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LM25061: Low impedance between Vin and Sense pin on few units.

Part Number: LM25061

Hi,

On few boards, LM25061 is showing low impedance b/w pin 1(Sense) and pin 2(Vin).

On failing samples with initial analysis following things are found:

  1. Impedance b/w pin 1 & 2 of failing samples is in range of 2 to 5ohm outside board, whereas for a passing IC it is around 100Kohms.
  2. Power-on inrush control is happening in failing IC, but current is shooting as high as 1.5A  whereas for a passing IC it is around 0.7A.
  3. Continuous current limit is set to 452mA in our circuit. I have tested failing IC for around 600mA. Gate is not being pulled low. Same test is done with passing IC, gate is latched low.

Values used in ckt: Rpwr=150k, Rsense 110mohm, Ctimer= 1uf, LM25061-1 part. Vin=12V

What could be the reason for this low impedance?

  • Vin cap placement.pdfNeeti,

    The issue may be damaged Vin_Sense due to voltage surge on the input.  Two things could cause.  1)  Lack of a front end TVS and output Schottky, and 2) the location of Vin capacitor (tyically 0.1uF).  An input TVS is required to protect both the LM25061 and the FET in the case of a hard short on the ouput where the FET is opened with a high di/dt current, causing a very high input voltage surge and a negative Vout (clamped by a Schottky).  Enough to exceed the 20v Vin rating and go below gnd on the OUT pin.  For this current level, an SMA should suffice.  The Vin cap location should be right at the input of Rsns, not at the Vin pin of the LM25061.  See attached explanation.   Either case 1 or 2 here can damage the Vin-Sense antiparallel diodes, internal Vin circuitry and with case 1 the OUT pin also.  Vin_Sns is rated for +/- 0.3v in the abs max table.

    Brian