What determines VAUX current limitation in FS45/FS65?

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What determines VAUX current limitation in FS45/FS65?

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Contributor III

I would like to start off with the fact that I am still waiting on receiving access to AN5238 and FS6500-FS4500SMUG. Maybe the answer to my questions is listed in there, but given the fact that I've already been waiting a few weeks, I'd like to ask my questions here.

The FS6500-FS4500 datasheet (Publicly available, thankfully) lists that VAUX requires an external PNP as it has no internal current handling capabilities. The max rating for IAUX is 400 mA. Current limitation kicks in between 400 and 800 mA. The datasheet in no way explains how this current detection is implemented, leaving me with the following questions:

  • How does the SBC detect how much current is being drawn, if the only information it has is the PNP's base current and the voltage on VAUX? It does, after all, not know the PNP's hFE.
  • What's to stop me from choosing any arbitrarily large hFE to increase the current handling capability, other than heat generation and maximum supply current constraints?
  • How does the hFE of the transistor affect accuracy of VAUX? Load regulation for VAUX is listed as 15 mV static and 1% transient, rated between IAUX of 10 mA to 400 mA. I would assume the hFE has more impact on these tolerances than the current flowing at any moment. Leading me to my last question;
  • At what accuracy can the SBC drive the base current of the PNP?

So in short and in general: What does the internal circuitry of the VAUX function block look like? How does it work?

Kind regards,

Joey

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NXP TechSupport
NXP TechSupport

The basic control theory below shows:

In the NPN of LDO works on saturation condition,when over current happened VBE drop down and IB decrease then IC decrease.

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Contributor III

Hi G.w. Sun,

Thank you for your response. Are you talking about the internal NPN of the VAUX LDO inside the FS65&FS45? If so, does that mean the limitation for IAUX listed in the datasheet is based on assumptions about a certain range of hFE for the external PNP?

What limits exist for the hFE of the external PNP? This is listed nowhere in the datasheet, and I am missing any other rationale for the value of 400 mA that is listed in the datasheet.

Kind regards,

Joey

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NXP TechSupport
NXP TechSupport

For AUX there is only one NPN external.

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Contributor III

Hi G.w.,

According to the datasheet, the external transistor should be PNP, not NPN. That's what got me confused.

For VCCA, the datasheet lists 100 < hFE < 450. For VAUX no limits are listed. Should I assume the same limits apply, and the datasheet is lacking?

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NXP TechSupport
NXP TechSupport

Sorry that for the typo,you can consider that 100<hFE<450 is same with VAUX.

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Contributor III

Hi G.w.,

Thank you. I've now also received access to AN5238 and this confirms that hFE for PNP should be between 100 and 450. I still have a few questions about how this influences the current limitation range, but I believe this platform is not the right place to ask, so I've opened a case through NXP support.

Thanks for your time,

Joey

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