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TCAN2450-Q1: minimum PWM duty cycle on HSS?

Jonathan Jank
Intellectual 1045 points
Part Number: TCAN2450-Q1

Tool/software:

I noticed some issues when observing the PWM output and would like to ask for your clarification on the following.

 

Here are the details of my configuration: the PWM frequency is set to 200Hz, and the PWM width is configured as 10 bits. During testing, I noticed that when the duty cycle is set to 10 (approximately 1%), the connected LED lights up normally, and the oscilloscope shows a valid PWM waveform. However, when the duty cycle is set between 0 and 9, there is no PWM waveform detected on the oscilloscope, and the LED remains off.

 

Given the above phenomenon, there seems to be an inconsistency with the confirmed minimum output duty cycle of 0.5%. Do I need to configure any additional special registers to achieve the minimum duty cycle output?

if duty cycle is set to 9, that would be approx 350us On time.   That seems fairly long as it relates to slew rate.

thanks,

JJ

  • 0
    TI__Guru 52482 points

    Hi JJ,

    So I think the math is a little off - a 10 bit PWM at 200Hz will have each step be ~4.88us ( 1/200 = 0.005; 2^10 = 1024, 0.005 / 1024 ~ 4.88us; 1024 * 4.88us ~ 5ms which is 200Hz period) - so 9 would be ~45us not 450us (for 450us you would want 90) 

    That being said you can program the 10-bit PWM to values that are not possible due to switch on/off time. The spec we have is 30us to 90us for off/on time - but that is assuming 220 ohm loading. At 200Hz values less than 6 will be under the minimum 30us and values and less than 19 may still be caught with PWM - so hitting 10 you are in the range that is more typical - but I wouldn't do a PWM less than 20 (0b0000010100) for best performance. 

    I am not sure where you are seeing the 0.5% min  duty cycle - but you won't hit that with the switch times being what they are - as 0.5% duty cycle at 200Hz is going to be an on time of 25us - where the switch could take a minimum of 30us for an on time - really you are looking at probably about a minimum of 2% duty cycle at 200Hz for best performance. 

    If you can share the test setup with loading I may be able to quickly run a test in the  lab tomorrow to confirm - but I do think it is just a limitation of the device. Based on what you are reporting - the duty cycle doesn't work until you hit 10 or ~48.8us - which is in the 30us to 90us range - so it is possible for it to show up - but it could be longer i.e. why I said 20 because up until 19 you may miss it - the test condition will impact this slightly as well - our test setup is 220 ohm load, no decoupling cap (purely resistive load so no decoupling cap needed) at a VSUP of 14V

    Please let me know on the test setup so I can verify - but I do think that is what is happening. 

    Best,

    Parker Dodson