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TPS2420EVM-01: current limit and EN voltage

Jason Jiang0
Prodigy 30 points
Part Number: TPS2420EVM-01
Other Parts Discussed in Thread: TPS2420

We have two questions during the test of TPS2420EVM.

1. The EN pin is open and the input voltage is 2.85V. The voltage of the EN pin is 1.2V. which is different from the caculation.

2. IMAX is set to 5A,and the load current is 6A(less than 1.6*IMAX).Why the  TPS2420 device immediately shuts off. 

  • 0
    TI__Guru 57737 points

    Hi Jason,

    1. What do you mean by EN pin is open ? Do you mean the pin is left floating ?
      • The ENABLE pin voltage needs to be less than 1V for the part to be enabled.

    2. The TPS2420 device limits current to 5A when you try to draw load current > 5A. There will be power dissipation in the FET while current limiting. The internal FET is turned OFF when the junction temperature increases to 160C.

      • Can you share waveforms with Vin, Vout, Iin and VCT ?

  • 0 in reply to Praveen GD
    TI__Intellectual 830 points

    Hi Praveen.

         Customer uses EVM to do the test, EN pin floating, remove C2.  The EN pin  left floating. VIN=5V.

        We find when EN = 1.21V, the internal MOSFET close and we could see VOUT output.  This does not match the EN less than 1V , enable the IC.   Jason will upload the waveform later.  You could refer to waveform and give some comment.

         Another question from my side is, when VIN = 2.85V, and according to block in the DS, resistor devider , we calculate the EN voltage is 1.8V. But when we use OSC check EN volatge, we find it is 1.2V, this is very strange. Please help to check too.

       For current limit question,  do you mean internal MOSFET will turn off when temperature ups to 160C? We get information from DS, shutdown mode chapter, when the current is 1.6*( max limit current ), the FET will turn off, but we try the current is up to 6A ( current limit is set to 5A ),  the MOSFET is off. this is not same with DS. Do you mean MOSFET off is not controlled by 1.6*5A only, and temperature also impact the MOSFET behavior?

    Thanks.

     

  • 0 in reply to Jason Jiang0
    TI__Intellectual 830 points

    Hi Xin,

        Could you please to check if VIN ==5V, EN voltage?

         Also, please help to check if VIN=2.85V, EN voltage. Not measure it when VIN start up, I mean you use a DC source force a voltage to VIN, check EN volatge, and update the information here. Thanks.

  • 0 in reply to April Jiang
    Prodigy 30 points

    if VIN ==5V, EN voltage is 2.34V

    if VIN=2.85V,EN voltage is 1.38v

  • 0 in reply to Jason Jiang0
    TI__Guru 57737 points

    Hi Jason, April,

    The EN/ pin should not be left floating. Tie it to GND when unused.

    When the load current goes above IMAX, the TPS2420 device limits current to IMAX. here you have set IMAX = 5A.

    As you have set the load current to 6A the current will be limited to 5A and in this process, there will be a drop in output voltage. Now, the power dissipation in the device = (Vin -Vout)*IMAX. This power dissipation increases the temperature of the IC junction. If this temp. reaches a threshold of 160C, the internal FET is turned OFF.

    Do not use electronic load in CC mode to apply load. Always use resistive loads to evaluate eFuse devices.

    • With resistive loads, the output operating point will be decided by the eFuse in current limit mode of operation. In case of active loads (e-load), the output voltage & current will be decided by both the e-load characteristics and eFuse characteristics in current limit mode. So, load current cannot be limited by the set current limit of the eFuse. You will observation different current than the current limit value.
    • Alternate explanation: When the eFuse device reaches current limit, the device acts as a current source. If an electronic load is used for testing in CC mode, which turns out to be placing two current sources in series in the network. When the electronic CC load exceeds set current limit of eFuse device, the electronic load tries to pull down the output voltage and shifts the operating current other than the current limit value.