 2 mA that charges the
external slow-start capacitor. The calculations for the slow-start time (10% to 90%) are shown in Equation 4. The
voltage reference (V
ref
) is 0.8 V and the slow-start current (I
SS
) is 2 mA. The slow-start capacitor should remain
lower than 0.47 mF and greater than 0.47 nF.
(4)
At power up, the TPS5401 does not start switching until the slow-start pin is discharged to less than 40 mV to
ensure a proper power up; see Figure 29.
Also, during normal operation, the TPS5401 stops switching and SS/TR must be discharged to 40 mV when the
VIN UVLO is exceeded, the EN pin is pulled below 1.25 V, or a thermal shutdown event occurs.
The VSENSE voltage follows the SS/TR pin voltage with a 45-mV offset up to 85% of the internal voltage
reference. When the SS/TR voltage is greater than 85% of the internal reference voltage, the offset increases as
the effective system reference transitions from the SS/TR voltage to the internal voltage reference (see
Figure 23). The SS/TR voltage ramps linearly until clamped at 1.7 V.
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