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Programming the TPS79401 Adjustable LDO Regulator

V

OUT +

V

REF

1 )

R1

R2

(3)

R1 +

V

OUT

V

REF

* 1

R2

(4)

C1 +

(R1 x R2)

(5)

22 pF

15 pF

15 pF

OUTPUT VOLTAGE

PROGRAMMING GUIDE

OUTPUT

VOLTAGE

R1

R2

2.5 V

3.3 V

3.6 V

C1

31.6 k

Ω

49.9 k

Ω

59 k

Ω

30.1 k

Ω

30.1 k

Ω

30.1 k

Ω

TPS794xx

GND

NR

FB

IN

OUT

EN

V

IN

V

OUT

R1

C1

R2

1

µF

2.2

µF

0.01

µF

Regulator Protection

TPS79401, TPS79418

TPS79425, TPS79428

TPS79430, TPS79433

SLVS349D – NOVEMBER 2001 – REVISED OCTOBER 2004

The output voltage of the TPS79401 adjustable regulator is programmed using an external resistor divider as

shown in Figure 24. The output voltage is calculated using:

where:

•

Resistors R1 and R2 should be chosen for approximately 40-µA divider current. Lower value resistors can be

used for improved noise performance, but the device wastes more power. Higher values should be avoided as

leakage current at FB increases the output voltage error. The recommended design procedure is to choose

R2 = 30.1 k

Ω to set the divider current at 40 µA, C1 = 15 pF for stability, and then calculate R1 using:

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:

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.

Figure 24. TPS79401 Adjustable LDO Regulator Programming

The TPS794xx 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 TPS794xx features internal current limiting and thermal protection. During normal operation, the TPS794xx

limits output current to approximately 925 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 or the absolute maximum

voltage rating of the device. 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.

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