LM138QML

ADJ

R2

R1*

240

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14

LM138QML

SNVSAA9 – OCTOBER 2017

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Copyright © 2017, Texas Instruments Incorporated

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 will eliminate the possibility of problems.

The adjustment terminal can be bypassed to ground on the LM138QML 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; however,

some types have a large decrease in capacitance at frequencies around 0.5 MHz. For this reason, a 0.01-μF disc

may seem to work better than a 0.1-μF disc as a bypass.

Although the LM138QML 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 LM138QML device 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) should 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 will have a load regulation

be 0.05 Ω (1 + R2/R1) or in this case, 11.5 times worse.

Figure 16 shows the effect of resistance between the regulator and 240-Ω set resistor.

Figure 16. Regulator With Line Resistance in Output Lead

With the TO-3 package, it is easy to minimize the resistance from the case to the set resistor by using two

separate leads to the case. The ground of R2 can be returned near the ground of the load to provide remote

ground sensing and improve load regulation.

8.2.1.2.3

Protection Diodes

When external capacitors are used with any IC regulator, it is sometimes necessary to add protection diodes to

prevent the capacitors from discharging through low current points into the regulator. Most 20-μF capacitors have

low enough internal series resistance to deliver 20-A spikes when shorted. Although the surge is short, there is

enough energy to damage parts of the IC.