Question, i.MX28 DCDC converter setting

[Aemj]

Dear All,

I would like to ask about USE_VDDXTAL_VBG setting for i.MX28 DCDC converter.

In i.MX28 reference manual, the description of USE_VDDXTAL_VBG bit is said as follows.

“Change the reference used in the dcdc converter to a low power, less accurate reference.”

Could you show me more details about the “change” by USE_VDDXTAL_VBG bit setting?

And,

Does it mean the accuracy gets worse than +/-3% that is described in datasheet when the bit set to low power?

BR,

Miyamoto

[Aemj]

Dear All,

I am still waiting for your response.

Miyamoto

[arthur_lai]

Hi Miyamoto,

Yes, the DCDC output accuracy will be worse than +/-3% as specified in the datasheet if the bit is set.

Why do you need to set this bit? What is the total system power consumption?

Thanks,

Arthur

[Aemj]

Hi Arthur,

Thanks for your reply.

The customer is searching for every possibility to reduce power consumption.

Reducing power consumption is very important for the customer's system.

Is there the specification for the output current of DCDC and actual value of the accuracy in the case of setting the bit to 1?

BR,

Miyamoto

[arthur_lai]

Hi Miyamoto,

I am afraid we do not have the specification with the bit set to 1.

However, how much power does the customer want to save? There should not may be saving on setting this bit.

Thanks,

Arthur

[Aemj]

Hi Arthur,

Actually, the customer already tried with the bit set to 1 on his board. And he saw the power consumption of 10-15mW lower, although DCDC output voltage got lower as well.

For his system, 10mW of saving power is NOT small.

He wants to use this bit=1 if the condition(accuracy, output current/voltage) for this bit=1 is specified and the condition is acceptable for his system.

Could you double check whether you have the specification for the bit=1?

BR,

Miyamoto

[arthur_lai · Accepted solution]

Hi Miyamoto,

Please understand that all the characterized data are found on datasheet.

For the 10-15mW power saving after setting the bit, it is more likely to to due to the reduced output power. As you mentioned, the output voltages are lowered. Therefore the output and thus the input power will also be lowered. Furthermore, the current consumption ususally decreases with supply voltage. This will further reduces the power consumption. So, it is hard to conclude how much power could be save by changing the bandgap reference.

In addition, i.MX28 itself is powered by the DCDC. Therefore, we have to guarantee the DCDC voltage output for the i.MX28 to operate properly itself. As all our characterization are done with this bit = 0 (our default configuration), it is not possible for the customer to tell which condition is acceptable.

Is any 3.3V peripherals powered by i.MX28 PMU? And what is the VDDIO voltage now? One quick way to reducing system power consumption is to lower the 3.3V peripherals supply voltage to 3.15 or 3V.

Thanks,

Arthur