Datasheet

13
LM138 , LM338
www.ti.com SNVS771C –MAY 1998–REVISED DECEMBER 2016
Product Folder Links: LM138 LM338
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Table 1. Design Parameters
PARAMETER VALUE
Feedback resistor 1 (R1) 270 Ω
Feedback resistor 2 (R2) 820 Ω
Input capacitor (C
IN
) 0.1 µF
Output capacitor (C
OUT
) 1 µF
Adjust capacitor(C
ADJ
) 10 µF
8.2.1.2 Detailed Design Procedure
8.2.1.2.1 External Capacitors
An input bypass capacitor is recommended. A 0.1-µF disc or 1-µF solid tantalum on the input is suitable input
bypassing for almost all applications. The device is more sensitive to the absence of input bypassing when
adjustment or output capacitors are used but the above values eliminate the possibility of problems.
The adjustment terminal can be bypassed to ground on the LM138 to improve ripple rejection. This bypass
capacitor prevents ripple from being amplified as the output voltage is increased. With a 10-µF bypass capacitor,
75-dB ripple rejection is obtainable at any output level. Increases over 20 µF do not appreciably improve the
ripple rejection at frequencies above 120 Hz. If the bypass capacitor is used, it is sometimes necessary to
include protection diodes to prevent the capacitor from discharging through internal low current paths and
damaging the device.
In general, the best type of capacitors to use are solid tantalum. Solid tantalum capacitors have low impedance
even at high frequencies. Depending upon capacitor construction, it takes about 25 µF in aluminum electrolytic to
equal 1-µF solid tantalum at high frequencies. Ceramic capacitors are also good at high frequencies; but some
types have a large decrease in capacitance at frequencies around 0.5 MHz. For this reason, 0.01-µF disc may
seem to work better than a 0.1-µF disc as a bypass.
Although the LM138 is stable with no output capacitors, like any feedback circuit, certain values of external
capacitance can cause excessive ringing. This occurs with values between 500 pF and 5000 pF. A 1-µF solid
tantalum (or 25-µF aluminum electrolytic) on the output swamps this effect and insures stability.
8.2.1.2.2 Load Regulation
The LM138 is capable of providing extremely good load regulation but a few precautions are needed to obtain
maximum performance. The current set resistor connected between the adjustment terminal and the output
terminal (usually 240 Ω) must be tied directly to the output of the regulator (case) rather than near the load. This
eliminates line drops from appearing effectively in series with the reference and degrading regulation. For
example, a 15-V regulator with 0.05-Ω resistance between the regulator and load has a load regulation due to
line resistance of 0.05 Ω × I
L
. If the set resistor is connected near the load, the effective line resistance is 0.05 Ω
(1 + R2/R1) or in this case, 11.5 times worse.
Figure 17 shows the effect of resistance between the regulator and 240-Ω set resistor.