ADJ

LM138QML

GND

‚

1…F

0.1…F

R1

270

R2

820

10…F

D1**

1N4002

D2

1N4002

‚‚

*Needed if device is more than 6 inches from filter

capacitors

‚2SWLRQDO±±improves transient response

‚‚

2

I

1

2

1

25

.

1

V

ADJ

OUT

R

R

R

V

¸

¹

·

¨

©

§

Copyright © 2017, Texas Instruments Incorporated

13

LM138QML

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SNVSAA9 – OCTOBER 2017

Product Folder Links: LM138QML

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Copyright © 2017, Texas Instruments Incorporated

8.2 Typical Applications

8.2.1 Constant 5-V Regulator

Figure 15. Constant 5-V Regulator

8.2.1.1 Design Requirements

Table 1. Design Parameters

PARAMETER

PART

NUMBER/VALUE

DESCRIPTION

Feedback resistor 1 (R1)

270 Ω

The LM138QML produces a typical 1.24-V potential between the OUTPUT and ADJUST

pins; therefore, placing a 270-Ω resistor between the OUTPUT and ADJUST pins causes

4.6 mA to flow through R1 and R2

Feedback resistor 2 (R2)

820 Ω

To achieve a 5-V output, the sum of the voltages across R1 and R2 must equal 5 V.

/ 4.6 mA = ~820 Ω.

0.1 µF

0.1 µF of input capacitance helps filter out unwanted noise, especially if the regulator is

located far from the power supply filter capacitors.

1 µF

The regulator is stable without any output capacitance, but adding a 1-µF capacitor

improves the transient response.

10 µF

A 10-µF capacitor bypassing the ADJUST pin to ground improves the regulators ripple

rejection.

Protection diode 1 (D1)

1N4002

impedance discharge path to prevent the capacitor from discharging into the output of the

regulator (see Protection Diodes).

Protection diode 2 (D2)

1N4002

discharge path to prevent the capacitor from discharging into the output of the regulator

(see Protection Diodes).