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Power Dissipation and Junction Temperature

P

T

R

Q

JA

(1)

P

D

I

OUT

(2)

Programming the TPS79301 Adjustable LDO Regulator

V

OUT

1 )

R

1

R

2

(3)

R

1

+

V

OUT

V

ref

* 1

R

2

(4)

C

1

+

1

(R

(5)

TPS79301, TPS79318

TPS79325, TPS79328, TPS793285

TPS79330, TPS79333, TPS793475

SLVS348H – JULY 2001 – REVISED OCTOBER 2004

APPLICATION INFORMATION (continued)

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 Equation 1:

Where:

• T

• T

The regulator dissipation is calculated using Equation 2:

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

protection circuit.

The output voltage of the TPS79301 adjustable regulator is programmed using an external resistor divider as

shown in Figure 23. The output voltage is calculated using Equation 3:

Where:

• V

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 in-

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 Equation 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 shown in Equation 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 4.7 µF instead of 2.2 µF.

10