Datasheet

+
Over Temp &
Over Current
Protection
Input
Adj.
Output
1.25V
I
adj
LM317M
www.ti.com SLVS297P –APRIL 2000–REVISED MARCH 2014
7 Detailed Description
7.1 Overview
The LM317 is a 500-mA linear regulator with high voltage tolerance up to 40-V. The device has a feedback
voltage that is relative to the output instead of ground. This ungrounded design allows the LM317 to have
superior line and load regulation. It also allows the LM317 to be used as a current source or current sink using a
single resistor. Any output voltage output voltage from 1.25-V to 37-V can be obtained by using two resistors.
The bias current of the device, up to 10-mA, flows to the output; this current must be used by the load or the
feedback resistors. The power dissipation will be the product of pass element voltage and current,
mathematically:
(V[input] – V[output]) × I[output] (1)
The application heat sink must be able to absorb this power.
7.2 Functional Block Diagram
7.3 Feature Description
7.3.1 NPN Darlington Output Drive
NPN Darlington output topology provides naturally low output impedance and an output capacitor is optional. To
support maximum current and lowest temperature, 2.5-V headroom is recommended (V
I
– V
O
).
7.3.2 Overload Block
Current limit scales with higher voltage to provide a safe operation area for the NPN pass elements. Over
temperature shutdown protects against overload or under heat sinking.
7.3.3 Programmable Feedback
Op amp with 1.25-V offset input at the ADJUST terminal provides easy output voltage or current (not both)
programming. For current regulation applications, a single resistor whose resistance value is 1.25V/IOUT and
power rating is greater than (1.25V)
2
/R should be used. For voltage regulation applications, two resistors set the
output voltage. See the Typical Application section for schematic and resistor formula.
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