TPS79101Q1,TPS79118Q1

TPS79133Q1,TPS79147Q1

SGLS160B − APRIL 2003 − REVISED SEPTEMBER 2008

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11

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,

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

P

D(max) +

T

J

max

* T

A

RqJA

Where:

(1)

The regulator dissipation is calculated using:

P

D +

V

I *

V

O

I

O

(2)

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

thermal protection circuit.

PROGRAMMING THE TPS79101 ADJUSTABLE LDO REGULATOR

The output voltage of the TPS79101 adjustable regulator is programmed using an external resistor divider as

shown in Figure 33. The output voltage is calculated using:

V

O +

V

ref

1

) R1

R2

(3)

Where:

Resistors R1 and R2 should be chosen for approximately 50-

µA divider current. Lower value resistors can be

used for improved noise performance, but the solution consumes more power. Higher resistor values should

be avoided as leakage current into/out of FB across R1/R2 creates an offset voltage that artificially

design procedure is to choose R2 = 30.1 k

Ω to set the divider current at 50 µA, C1 = 15 pF for stability, and

then calculate R1 using:

R1

+

V

O

V

ref

* 1

R2

(4)

In order to improve the stability of the adjustable version, it is suggested that a small compensation capacitor

be placed between OUT and FB. For voltages <1.8 V, the value of this capacitor should be 100 pF. For voltages

>1.8 V, the approximate value of this capacitor can be calculated as:

C1

+

(3

(R1

) R2)

(R1

R2)

(5)

The suggested value of this capacitor for several resistor ratios is shown in the table below. If this capacitor is

not used (such as in a unity-gain configuration) or if an output voltage < 1.8 V is chosen, then the minimum

recommended output capacitor is 2.2

µF instead of 1 µF.