Datasheet

Regulator Protection
Power Dissipation and Junction Temperature
P =
D(max)
R
qJA
T
J(max)
T-
A
1
deratingfactor
from the dissipation rating tables
P =(V V )xI-
D IN OUT OUT
TPS775xx, TPS776xx
www.ti.com .............................................................................................................................................. SLVS232J – SEPTEMBER 1999 – REVISED MARCH 2009
The TPS775xx and TPS776xx PMOS-pass transistors have a built-in back diode that conducts reverse currents
when the input voltage drops below the output voltage (for example, during power down). Current is conducted
from the output to the input and is not internally limited. When extended reverse voltage is anticipated, external
limiting may be appropriate.
The TPS775xx and TPS776xx also feature internal current limiting and thermal protection. During normal
operation, the TPS775xx and TPS776xx limit output current to approximately 1.7A. When current limiting
engages, the output voltage scales back linearly until the overcurrent condition ends. While current limiting is
designed to prevent gross device failure, care should be taken not to exceed the power dissipation ratings of the
package. If the temperature of the device exceeds +150 ° C(typ), thermal-protection circuitry shuts it down. Once
the device has cooled below +130 ° C(typ), regulator operation resumes.
Specified regulator operation is assured to a junction temperature of +125 ° C; the maximum junction temperature
should be restricted to +125 ° C under normal operating conditions. This restriction limits the power dissipation the
regulator can handle in any given application. To ensure the junction temperature is within acceptable limits,
calculate the maximum allowable dissipation, P
D(max)
, and the actual dissipation, P
D
, which must be less than or
equal to P
D(max)
.
The maximum-power-dissipation limit is determined using the following equation:
where:
• T
J(max)
is the maximum allowable junction temperature
• R
θ JA
is the thermal resistance junction-to-ambient for the package, and is calculated as
• T
A
is the ambient temperature
The regulator dissipation is calculated using:
Power dissipation resulting from quiescent current is negligible. Excessive power dissipation will trigger the
thermal protection circuit.
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