LM5026EVAL: Power management forum

Irene Perez

Part Number: LM5026EVAL
Other Parts Discussed in Thread: LM5026

Hello

I would like to control the controller of the eval board converter from the comp pin, in open loop, skipping the compensator.

So I have removed the optocoupler from the PCB and connected a voltage supply to com pin. The datasheet indicates:

It says that duty cycle is maximum when current is 0A, is this maximum duty cycle 80% or 100%?

Also in regards to the com pin the datasheet says:

From that I understand that inside the controller there is a resistor of 1700ohm as per the following scheme:

Is this assumption correct? I have used this resistor value to calculate the proper voltage over the comp pin.

For an input voltage of 40V and applying 780mV on the COMP pin I got an output voltage of 3V. But I´m not sure about my calculations. Could you please let me know the proper way to calcualte the voltage value to be applied on the comp pin to set the desired duty cycle? what is the voltage value in order to achieve a 50% of duty cycle for instance?

I´m doing this test because I’m going to change the components of the eval board in order to move the output voltage from 3-3V to 5V, so once the HW changes are completed, before closing the loop I prefer to set the duty cycle from the comp pin working in open loop and measure the plant to calculate the compensator.

Thanks in advance !!

over 4 years ago

0 Aidan Davidson1 over 4 years ago

TI__Intellectual 2640 points

Hi Irene!

Thank you for reaching out and for your interest in the LM5026! When the COMP pin is demanding maximum duty cycle, the limit is either determined by the user programmed max duty cycle or the voltage-dependent duty cycle limit (which varies inversely with input line voltage). See below images from the datasheet for more information.

Chances are that your maximum duty cycle is ~68% due to R11 & R10 and the clamping mechanism is the resistors and not the input voltage (for your input of 40V).

Let me confirm with one of my colleagues on the COMP small signal impedance and I will get back to you with more information by Thursday latest.

Thanks again for getting in touch, I look forward to talking more soon!

Aidan Davidson

0 Aidan Davidson1 over 4 years ago in reply to Aidan Davidson1

TI__Intellectual 2640 points

Hi Irene!

Apologies for the delayed response! It seems my original images didn't post correctly, please see below.

I still haven't heard back regarding the COMP impedance. If you vary the voltage directly applied to the COMP pin, does the output voltage vary correspondingly? I would expect the set duty cycle to be equal to ( COMP current * max duty cycle / 1mA ), so a duty cycle of 50% could be achieved with a COMP pin current of ~735 uA. If you have access to a current probe that would be an easy way of confirming your open-loop behavior.

Best,

Aidan

0 Irene Perez over 4 years ago in reply to Aidan Davidson1

Prodigy 150 points

Hi Aidan,

Thanks for your response. The output voltage of the converter varies correspondingly to the voltage applied at the COMP pin.

In any case, I would appreciate if you can provide a answer regarding the comp pin resistance: 1700ohm,

Thanks a lot,

0 Aidan Davidson1 over 4 years ago in reply to Irene Perez

TI__Intellectual 2640 points

Hi Irene!

Unfortunately I'm still waiting on an answer from one of my colleagues, as soon as I hear back from him I will update you here!

Thank you for your patience!

Aidan

0 Aidan Davidson1 over 4 years ago in reply to Aidan Davidson1

TI__Intellectual 2640 points

Hi Irene!

Since you're running the LM5026 in an open-loop configuration, the small signal impedance will not be a factor here. This means that you can choose your COMP current with a 5V voltage source and a resistor of your choice on the COMP pin. Reference the chart below from the LM5026 datasheet for help choosing your COMP current.

Thanks again for reaching out Irene, I hope this information helps!

Aidan

0 Irene Perez over 4 years ago in reply to Aidan Davidson1

Prodigy 150 points

Hi Aidan,

Since there is not any voltage value limit on the datasheet for the COMP PIN, what is the maximum voltage value I can apply?

If I apply 5V through a 10kohm resistor, so 500uA current, What should be the duty cycle?

Do you confirm that the diagrama I sketched in the first message of the Com Pin (with the series resistance-1700ohm) is not correct?

Many thanks,

0 Aidan Davidson1 over 4 years ago in reply to Irene Perez

TI__Intellectual 2640 points

Hi Irene!

The maximum voltage value is 7V.

I got a bit more information here: the on time of the PWM pulse is determined using the CS waveform in conjunction with the chart (Figure 13) that I referenced above. Inputting a COMP pin current of 500uA would yield a threshold voltage of ~0.275V (from Figure 13 above). The PWM output will turn on when the CS waveform passes that 0.275V threshold. If you were applying a generated sawtooth to the CS pin with an amplitude of 0.35V you could expect a duty cycle of ~21.4%. Calculated using (max ramp value - threshold voltage) / max ramp value.

Yes, I confirm that this diagram is incorrect. Since we are injecting DC current the small-signal model does not apply so we can disregard this impedance.

Thanks, I hope this information helps!

Aidan

0 Irene Perez over 4 years ago in reply to Aidan Davidson1

Prodigy 150 points

Hi Aidan,

Last question :)

If I leave the COMP PIN opened, will the controller work at the maximum duty cycle set bu the DCL?

Many thanks!

0 Aidan Davidson1 over 4 years ago in reply to Irene Perez

TI__Intellectual 2640 points

Hi Irene!

Per Figure 18 shown above, this depends on your UVLO pin voltage (which depends on your input voltage). If your UVLO pin voltage is 1.25V - ~2.25V the max duty will be set by the resistors around the DCL pin, if the UVLO pin voltage is greater than ~2.25V the max duty will be set by the line voltage duty limiter.

Thanks again, I hope this helps!

Aidan