TPS 40304 Start up problem

Hi Behzad,

I think the part may possibly hit the maximum duty cycle. There is no good solution but increasing input voltage.

Regards,

Na 

According to WEBENCH the duty cycle for an input of 6.4V should be 85%. That is below the specs of the TPS40304 (Maximum duty cycle for TPS40304 is MINimal 90% or it is higher).

Could you check what the duty cycle is for the input voltage of 6.7V?

Maybe the mosfets are not well enough driven?

Could you check voltage is on pin BP with respect to GND? (According to the BP REGULATOR section in the D/S it should be the input voltage minus about 0.1V)

Could you check voltage is on pin BOOT with respect to pin SW? (According to the BLOCK DIAGRAM  in the D/S and the BOOT DIODE section in the D/S it should be the voltage on BP minus about 0.8V)

Note that WEBENCH adds an schottky diode between BP and BOOT when you enter the design values Vin_min = 6.4 and Vin_max = 6.7V

Hi Na,

Thanks for your answer. That is what I think too but my duty cycle according to Webench design is 85% which is still below 90%. I did another experiment today. When I started up the device to operate with a higher input voltage, I decreaded the input voltage to 6.5 V and everything was fine. The measured duty cycle was 85.3% in my case. Unfortunately I cannot increase the input voltage in the design. Do you thing changing the feedback circuit will help me to use the original input voltage?

Bests,

Behzad

Thank you Jurjen.

I measured the duty cycle for 6.7V input 82.3%. As I mentioned in Relp to Na, I did another experiment. When I started up the device to operate with a higher input voltage, I decreaded the input voltage to 6.5 V and everything was fine. The measured duty cycle was 85.3% in my case. I measured BP 6.45V and SW with respect to BOOT is 5.525.

Behzad

Hi Behzad,

If the 6.5Vin only cause problem at start-up, you may try to increase the SS cap to extend the soft-start time and reduce the start-up inrush current. The start-up issue may be caused by the overcurrent fault.

Regards,

Na

That did not help eaither :( I changed from 3 nF up to 100 nF but not any change in the response.

Na,

I believe this part cannot do what we are looking for. Can you please help us to find alternative solution handeling our power need i.e. +5.5V, 11A out of +6.5 V power supply with the highest efficiency.

Behzad

No output at all?  CAn you post some start up waveforms including the switching node?

Hey John,

Nothing at all! There is nothing to show LOL.

So during the slow start sequence there are never any switching pulses at all?  Just like the device was never enabled or Vin rose above the UVLO?

Exactly. Nothing happens until I increase the input voltage to 6.7 or above.

I would suggest checking the section "Enable Functionality, Startup Sequence and Timing" of the D/S on the chip for the 6.5V input voltage.

So, to measure the voltage on the EN/SS pin and when this is about 0.7V to check if the BP regulator starts and then to check the voltage across Rs (on the LDRV pin).