TPS78825, TPS78833

SLVS382A – JUNE 2001 – REVISED JULY 2001

7

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APPLICATION INFORMATION

external capacitor requirements (continued)

The external bypass capacitor, used in conjunction with an internal resistor to form a low-pass filter, should be

a low ESR ceramic capacitor. For example, the TPS78833 exhibits only 56

µVRMS of output voltage noise using

a 0.01

µF ceramic bypass capacitor and a 10-µF ceramic output capacitor. Note that the output will start up

slower as the bypass capacitance increases due to the RC time constant at the bypass pin that is created by

the internal 150-k

Ω resistor and external capacitor.

power dissipation and junction temperature

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, PD(max), and the actual dissipation, PD, which must be less than

or equal to PD(max).

The maximum-power-dissipation limit is determined using the following equation:

P

D(max) +

T

J

max

* T

A

RθJA

Where:

TJmax is the maximum allowable junction temperature.

RθJA is the thermal resistance junction-to-ambient for the package, see the dissipation rating table.

TA is the ambient temperature.

The regulator dissipation is calculated using:

P

D +

V

I *

V

O

I

O

Power dissipation resulting from quiescent current is negligible. Excessive power dissipation will trigger the

thermal protection circuit.

regulator protection

The TPS788xx PMOS-pass transistor has a built-in back diode that conducts reverse current when the input

voltage drops below the output voltage (e.g., during power down). Current is conducted from the output to the

input and is not internally limited. If extended reverse voltage operation is anticipated, external limiting might

be appropriate.

The TPS788xx features internal current limiting and thermal protection. During normal operation, the TPS78833

limits output current to approximately 350 mA. 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 approximately 165

°C, thermal-protection circuitry shuts it down. Once the device has cooled down to

below approximately 140

°C, regulator operation resumes.