Datasheet

Copyright © 2016, Texas Instruments Incorporated
12
LM138 , LM338
SNVS771C –MAY 1998–REVISED DECEMBER 2016 www.ti.com
Product Folder Links: LM138 LM338
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8.2 Typical Applications
8.2.1 Constant 5-V Regulator
Figure 16. Constant 5-V Regulator
8.2.1.1 Design Requirements
R1: Because the LM138 produces a typical 1.24 V potential between the OUTPUT and ADJUST pins, placing a
270-Ω resistor between them causes 4.6 mA to flow through R1 and R2.
R2: To achieve a 5-V output, the sum of the voltages across R1 and R2 must equal 5 V. Therefore, Vr2 must
equal 3.76 V when 4.6 mA is flowing through it. R2 = Vr2 / I = 3.76 V / 4.6 mA = ~820 Ω.
C
IN
: 0.1 µF of input capacitance helps filter out unwanted noise, especially if the regulator is located far from the
power supply filter capacitors.
C
OUT
: The regulator is stable without any output capacitance, but adding a 1-µF capacitor improves the transient
response.
C
ADJ
: A 10-µF capacitor bypassing the ADJUST pin to ground improves the regulators ripple rejection.
D1: Protection diode D1 is recommended if C
OUT
is used. The diode provides a low-impedance discharge path to
prevent the capacitor from discharging into the output of the regulator (see Protection Diodes).
D2: Protection diode D2 is recommended if C
ADJ
is used. The diode provides a low-impedance discharge path to
prevent the capacitor from discharging into the output of the regulator (see Protection Diodes).
Table 1 lists the design parameters for this typical application.