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TPS92643-Q1: questions

Ivy Jin
Expert 4575 points
Part Number: TPS92643-Q1

Tool/software:

Hi team,

Below questions from my customer who is evaluating TPS92643-Q1 in their system, could you please help check and give some comments? Thanks.

        The questions related pin in the schematic , their setting is like AGND and PowerPAD connect to PCB GND , PGND connect to the LOAD GND which connects to PCB GND through a mosfet.

        Q1:In the specification, it notes the AGND and PGND pin must be connected to the Power Pad. Why should it be ? I test out that a 0.2V voltage exists between PGND and AGND, if I don't do as the specification suggested . Further more,  the max rating for voltage between PGND and AGND is 0.5V.

         Q2:As the Q1 said, all the other pin setted well , if I set IADJ a certain voltage but open load , the chip'll get heated. Is that caused by the chip still force mosfet to work in fixed on-time and min off-time  ( output capacitance is 20uF ) , or something else ? and the exact rates about these two kinds of time ?

         Q3:For the internal current sensing circuit, having input resistor 10kR and suggesting the Rcs directly connect to chip . Do you recommend placing  external RC to filter noise ? In the other specific current sensing chips , placing external RC is preferred. 

         Q4:For the fault indicator , all the other pin setted well , if I input 0V to IADJ , the indicator is 0V ( external lift up to 3.3V through a 10KR resistor). However , setting a certain voltage to IADJ , the fault disappears . Under 0V IADJ condition, I test all the fault factors written in the specification , that's all proper . Just COMP is 0V . Why would the fault occurs ?

         Q5:Can't the chip drives 3A current ? The APWM is 3V and IADJ is 3V ,but it only emerges about 2A current ,it ought to be 3A due to the current limitation.

Regards,

Ivy

  • 0
    TI__Expert 3550 points

    Hi Ivy, 

    Q1: If there is some long trace or other impedance between AGND or PGND it's possible to have some potential between the two pins violating the max rating. Connecting both to Power PAD provides a extremely low impedance path which makes this impossible. 

    Q2: The device should not be getting hot in this condition. When there is an open load fault the output current will be very low and the output voltage will rise up to the input voltage. There will be very little power dissipation on the device. The customer might be doing something wrong, can you provide a schematic with BOM? Also you can check the output current by looking at the inductor current. Again it should be very low in an open load condition. 

    Q3: If you are recommending adding a series resistor between Rcs and the CSN/CSP pins, that is not recommended and is a bad idea. Adding any resistor will affect the Iadj resistor divider ratio that the device expects. In the past no one has used any filtering on the node and it has never been an issue. 

    Q4: What fault/indicator specifically are you referencing? There is a minimum voltage threshold needed on the Iadj pin to guarantee accurate fault coverage. Below that threshold the output voltage will be lower than the short threshold and you will get a false positive short fault. 

    Q5: The part is rated up to 3A continuous. But the output current is not only a function of APWM and IADJ, please refer to equation (15) in the datasheet. It also depends on your current sense resistor, Rcs. What are you using for Rcs? In order to achieve 3A output max. Rcs is ~58 mOhm. 

    Thanks,

    Jaron