TPS62932: voltage regulated is higher than expected and transient loads problem

Hi Gui He,

thanks for your answer. I'll try with an inductor around 6.8uH and I'll come back with a feedback.

Could you tell me the reason why I should lower the inductor value? The current ripple shoul decrease with a larger value and also the Vout ripple.

Maybe the reason is that with a larger inductor value I slow down the frequency response of the Vout/Vin real transfer decreasing the fp introduced by L-C network?

Thank you for your support,

Alessandro

Hi Alessandro

Can you please share the waveforms for the SW and Vout ? 

Thank you

Hi Gui He,

Next Tuesday I'll be in laboratory :I'll capture the waveforms and I'll update you.

Meanwhile, regarding my theoretical question from the earlier post:

"Could you tell me the reason why I should lower the inductor value? The current ripple shoul decrease with a larger value and also the Vout ripple.

Maybe the reason is that with a larger inductor value I slow down the frequency response of the Vout/Vin real transfer decreasing the fp introduced by L-C network?"

Could you give me an answer?

Thank you for your support,

Alessandro

Hi Alessandro

Sorry for late reply. Did you got any waveforms about this? 

Thanks and best regards/

Hi Gui He,

I have changed the inductor to 6.8uH as you suggested but the problem still remains.

Attached you can see the waveforms of VOUT(yellow trace) and SW (purple trace): it seems that SW doesn't switch and VOUT follows SW pin jumping from 3.8V to 4V.

In the second screen you can see also VIN waveform in green (about 19 V, quite stable).

Addiotionally I have discovered that inserting a minimum load on Vout (100 ohms, hence about 30 mA load) IC3 starts working correctly and Vout is around 3.3V.

So it seems like a problem for IC3 the low current condition. The current absorbed by Vout is about 10 mA medium (measured by connecting a PWS on Vout);downstream there is a MCU with a basic firmware which drives periodically (every seconds) two leds (Part number :LTST-C150YKT).
From datasheet they have a peak current of 80 mA.

I attach also the waveforms of SW pin with a 100 0hm load: zooming on SW I see a DCM behaviour with a lot of ringing in the dead zone where the inductor current reaches zero and it is not yet started the next switching cycle.
How can I reduce it?

Could you help me with these two issues?

Thanks for your support
Alessandro  

Hi

I just got the TPS62933EVM on hand and I tested on the EVM. Changing R4 to 41.2k to set Vo=3.3V.

And I tested the Vout under different Vin/Iout:

The SW(CH3), Vo(CH1) waveform is seen below

:

Can you please check that can there be any power applied to the Vout side of TPS62933 when the VIn feeds from the +19V rail?

For the questions: 

1) TPS62933 has a minimum peak inductor current limit (0.75A specified in datasheet), in each switching cycle, iL_peak will be at least 0.75A, so under light load the device will decrease the frequency and the Vo_ripple gets higher than under CCM mode.

2)The ringing during the dead time is caused by the resonant by L and Cout, which is quiet normal during DCM and no need to reduce.

Thanks and best regards.

Hi Gui He,

the circuit downstream of VOUT is the same for both VIN cases (+5V or +19V);I wouldn't expect different behaviours.

I attach the downstream load  of Vout: a MCU IC5 , A Lora Module U2 (currenty in sleep mode),a RTC module(U3) which has a back-up battery(VBATT)  not used when the 3.3V is present.

Furthermore I have discovered that disconnetting the +19V rail of the board and applying here an external PWS with +19V,1A settings the VOUT works fine.

Have you got any other suggestions for this issue?

Thanks for your support

Alessandro

Hi Alessandro

Did you measure the +19V rail on the board and is that a stable +19V power supply ? 

Thanks and best regards.

Hi Gui He,

The +19V seems quiet stable. I reattach the picture of the previous message where you can see the the +19V rail on the green line (C3): it's about +1V max ripple. In the picture VOUT is the yellow trace and SW is the purple trace: SW doesn't switch and VOUT follows SW pin jumping from 3.8V to 4V.

As written above, TPS62932 works fine if I apply a load of 100 ohm (hence 33 mA current) on its 3.3V output.

Have you got any other suggestions for this issue?

Thank you

Alessandro

Hi Gui He,

I found the problem: there's a low impedence path between the +5V rail and the +3.3V output of TPS62933EVM.

Attached you can see the part of schematic where this happens: D1 connected on R pin of THVD1406DR transceiver creates the path between +5V and +3.3V..

I think that one solution should be change the supply of THVD1406DR to 3.3V..

Or maybe add a series resistor between R and anode of D1 to make a voltage divider..

Could be this the solution?

Thank for your help

Alessandro

Hi Alessandro

Please let me check and get back later.

Thanks and best regards.

Hi Alessandro

Sorry that I am not very sure about the configuration for the TVD1406, you might need to check with the expert about TVD1406 about how to avoid the +5V been passed through to the +3.3V .

Hi Gui He,

ok, so I can wait for the answer remaning on this thread or I should create a new question?

Thanks

Alessandro