Datasheet

dVńdt +
V
OUT
C
OUT
80kW ø R
LOAD
dVńdt +
V
OUT
C
OUT
80kW ø
(
R
1
) R
2
)
ø R
LOAD
TPS731xx
www.ti.com .......................................................................................................................................... SBVS034M –SEPTEMBER 2003–REVISED AUGUST 2009
TRANSIENT RESPONSE After the EN pin is driven low, no bias voltage is
needed on any pin for reverse current blocking. Note
The low open-loop output impedance provided by the
that reverse current is specified as the current flowing
NMOS pass element in a voltage follower
out of the IN pin due to voltage applied on the OUT
configuration allows operation without an output
pin. There will be additional current flowing into the
capacitor for many applications. As with any
OUT pin due to the 80kΩ internal resistor divider to
regulator, the addition of a capacitor (nominal value
ground (see Figure 1 and Figure 2).
1μF) from the output pin (OUT) to ground will reduce
undershoot magnitude but increase its duration. In For the TPS73101, reverse current may flow when
the adjustable version, the addition of a capacitor, V
FB
is more than 1.0V above V
IN
.
C
FB
, from the OUT pin to the FB pin will also improve
the transient response.
THERMAL PROTECTION
The TPS731xx does not have active pull-down when
Thermal protection disables the output when the
the output is over-voltage. This allows applications
junction temperature rises to approximately +160°C,
that connect higher voltage sources, such as
allowing the device to cool. When the junction
alternate power supplies, to the output. This also
temperature cools to approximately +140°C, the
results in an output overshoot of several percent if the
output circuitry is again enabled. Depending on power
load current quickly drops to zero when a capacitor is
dissipation, thermal resistance, and ambient
connected to the output. The duration of overshoot
temperature, the thermal protection circuit may cycle
can be reduced by adding a load resistor. The
on and off. This limits the dissipation of the regulator,
overshoot decays at a rate determined by output
protecting it from damage due to overheating.
capacitor C
OUT
and the internal/external load
Any tendency to activate the thermal protection circuit
resistance. The rate of decay is given by:
indicates excessive power dissipation or an
(Fixed voltage version)
inadequate heatsink. For reliable operation, junction
temperature should be limited to +125°C maximum.
To estimate the margin of safety in a complete design
(4)
(including heatsink), increase the ambient
temperature until the thermal protection is triggered;
(Adjustable voltage version)
use worst-case loads and signal conditions. For good
reliability, thermal protection should trigger at least
+35°C above the maximum expected ambient
(5)
condition of your application. This produces a
worst-case junction temperature of +125°C at the
REVERSE CURRENT
highest expected ambient temperature and
worst-case load.
The NMOS pass element of the TPS731xx provides
inherent protection against current flow from the
The internal protection circuitry of the TPS731xx has
output of the regulator to the input when the gate of
been designed to protect against overload conditions.
the pass device is pulled low. To ensure that all
It was not intended to replace proper heatsinking.
charge is removed from the gate of the pass element,
Continuously running the TPS731xx into thermal
the EN pin must be driven low before the input
shutdown degrades device reliability.
voltage is removed. If this is not done, the pass
element may be left on due to stored charge on the
gate.
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