AMC6821: Application: FAN speed control abnormal

Kay Chen53

Expert 2225 points

Part Number: AMC6821

Hi there:

Our customer would like to do FAN speed control (ex: 100%, 90%, 80% or else...) , when 100% speed control, it’s OK;

but when in 90% speed control, we found some issue as following

Each PWM output signal with about 680ms interval. register set in DCY control mode.

Or any suggestion we could check from register viewing?

Question 1:

Does this 680ms interval case an normal symptom?

Question 2:

When zoom-in the PWM waveform, we found 2 different duty frequency; Does that normal happen in one PWM output?

over 5 years ago

0 Kay Chen53 over 5 years ago

Expert 2225 points

PS: attache waveform & schACM6821 FAN control.docx

0 Kelvin Le THS over 5 years ago in reply to Kay Chen53

TI__Expert 6475 points

Kay,

Can you send me a screen shot zoomed in a bit more. The duty cycle is 90%?

-Kelvin

0 Kay Chen53 over 5 years ago in reply to Kelvin Le THS

Expert 2225 points

Hi Kelvin:

Could you give us an example and location to check? or which register reading should be checking as well?

Thanks again.

also update the log message from customer

0 Kelvin Le THS over 5 years ago in reply to Kay Chen53

TI__Expert 6475 points

Kay,

Can you tell me what's your configuration register 2 setting is? Specifically, the TACH-MODE setting is important (pg 37 of datasheet).
When the TACH-MODE bit is cleared ('0'), the PWM-Out pin is forced ON during RPM
measurement, and internal correction circuitry is enabled to correct the error caused by
this extra duty cycle. This may be the reason why you're seeing different duty cycles.

I will need to go in the lab and check on the PWM output based on your configuration reg 1 setting of 0x15 to see if I get the 680ms period also.

-Kelvin

0 Kelvin Le THS over 5 years ago in reply to Kelvin Le THS

TI__Expert 6475 points

Kay,

I got the same result in the lab as you. The 680ms period is due to the TACH-MODE=0. If you set the TACH-MODE=1, this will disable 680ms period. The duty cycle variation you saw is probably due to this setting as well.

-Kelvin

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AMC6821: Application: FAN speed control abnormal