Datasheet

dV
dT
+
V
OUT
C
OUT
80kW ø
(
R
1
) R
2
)
ø R
LOAD
dV
dT
+
V
OUT
C
OUT
80kW ø R
LOAD
TPS737xx
SBVS067P –JANUARY 2006–REVISED JULY 2013 www.ti.com
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 because of voltage applied on the
configuration allows operation without a 1.0µF output
OUT pin. There will be additional current flowing into
capacitor. As with any regulator, the addition of
the OUT pin as a result of the 80kΩ internal resistor
additional capacitance from the OUT pin to ground
divider to ground (see Figure 1 and Figure 2).
reduces undershoot magnitude but increases its
duration. In the adjustable version, the addition of a For the TPS73701, reverse current may flow when
capacitor, C
FB
, from the OUT pin to the FB pin will V
FB
is more than 1.0V above V
IN
.
also improve the transient response.
THERMAL PROTECTION
The TPS737xx does not have active pull-down when
the output is over-voltage. This architecture allows
Thermal protection disables the output when the
applications that connect higher voltage sources,
junction temperature rises to approximately +160°C,
such as alternate power supplies, to the output. This
allowing the device to cool. When the junction
architecture also results in an output overshoot of
temperature cools to approximately +140°C, the
several percent if the load current quickly drops to
output circuitry is again enabled. Depending on power
zero when a capacitor is connected to the output. The
dissipation, thermal resistance, and ambient
duration of overshoot can be reduced by adding a
temperature, the thermal protection circuit may cycle
load resistor. The overshoot decays at a rate
on and off. This cycling limits the dissipation of the
determined by output capacitor C
OUT
and the
regulator, protecting it from damage due to
internal/external load resistance. The rate of decay is
overheating.
given by:
Any tendency to activate the thermal protection circuit
(Fixed voltage version)
indicates excessive power dissipation or an
inadequate heatsink. For reliable operation, junction
temperature should be limited to +125°C maximum.
(4)
To estimate the margin of safety in a complete design
(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 highest
REVERSE CURRENT
expected ambient temperature and worst-case load.
The NMOS pass element of the TPS737xx provides
The internal protection circuitry of the TPS737xx has
inherent protection against current flow from the
been designed to protect against overload conditions.
output of the regulator to the input when the gate of
It was not intended to replace proper heatsinking.
the pass device is pulled low. To ensure that all
Continuously running the TPS737xx into thermal
charge is removed from the gate of the pass element,
shutdown degrades device reliability.
the EN pin must be driven low before the input
voltage is removed. If this is not done, the pass
element may be left on because of stored charge on
the gate.
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