Datasheet

23
LM138 , LM338
www.ti.com SNVS771C –MAY 1998–REVISED DECEMBER 2016
Product Folder Links: LM138 LM338
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9 Power Supply Recommendations
The input supply to LM138 and LM338 must be kept at a voltage level such that its maximum input to output
differential voltage rating is not exceeded. The minimum dropout voltage must also be met with extra headroom
when possible to keep the LM138 and LM338 in regulation. TI recommends a capacitor be placed at the input to
bypass noise.
10 Layout
10.1 Layout Guidelines
Some layout guidelines must be followed to ensure proper regulation of the output voltage with minimum noise.
Traces carrying the load current must be wide to reduce the amount of parasitic trace inductance and the
feedback loop from V
OUT
to ADJ must be kept as short as possible. To improve PSRR, a bypass capacitor can
be placed at the ADJ pin and must be placed as close as possible to the IC. In cases when V
IN
shorts to ground,
an external diode must be placed from V
OUT
to V
IN
to divert the surge current from the output capacitor and
protect the IC. Similarly, in cases when a large bypass capacitor is placed at the ADJ pin and VOUT shorts to
ground, an external diode must be placed from ADJ to V
OUT
to provide a path for the bypass capacitor to
discharge. These diodes must be placed close to the corresponding IC pins to increase their effectiveness.
10.2 Layout Example
Figure 45. LMx38 Layout