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TPS61253 output voltage smaller than 5V at heavy load

Other Parts Discussed in Thread: TPS61253

Hi all,

I tested the tps61253evm board, input voltage Vin 4V, EN = 1.

At output load Iout smaller than 180mA, Vout was stable at about 5,17V. When I raised Iout to around 300mA, the output voltage Vout fell down to 3,5V (power supply at Vin provides max 4A). Arcording to figure 12 of datasheet ,tps61253 should be able to provide up to 1500mA at Vout = 5V.

Do you have any ideas for this strange behaviour? 

Thanks in advance

  • Hi Bernd,

    You are right that Vout should not droop to 3.5V. Since it is unlikely the board is defective (since you can see it is stable with low load) then let us first examine the test setup.

     

    What type of load are you connecting to draw 300mA? Do you have a large resistor available to try a resistive load? For example, if you have a 10ohm >3W resistor then you could temporarily test a 500mA load (be careful as it may get hot).

     

    Thanks,

    Alex

  • hi Bernd

    i guess the cable between Power supply and the EVM is too long, so the input voltage is actually much lower than 4V. 

    BSC team

    jasper li

  • Hello Jasper and Alex,

    I supplied the board over a 10 cm cable. The input voltage was measured directly at VIN and GND of the evm board. It was stable at VIN = 4V during the measurement. The load is 20 Ohm, 5W. Unluckily, the result didn't change.

    Did you have any case of defective board till now? 

  • no, i didn't see this before.

    could you share the waveform of SW pin and Vout, and also the inductor current if possible.  does the device return to 5V when you remove the load? and please check the marking of this device.

    Jasper 

  • Hi Jasper,

    below are the waveform for 3 loads 1KOhm, 50Ohm and 20Ohm. As you can see, at 20Ohm load, the device didn't switch anymore.

    The marking on the component of evm board is 24I-58F-C433.

    Regards

    Bernd  

  • Bernd

    it seem the device of the EVM is damaged. Could you apply another EVM? i think you can also apply some samples to replace the damaged IC. 

    Jasper

  • I took measurement with another IC, but the behaviour of the evm board still be the same. No 5V output at high load.

    Could you please test the board and tell me your result ?

    Thanks in advance

    Bernd

  • Hi Jasper,

    yesterday I tested the evm board using a potentiometer as load. When I slowly increased Iload from 50mA to 1000mA, the IC gave out 5V. But when I used a fixed heavy load (Iload = 300mA) at the beginning, the IC didn't boost up to 5V. Vout is equal to Vin, Iout  200mA.

    In this case, I think there are something wrong in the soft start phase of the device (datasheet page 19).

  • the feature is special designed. almost all the boost converter will limit the output current during the start-up, and the current limit  is lower than the value in normal operation. so this device can't support start up with heavy load. 

    i don't think this feature impact the real application, because the load would not work during the boost start-up.

    Jasper