Datasheet

Tss(ms) Iss( A)
Css(nF) =
Vref (V) 0.8
m´
´
VIN
R1
R2
Node
5.8V
10kohm
ENA
ENA
START ENA
ENA
1
V
R2
V V
V
I
R1 R3
=
-
+ -
TPS54040A
SLVSB58A –MARCH 2012–REVISED JANUARY 2014 www.ti.com
DETAILED DESCRIPTION (continued)
(5)
Do not place a low-impedance voltage source with greater than 5 V directly on the EN pin. Do not place a
capacitor directly on the EN pin if V
EN
> 5 V when using a voltage divider to adjust the start and stop voltage.
The node voltage, (see ) must remain equal to or less than 5.8 V. The zener diode can sink up to 100 µA. The
EN pin voltage can be greater than 5 V if the V
IN
voltage source has a high impedance and does not source
more than 100 µA into the EN pin.
Figure 30. Node Voltage
Slow Start/Tracking Pin (SS/TR)
The TPS54040A effectively uses the lower voltage of the internal voltage reference or the SS/TR pin voltage as
the power-supply's reference voltage and regulates the output accordingly. A capacitor on the SS/TR pin to
ground implements a slow start time. The TPS54040A has an internal pull-up current source of 2 μA that charges
the external slow start capacitor. The calculations for the slow start time (10% to 90%) are shown in Equation 6.
The voltage reference (V
REF
) is 0.8 V and the slow start current (I
SS
) is 2 μA. The slow start capacitor should
remain lower than 0.47 μF and greater than 0.47 nF.
(6)
At power up, the TPS54040A will not start switching until the slow start pin is discharged to less than 40 mV to
ensure a proper power up, see Figure 31.
Also, during normal operation, the TPS54040A will stop switching and the SS/TR must be discharged to 40 mV,
when the VIN UVLO is exceeded, EN pin pulled below 1.25 V, or a thermal shutdown event occurs.
The VSENSE voltage will follow 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% on 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 will ramp linearly until clamped at 1.7 V.
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