LM138 STMicroelectronics, LM138 Datasheet - Page 14

• Manufacturer Part Number: LM138
• Description: Three-terminal 5 A adjustable voltage regulators
• Manufacturer: STMicroelectronics

7.2
7.3
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0.1 µF disc as a bypass. Although the LM338 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 mF solid tantalum (or 25 µF aluminium
electrolytic) on the output swamps this effect and insures stability.
Load regulation
The LM338 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 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
due to line resistance of 0.05 Ω x I
effective line resistance will be 0.05 Ω (1 + R
on page 8
the TO-3 package, it is easy to minimize the resistance from the case to the set resistor, by
using 2 separate leads to the case. The ground of R
load to provide remote ground sensing and improve load regulation.
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. When an output capacitor is connected to a regulator and the input is
shorted, the output capacitor will discharge into the output of the regulator. The discharge
current depends on the value of the capacitor, the output voltage of the regulator, and the
rate of decrease of V
able to sustain 25 A surge with no problem. This is not true of other types of positive
regulators. For output capacitors of 100 µF or less at output of 15 V or less, there is no need
to use diodes.The bypass capacitor on the adjustment terminal can discharge through a low
current junction. Discharge occurs when either the input or output is shorted. Internal to the
LM338 is a 50 Ω resistor which limits the peak discharge current. No protection is needed
for output voltages of 25 V or less and 10 µF capacitance.
LM338 with protection diodes included for use with outputs greater than 25 V and high
values of output capacitance output capacitance an LM338 with protection diodes included
for use with outputs greater than 25 V and high values of output capacitance.
shows the effect of resistance between the regulator and 140 Ω set resistor. With
I
. In the LM338 this discharge path is through a large junction that is
L
. If the set resistor is connected near the load the
2
/R
1
) or in this case, 11.5 times worse.
2
can be returned near the ground of the
Figure 5 on page 8
shows an
Figure 4