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TPS746-Q1: Foldback Current Limit

wang wan
Prodigy 120 points
Part Number: TPS746-Q1

Hi,

   From the datasheet，we see the relationship between IOUT and foldback current limit. First, IOUT reaches current limit which is 1.5A. Then Vout decreases to 0.4*3.3V, IOUT is limited to short current which is 680mA typically.

However, the waveform we tested is different from what is shown above. 

We used E-load of which slew rate is 0.01A/us. IOUT is limited to 954mA finally. The value is out of range of current limit. I am confused about it. Could you give me any advice?

 

  • 0
    TI__Expert 4350 points

    Hi Wang,

    Thanks for sending a scope shot. If you look at the point where Vout just starts to decrease at approximately 220 usec, the current out of the LDO is approximately 1.5 A. From the datasheet Electrical Characteristics (EC) Table, for Vout > 1 V, the current limit is tested at 85% of Vout(nom), which is 1.615 V for your case. Looking at the scope shot at approximately 250 usec, the Vout = 1.6 V and output current is approx 1.53 A. The typical current limit in the EC table is 1.5 A, so this matches well.

    Since this part has a foldback current limit, the current limit reduces as Vout continues to decrease past the 85% point. Your E-load draws more than 1.5 A and continues to ramp up. As a result the current limit circuit causes the output voltage to decrease. Since the current limit is a function of % of Vout, the current limit adjusts at the new Vout point. 

    As your load current reaches 1.79 A, the Vout comes close to 0 V. Your cursors on the current waveform sit above the top of the fuzz in the waveform. If you center your scope cursors on the middle of the output current waveform, you may find that the Iout is within the 850 mA max spec for short circuit current limit. 

    Let me know if you have further questions.

    Thanks, Eric