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TPS7B7702-Q1: no output issue

Part Number: TPS7B7702-Q1
Other Parts Discussed in Thread: TPS7B7702

Hi team,

My customer use TPS7B7702 power for camera PoC circuit and now they meet an issue, the issue details is as below

  • They use TPS7B7702 power for DMS camera(out1) and DVR camera(out2), DMS camera current need is large than DVR camera
  • When 7702 powered on, out1 is normal, out2 has no output and SENSE2 is about 3.2V
  • The issue is followed DMS camera, if DMS camera connect to out2, out1 will meet no output issue.

The schematic is attached.

Could you kindly tell me why this happen? Thanks!

B.R

Jiawei

  • Hi Jiawei,

    3.2V on SENSE corresponds to a sensed output current of 528mA while the current limit has been set at 489mA. So the current is sensed at above the set current limit and the output is at 0V so it seems to me that the output is shorted to ground in this scenario. The customer will need to figure out what is causing the short to ground condition.

    Regards,

    Nick

  • Hi Nick,

    Thanks for your reply! As you can see, customer use two TPS7B7702 to power 4 cameras, DMS/DVR/OMS/RMS. And I also mentioned that the issue follows DMS camera, which need more larger current. What confused me is as below:

    • TPS7B7702 two outputs for DMS, DVR, the DVR out will meet no output issue.
    • But if TPS7B7702 two outputs for DVR, RMS, TPS7B7702 will work normally.

    Another I noticed is if sense pin voltage is around 3.2V, it means it meet short-battery or reverse current? Could you explain it? Thanks!

  • Hi Jiawei,

    The last paragraph of Section 7.3.3 seems to describe the scenario the customer is seeing, where one of the channels demands more current than the other and causes the input to dip. Can you ask them to increase the amount of input capacitance? 

    Regards,

    Nick

  • Hi Nick,

    Thanks for your answer, increase capacitance works, but how to calculate input cap? And how to explain why no output issue happened another output? Thanks!

    B.R

    Jiawei

  • Hi Jiawei,

    Can you please share the input and output voltages the customer measures under normal operation? The datasheet implies that the output should be a maximum of 500mV below the input, and the input to the LDO looks to be a diode drop less than 12V from the schematics. If I know the input voltage that the LDO sees and the output voltage I can provide a method to calculate the input capacitor. 

    Is this issue happening at startup? I assume that the battery is always tied to the input - is that true? 

    The issue that is happening here is that one of the channels draws a lot of current so the input droops. This drooping input voltage causes a reverse current condition in the channel that was not drawing so much current and shuts the output of that channel off. So it is not the channel that was drawing a lot of current that is affected by this phenomena; it is the other channel that is affected. Does that explanation make sense?

    Regards,

    Nick

  • Hi Nick,

    Pls notice that customer connect FB to GND so that 7702 works at power switch mode, and 4 cameras output current and voltage is as below:

    • 140mA, 10.5V
    • 40mA, 11V *3

    The input is constant 12V, and the 7702 input voltage is less than 12V due to the voltage loss of diode.

    The issue is happening at start up. I think you can verify this with designer, and I will test on customer board to verify this, thanks~

    BR

    Jiawei

  • Hi Jiawei,

     

    To calculate the input cap we can use t_(PD_RC), which is the specification for the delay to shut down the output when there is reverse current detected, Ilim, Vin, and I will assume a short-to-ground condition. The maximum delay time at 25C is 20us, the maximum current limit over the full operating temperature range is 550mA, and Vin = 12V. I will assume that during the duration of the short-to-ground condition before the output is shut off, the output current is always at the maximum current limit, 550mA, to provide margin. I will also assume that the input to the LDO is 12V - 0.7V = 11.3V and that the most deviation we want to tolerate is ΔVin = 0.2V (so that Vin never goes below 11V). With these specifications, this is my calculation of the input capacitance requirement:

    ΔQin = t_(PD_RC) * Ilim

    Cin = ΔQin/ΔVin = t_(PD_RC) * Ilim / ΔVin = 20us * 550mA / 0.2V = 55uF

    Does that make sense?

    Regards,

    Nick