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TPS92691-Q1: TPS92692-Q1

Part Number: TPS92691-Q1
Other Parts Discussed in Thread: TPS92691, TPS92692

Hi All,

I have designed an application using the TPS9262-Q1. The application drives two high power LEDs. There are two drivers on the PCB , one for each LED. The LEDs work at 32V and 0.6A each.

The design seems to work fine ,it drives the right current. The problem is that it is making a noise. When we power up just one driver it doesn't make any noise, but when both are driving current at the same time it makes a noise. 

I understand that if both drivers individually don't make any noise the circuit should be working fine. I don't know why when both are switching it is making a noise. Could it be because both are switching at the same time but asynchronized and then it is making  the PCB vibrate?. Could it be any other reason other  like coupling, resonance of inductors ( but individually don't make noise) or any other thing?

Any help would be very appreciated. 

  • Hi All again,

    The switching frequency of both drivers is 80khz,the inductor value is 150khz , I have put more capacitance that what is required according to the TPS92691 design calculator. I have chosen the component for the application using the TPS92691 design calculator and the compensation network is PI( TYPE 2).

    Any help would be appreciated, thanks.

  • Hi Daniel,

    Can you get a scope capture of the input current and output currents of the LED's for us along with the input voltage at the power stage?  We would also like to have the schematic and spread sheet calculator that you use.  I assume you're not doing any PWM dimming.

    Thanks Tuan

  • Thanks Tran.

    Unfortunately I can not share technical information. I have done a further investigation and I have found out a couples of things.

    The PCB has two drivers that are identical, just one of them is making noise. The one which is making noise is drawing less current than what is suppose to be ( the second one is working as expected).

    I have scoped the gate of the mosfet that is making noise and it seems like every X time the IC is pulling down the gate to 0V. I though it could be the overvoltage protection triggering because of tolerance of resistors, so I changed the values to be sure it was out of OVP protection range but  it still is making noise and not drawing the right current.

    I think the IC is getting into a fault mode so that is why it is pulling down the gate to 0V, but I am not sure which one is being triggered as I have followed the calculator spread sheet and the second driver is identical and it works fine.

    Any idea of what next to test or investigate would be beneficial, thanks

     

  • Hi Again,

    We have done further investigation and it seems like the resistor R IS is not getting the current that it is supposed to get. I have measured the voltage with an oscilloscope of the gate of the mosfet and the voltage of R Is and it is getting less current than the driver that is working fine. Channel two is the voltage at R IS and channel 1 is the voltage at the gate of the mos fet.It seems like the current is not flowing for some reason. We have changed the mosfet in case it was the problem but it keeps having the same problem. The IC has also been changed with not good results.

    Below is the oscilloscope capture of the the gate of both mos fet ( the driver that works well is the blue trace and the driver that doesnt work well is the yellow traces). It seems like the gate pin of the IC is not opening the mos fet for some reason.

    This is the oscilloscope capture of the capacitor connected to SS. It seems like a capacitor charging ramp. Could it be because the IC is switching off for some reason?

    This is the voltage at Vin, so the IC should be getting power with no problems.

    This is the voltage at Vcc:

    This is the voltage at OVP:

    This is the voltage at output capacitors:

    I think there are here two things that are not right. The SS should be a constant voltage and it is actually a charge/discharge graph of a capacitor.  The second one is that the R IS is not getting enough current.

    Could you let mw know where could be the problem? Please I need some help as I am running out of ideas.

  • Hi Daniel,

    I cannot see any of the scope captures on your post at all.  You will need to post them for us again here.

    Thanks Tuan

  • Hi Truan,

    Please find attached the oscilloscpe captures: 

    Yellow channel is the gate of the driver which doesn't work properly, Blue channel is the gate of the driver that works properly.

    From here every oscilloscope capture is for the driver which doesn't work properly.

    This is the capture of the capacitor which is connected to SS pin. It looks like the charge/discharge trace of a capacitor. 

    The yellow channel is the capture of the gate of the mosfet, the blue channel is the capture of the voltage across IS resistor.It seems like the IS voltage doesn't reach what is supposed to reach. According to the calculator the peak current on the inductor would be 1.1A, but according to the blue trace I get about 300mA. I am not sure why, as I have tested the inductor/mosfet loop  activating the gate of the mosfet  with a external power supply   and having the IC disconnected and the inductor/mosfet loop works fine as I get the current expected on R is.

    Voltage on Vin seems fine to me.

    Voltage at Vcc is steady and it is the right level.

    What could be causing this strange behaviour?. Could it be that the controller is getting into an unstable state? Please any help or idea would be vert helpful

  • Hi Daniel,

    You're getting shut down of the TPS92692 due to over-current at 200Hz and that is where the audible noise is coming from.  The voltage that represent the current on IS does not look right at all.  You need to lift up one end of the inductor and put a current loop on it to see what the current is really with the current probe.  If the current on the inductor does not look like the voltage feeding back on VIS then you need to trouble shoot as to why.

    You mention that if just that one channel is ON like the fault one is on then there is no issue.  You need to verify that again and if it's no issue then there can be layout issue with one effecting another.  Again, hard for me to help you without schematic ect.

    Thanks Tuan