LM138 STMicroelectronics, LM138 Datasheet - Page 13

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

7
Figure 18. Application circuit
7.1
Application hints
In operation, the LM338 develops a nominal 1.25 V reference voltage, V
output and adjustment terminal. The reference voltage is impressed across program resistor
R
resistor R
V
Since the 50 µA current from the adjustment terminal represents an error term, the LM338
was designed to minimize I
this, all quiescent operating current is returned to the output establishing a minimum load
current requirement. If there is insufficient load on the output, the output will rise.
External capacitors
An input bypass capacitor is recommended. A 0.1 µF disc or 1 µF solid tantalum on the input
is suitable input by passing for almost all applications. The device is more sensitive to the
absence of input bypassing when adjustment or output capacitors are used by the above
values will eliminate the possibility of problems. The adjustment terminal can be bypassed
to ground on the LM338 to improve ripple rejection. This bypass capacitor prevents ripple
form 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
O
1
and, since the voltage is constant, a constant current I
= V
(REF)
2
, giving an output voltage of
(1+ R
2
/R
1
) + I
ADJ
ADJ
R
and make it very constant with line and load changes. To do
2
1
then flows through the output set
(REF)
, between the
13/23