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LM317L: Quesiton of LM317LIPK

Masaharu Takahashi
Expert 4170 points
Part Number: LM317L
Other Parts Discussed in Thread: LM317, , LM317A

Hi, Team.

This is MASA.

Could you answer for below the question my customer asked me?

Q1. In the datasheet shows that "output regulation" is 0.5%.

Q2. what does "reference voltage" mean even output regulation 0.5% shown on datasheet.

 the datasheet shows reference voltage as 1.2v ~ 1.3v

(13-12/12.5)*100=8%...

Q3how much minimum resister customer can choose and is there any drawback if customer use small resister ?

Customer wants to make LM317 output within 0.5% tight regulation.

currently customer uses 470ohm and 5.1kohm.

adjustment current is 50uA(typ),

0.255V is dropped with R2(5.1k) (50uA adustment current times 5.1kohm) which is already 1.7% changed voltage for 15V output

Q4. is Vdrop for LM317L 2.5v ?

Q5.why is there needed to have protection diode like dataasheet mentioned below?

9.2.2.4 Adjustment Terminal Capacitor The optional adjustment pin capacitor will improve ripple rejection by preventing the amplification of the ripple. When capacitor is used and VOUT > 6 V, a protection diode from adjust to output is recommended.

  • 0
    TI__Guru 55940 points

    Hi Masa,

    The more common term for what the LM317L datasheet refers to as "output regulation" is "load regulation."  This is the percent change in output voltage due to a change in Iout.

    The reference voltage is the voltage of our internal reference.  The internal reference is the primary source of inaccuracy for a linear regulator.  As you point out the linear regulator output can be +/-4% due to the variation of the internal reference.

    The smaller the resistor divider is, the more inefficient the overall application will be.  This is due to the extra current flowing through the resistor divider.  As long as your application can accept the additional power loss, you can reduce R2 and R1 as much as you want.  Keep in mind that it is the ratio between R2 and R1 that sets the output voltage.

    LM317L is a linear regulator that was designed, tested, and characterized for a minimum Vin - Vout differential of 2.5 V.

    The protection diode is to provide a robust discharge path for the capacitor.  A more full explanation is provided in the LM317A datasheet which is part of the same family of linear regulators.

    Very Respectfully,

    Ryan