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SBVS037I – AUGUST 2003 – REVISED MAY 2006
(Adjustable voltage version)
V
OUT
dV dt
+
C
OUT
80kW
ø
(R
1
)
R
2
)
ø
R
LOAD
(5)
REVERSE CURRENT
The NMOS pass element of the TPS732xx provides
inherent protection against current flow from the
output of the regulator to the input when the gate of
the pass device is pulled low. To ensure that all
charge is removed from the gate of the pass
element, the enable pin must be driven low before
the input voltage is removed. If this is not done, the
pass element may be left on due to stored charge on
the gate.
After the enable pin is driven low, no bias voltage is
needed on any pin for reverse current blocking. Note
that reverse current is specified as the current
flowing out of the IN pin due to voltage applied on
the OUT pin. There will be additional current flowing
into the OUT pin due to the 80kΩ internal resistor
divider to ground (see
and
For the TPS73201, reverse current may flow when
V
FB
is more than 1.0V above V
IN
.
35°C above the maximum expected ambient
condition of your application. This produces a
worst-case junction temperature of 125°C at the
highest expected ambient temperature and
worst-case load.
The internal protection circuitry of the TPS732xx has
been designed to protect against overload
conditions. It was not intended to replace proper
heatsinking. Continuously running the TPS732xx into
thermal shutdown will degrade device reliability.
POWER DISSIPATION
The ability to remove heat from the die is different for
each
package
type,
presenting
different
considerations in the PCB layout. The PCB area
around the device that is free of other components
moves the heat from the device to the ambient air.
Performance data for JEDEC low- and high-K boards
are shown in the Power Dissipation Ratings table.
Using heavier copper will increase the effectiveness
in removing heat from the device. The addition of
plated through-holes to heat-dissipating layers will
also improve the heat-sink effectiveness.
Power dissipation depends on input voltage and load
conditions. Power dissipation is equal to the product
of the output current times the voltage drop across
the output pass element (V
IN
to V
OUT
):
P
D
+
(V
IN
*
V
OUT
) I
OUT
(6)
Power dissipation can be minimized by using the
lowest possible input voltage necessary to assure
the required output voltage.
THERMAL PROTECTION
Thermal protection disables the output when the
junction temperature rises to approximately 160°C,
allowing the device to cool. When the junction
temperature cools to approximately 140°C, the
output circuitry is again enabled. Depending on
power dissipation, thermal resistance, and ambient
temperature, the thermal protection circuit may cycle
on and off. This limits the dissipation of the regulator,
protecting it from damage due to overheating.
Any tendency to activate the thermal protection
circuit indicates excessive power dissipation or an
inadequate heatsink. For reliable operation, junction
temperature should be limited to 125°C maximum.
To estimate the margin of safety in a complete
design (including heatsink), increase the ambient
temperature until the thermal protection is triggered;
use worst-case loads and signal conditions. For good
reliability, thermal protection should trigger at least
Package Mounting
Solder pad footprint recommendations for the
TPS732xx are presented in Application Bulletin
Solder Pad Recommendations for Surface-Mount
Devices
(AB-132), available from the Texas
Instruments web site at www.ti.com.
13
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