SN65HVS885: SN65HVS885 IPx and REx Frequence .

user4786735

Intellectual 460 points

Part Number: SN65HVS885

If I use SN65HVS885.

IF IPx have a 100K~200KHz PWM.

Can I get same frequence PWM on REx?

Shawn

over 5 years ago

0 Kate Dickson over 5 years ago

TI__Expert 5660 points

Hello Shawn,

Thank you for reaching out, I will be happy to help you. First, I will need more information.

What is the load capacitance and the load resistance on each REx pin?
What value of RLIM are you using in the system?

Once I collect this information I can further assist you.

Best,

Kate

0 user4786735 over 5 years ago in reply to Kate Dickson

Intellectual 460 points

Hi Kate

REx I Use 15K and 10K to get 3.3V

RLIM=13K//330K=12.5K

BTW.: Can I connect CE and Ln together? If I use SPI to read SOP data.

I don't have more GPIO to use for Ln.

Shawn

0 Kate Dickson over 5 years ago in reply to user4786735

TI__Expert 5660 points

Hi Shawn,

Thank you for that information.

I would like to point out that your system has a value of RLIM that is smaller than the value that was used in the test conditions in the data sheet (25 kOhm), which is resulting in a much larger I(IP-LIM) than the maximum of 4 mA . My suggestion is to increase the value of RLIM so that I(IP-LIM) falls within the specified range.

Could you help me understand if the anode or cathode node of the diode is connected to the input REx?

In regards to your second question, CE and LD cannot be connected to the same source. According to Table 2 in the data sheet, LD and CE must be opposite (LD high and CE low) in order to enable shifting to generate SOP data.

Regards,

Kate

0 user4786735 over 5 years ago in reply to Kate Dickson

Intellectual 460 points

REx connect to F280049CPMS , not LED.

So, REx can be run 100K from IPx?

attach is my schematic.

RLIM will change to 330k//30kohm =27.5K Ilim=3.25mA

Q2.

LD connect to GPIO is OK.

Thanks

ShawnDI_SN65HVS885-2.pdf

0 Kate Dickson over 5 years ago in reply to user4786735

TI__Expert 5660 points

Hi Shawn,

Thank you for sending the schematic. I would like to make some calculations. Do you know if there is any capacitance on the REx pin?

Thanks!

Kate

0 user4786735 over 5 years ago in reply to Kate Dickson

Intellectual 460 points

Hi Kate

I don't have any Capacitance on the REx.

It's needed? 10pF is OK?

Shawn

0 Kate Dickson over 5 years ago in reply to user4786735

TI__Expert 5660 points

Hi Shawn,

I have ordered some parts in order to run some tests in the lab. The parts should arrive within the next week, and I will need a few days to perform these tests. I will respond within 1 to 2 weeks.

Regards,

Kate

0 Kate Dickson over 5 years ago in reply to user4786735

TI__Expert 5660 points

Hi Shawn,

I was able to measure the waveforms of the input IP0 (shown below in yellow) and the output RE0 pin (shown below in green) with similar loading to your system.

The slope on the output waveform is due to the switching time associated with the parasitic capacitances of the current limiter circuit and load resistance. This return path (REx) was designed to be an LED driver and does not have the bandwidth to support this application at this frequency.

Regards,

Kate

0 user4786735 over 5 years ago in reply to Kate Dickson

Intellectual 460 points

Hi Kate

Thank you very much for your help.

0 user4786735 over 5 years ago in reply to Kate Dickson

Intellectual 460 points

Hi Kate

I don't put any Cap on REx side.

have any sugget to speed up the falling edge.

Use a buffer or Pivot circuit ? to get square wave.

0 Kate Dickson over 5 years ago in reply to user4786735

TI__Expert 5660 points

Hi Shawn,

My suggestion is to reduce the load resistance to reduce the fall time.

I would like to reiterate that this return path (REx) was designed to be an LED driver and does not have the bandwidth to support this application at this frequency.

Best Regards,

Kate

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SN65HVS885: SN65HVS885 IPx and REx Frequence .