TPS61169: TPS61169 current too high; does not respond to value of feedback resistor

Peter Borden

Part Number: TPS61169
Other Parts Discussed in Thread: LM3410, TPS61165, TPS61500

I am using three TPS61169s to drive three strings of 5 RGB LEDs. The respective string voltages are R=10.8, G=15.7 and B=14.6 volts, and I chose feedback resistors of R=.47, G=.56 and B=.68 ohms to give string currents of R=425 mA, G=360 mA and B=325 mA. I am using the circuit on page 1 of the data sheet with L=6.8 uH, Cout = 1 ufd, Cin = 10 ufd, and the Schottky diode is rated at 40V and 500 mA. I've attached a screen shot of the layout and the driver schematic. In the layout, the red channel is on the left, the green in the middle, and the blue is on the right. The PWM frequency is 10 kHz.

The channels for the three colors have the same layout. The blue channel works properly. However, while the red and green channels respond to the PWM input, the maximum currents are 1.25 and 1.35 A (which looks like the current limit for the TPS61169), regardless of the value of the feedback resistor. I have built several boards and they all do the same thing, so this appears to be a design problem.

Please suggest what might be happening and how to correct the problem.

Also, in the event that the TPS61169 is an inappropriate choice for this application, please suggest alternate parts. I have used the LM3410 in the past on other projects; is this an acceptable alternative?

Thanks,

Peter

over 4 years ago

0 Frank Qiu over 4 years ago

TI__Genius 15776 points

Hi Hardy, could you help Peter in this case.

0 Hardy Wu over 4 years ago

TI__Genius 12915 points

Hi Peter,

It looks like the FB pin shorted to GND. Could you help to check if FB resistor have the right value? Also we recommend this resistor close to FB pin.

0 Peter Borden over 4 years ago in reply to Hardy Wu

Prodigy 10 points

Hi Hardy:

Thank you for the fast response. I rechecked and the FB pins are connected to their resistors, not ground, as shown in the schematic. After lifting the FB resistors, the FB pins are open, so there is likely another problem.

I look forward to your thoughts on other possible causes.

Also, is the TPS61169 suitable for applications at this current (300-450 mA), or is a part like the TPS61165 (or another one you can recommend) preferred?

Thanks,

Peter

0 Frank Qiu over 4 years ago in reply to Peter Borden

TI__Genius 15776 points

Hi Hardy please help in the further discussion.

0 Hardy Wu over 4 years ago in reply to Peter Borden

TI__Genius 12915 points

Hi Peter,

I think TPS61169 is OK for the application with 300-450 mA output current. For higher current limit device, you could look at TPS61500

It is still hard for me to find out why all 3 devices have the same circuit but red and green have abnormal behavior. Could you help to measure the voltage on FB pins to see if they are 200mV?

0 Peter Borden over 4 years ago in reply to Hardy Wu

Prodigy 10 points

Hi Hardy:

The voltage on the FB pins is between 0 and 200 mV depending on the PWM fraction; I think this is normal. However, there is spike noise, and this could be a source of the problem. I'm laying out the board again following your recommendation of placing the feedback resistors next ot the feedback pin. I'll get back to you after I have been able to test this.

Thanks for your help,

Peer

0 Frank Qiu over 4 years ago in reply to Peter Borden

TI__Genius 15776 points

Hi Peter, please help tp capture the waveform which Hardy need thx.

0 Peter Borden over 4 years ago in reply to Frank Qiu

Prodigy 10 points

Frank:

These are the waveforms at the FB pin for the red (left) and green (right) LED drivers. These are the ones that are putting out full current, independent of the feedback resistor.

0 Frank Qiu over 4 years ago in reply to Peter Borden

TI__Genius 15776 points

hi hardy could you help Peter in this case.

0 Peter Borden over 4 years ago in reply to Frank Qiu

Prodigy 10 points

I re-laid out the circuit board with the feedback resistors next to the driver, as you recommended. I also added a second supply bus to better connect the drivers to the supply. These changes did not solve the problem. I tried larger feedback resistors to drop the current (1 ohm), and the circuit worked as expected. Although I appear to be within the upper limit of the current indicated on the spec sheet, I suspect I am asking the TPS61169 to supply too much current in this application

Thank you very much for your prompt assistance.

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TPS61169: TPS61169 current too high; does not respond to value of feedback resistor