ADAU1451WBCPZ-RL - 1.2V Rail connected to external IC (ADAU7118ACPZRL7)

HardwareDesign Question

Mar 30, 2022 00:00

Hi,
We are generated 1.2V using Vdrive pin & PNP transistor(DXTP3C60). This1.2V rail is connected to DVDD of DSP & the external IC (ADAU7118ACPZRL7 - it is also Analog device).

The Beta of transistor is 140. But actual beta required for linear operation is 45 only. The current of external IC (ADAU7118ACPZRL7) is 1.4mA. As per the configuration, this transistor can able to drive the external IC in addition to DSP.

While we checking with datasheet of DSP, it is not recommended to connect the 1.2V rail to external IC. Attached below for your reference. It is in 41 page no.

Could you please share the reason for not connecting 1.2V rail to external IC.

Thanks in advance

Thanks & Regards,

Thangaprabu S

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DaveThib Reply

Mar 30, 2022 00:00

Hello Thangaprabu,

This is because it will add extra capacitance and inductance to the DVDD power rail. In addition, an external part can inject noise onto a power rail and long traces can also inject noise. We carefully designed the voltage regulator in the part for a stable phase margin with a known capacitance on the line. Connecting to other parts introduces all these other factors that are out of our control and may cause a problem with the DSP core, the regulator, or other digital circuits that would make it seem like the problem is with our part when it could be an external factor causing the problem. So to guarantee the performance we say to not use this DVDD power for anything else.

The ADAU7118 is very low power but we cannot say that this is OK because chances are there will be a long trace to this part so that will add the factors I mentioned above.

Dave T

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HardwareDesign Reply

Mar 30, 2022 00:00

Hi Dave,

Thanks for the quick response.

Thangaprabu S

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