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TLV758P: Question about the foldback current limit

Wayne Wang20
Expert 4360 points
Part Number: TLV758P

Hi team,

When I read the TLV758P datasheet, I didn't quite understand the Foldback Current Limit part. For example, let's say Vin=5V and Vout(NOM)=3V. Normal load=6ohm. So the foldback voltage is 0.4*3V=1.2V.

So if the load becomes to 3ohm, the Io should be 3V/3ohm=1A. But 1A is higher than the Icl so Io is restricted to Icl(take Icl=600mA). So the real Vout is 0.6A*3ohm=1.8V. Because 1.8V is higher than foldback voltage 1.2V, this conclusion works.

So if the load becomes to 1ohm, the Io should be 3V/1ohm=3A. But 3A is higher than the Icl so Io is restricted to Icl(take Icl=600mA). So the the real Vout is 0.6A*1ohm=0.6V. Because 0.6V is lower than the foldback voltage 1.2V, so the final Io is not 600mA. So my question is how to decide the final Io and Vout when the load is 1ohm? Thanks.

Best regards,

Wayne

  • 0
    TI__Guru 55940 points

    Hi Wayne,

    Traditionally current limit is a fault protection circuit for a linear regulator.  It is not intended for the linear regulator to operate in current limit for a sustained period of time.  As the regulator is in current limit, there will be increased power dissipation due to the higher current.  As such, thermal shutdown may be triggered.

    In your example, the load current is initially set to the rated current value of TLV758P (500mA).  As such, the LDO will regulate as intended.  When the load current demand increases to 1 A, you will hit the "brickwall" portion of the current limit as you mention.  During this time the load will be demanding more current than the LDO can source (due to current limit); therefore, your output capacitors will discharge causing your output voltage to reduce.  When the output voltage reduces to 1.2 V, the current limit will begin to reduce as well (folding back).  

    With a resistive load, the current demand will reduce as Vout decreases.  We know that the short-circuit current limit is 350 mA from the Electrical Characteristics table.  With a 1 Ohm load, we would achieve this same current at an output voltage of 350 mV.

    A brief explanation of the two most common current limits can be found in Chapter 5 of our LDO Basics E-book:

    http://www.ti.com/lit/ml/slyy151a/slyy151a.pdf

    Very Respectfully,

    Ryan