CCS/TMS320F28379D: Using the SDFM to replace SAR ADCs for power electronics applications

Richard Tarbell56

Part Number: TMS320F28379D
Other Parts Discussed in Thread: CONTROLSUITE

Tool/software: Code Composer Studio

Greetings! We have a project using the 28335 that measures a total of 4 external ADCs (two voltages, two currents) in a power electronics AC-DC converter using SPI.

The simple layout is:

Timer fires at 100kHz --> at negative edge of timer, all 4 ADCs start their conversion ("simultaneous sampling") --> interrupt the 28335 on the ANDed "convert_done" signals --> read that ADC result in the interrupt routine

Now we are using the 28379D micro on the Launchpad development board. To save space and money, we would like to replace the 4 traditional SPI ADCs with delta sigma ADCs running at 10MHz clocks. We will still run the inner control loop at 100kHz, but now there is no "convert_done" signal to interrupt on - I understand that the delta sigma ADCs are updating at 10MHz (their clock speed).

--> In the SDFM, how can I temporarily "freeze" the ADC results every 100kHz, so that I can quickly read the four ADC results without their values changing on me?

over 7 years ago

0 Manoj Santha Mohan over 7 years ago

TI__Guru 55700 points

Richard,

SDFM filter results don't update the data on every SDCLK edge. It updates data at every (DOSR / SD modulator frequency). So, SDFM filter results are indirectly temporarily frozen.

Regards,
Manoj

0 Richard Tarbell56 over 7 years ago in reply to Manoj Santha Mohan

Intellectual 335 points

Thank you for your response Manoj! In the ControlSuite examples for the SDFM modules, the code shows how to setup the modules, but I don't see how to actually "read" the SDFM module results (that is, actually read the SDFM output, and convert it to some kind of voltage). Is there an example of that?

0 Manoj Santha Mohan over 7 years ago in reply to Richard Tarbell56

TI__Guru 55700 points

Richard,

No, we don't have an example exactly matching your requirement. The closest example is sdfm_pwm_sync_cpu which can be modified to your requirement.

Regards,
Manoj

0 Richard Tarbell56 over 7 years ago in reply to Manoj Santha Mohan

Intellectual 335 points

Thank you Manoj!

For understanding purposes:

Let's say I use a timer to generate a PWM clock of 10MHz for my sigma delta ADCs and the SDFM module.

I want an update rate of 100kHz (similar to our original setup using the SAR ADCs, we want our control law loops to run at 100kHz).

--> My OSR value should be OSR_100 (10MHz / 100 = 100kHz), is this correct?

--> Do I want to use OSR_100 in the comparator filter, or in the data filter? I don't exactly understand the difference.

0 Manoj Santha Mohan over 7 years ago in reply to Richard Tarbell56

TI__Guru 55700 points

Richard,

You can use PWM to generate SDCLK. This is effective than using CPU Timer. Also, I would advice you using 20 MHz PWM clock instead of 10 MHz as it provides better ENOB for a given OSR setting.

For Comparator filter, filter result is not memory mapped and COSR can be varied from 1 to 32.
For Data filter, filter result is memory mapped and DOSR can be varied from 1 to 256.

Comparator filter is used to detect over-/under-current condition and is not used in control law loop. Data filter output is used for control loop.

Regards,
Manoj

0 Richard Tarbell56 over 7 years ago in reply to Manoj Santha Mohan

Intellectual 335 points

Thank you again Manoj!

--> So let's say I use a 20MHz PWM from the 28379, along with an OSR rate of 200 (again, for a control loop rate of 100kHz).

What is the best way to trigger an interrupt every 100kHz, to "synchronize" the control law equations with the SDFM results?

0 Manoj Santha Mohan over 7 years ago in reply to Richard Tarbell56

TI__Guru 55700 points

PWM interrupt at every 100KHz is best method as the whole process is synchronized with respect to PWM which happens to be heart of control loop.

Regards,
Manoj

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CCS/TMS320F28379D: Using the SDFM to replace SAR ADCs for power electronics applications