VF5xx/about BCTRL of TWR-VF65GS10 Schematics Rev.G.

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VF5xx/about BCTRL of TWR-VF65GS10 Schematics Rev.G.

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soichiyamamoto
Contributor V

Hi,

I Have question about BCTRL of TWR-VF65GS10 Schematics Rev.G.

I not use VideoADC.

This circuit makes 1.2V from Q1 (NJD2873T4).

1.Do I need Q1 (NJD2873T4)?

2.This circuit makes 1.2V(For VDD1~VDD23) from Q1 (NJD2873T4)

  Where does the feedback to make 1.2V go this circuit?

3.Because a part area is very big, Q1 wants to use the small part.

Please tell me the criteria for selection of Q1

Best Regards,

soichi

.

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naoumgitnik
Senior Contributor V

Dear Soichi,

  • Necessity of having Q1 in the design is not related to just VideoADC - 1.2V is mostly used by the device core.
  • The VDD feedback is inside the device.
  • Since this board is intended for code development as well, the device might run a heavy-computation code, which translates into high core current. Since it is a linear voltage regulator and the transistor dissipates 2.1V (Vce = 3.3V - 1.2V) at Ic up to 0.8A at max. temperature (see Datasheet), it is ~1.7W. There ways to improve the situation - take a look at the App. Note (http://cache.freescale.com/files/32bit/doc/app_note/AN4807.pdf?fsrch=1&WT_TYPE=Application%20Notes&W...) and this thread - https://community.freescale.com/message/335416#335416. Please, pay attention to the requirement for the mutual timing of the 3.3V rail and the transistor collector voltage; the best way to meet it is using voltage regulators with a shut-down feature.

Regards, Naoum Gitnik.

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naoumgitnik
Senior Contributor V

Dear Soichi,

  • Necessity of having Q1 in the design is not related to just VideoADC - 1.2V is mostly used by the device core.
  • The VDD feedback is inside the device.
  • Since this board is intended for code development as well, the device might run a heavy-computation code, which translates into high core current. Since it is a linear voltage regulator and the transistor dissipates 2.1V (Vce = 3.3V - 1.2V) at Ic up to 0.8A at max. temperature (see Datasheet), it is ~1.7W. There ways to improve the situation - take a look at the App. Note (http://cache.freescale.com/files/32bit/doc/app_note/AN4807.pdf?fsrch=1&WT_TYPE=Application%20Notes&W...) and this thread - https://community.freescale.com/message/335416#335416. Please, pay attention to the requirement for the mutual timing of the 3.3V rail and the transistor collector voltage; the best way to meet it is using voltage regulators with a shut-down feature.

Regards, Naoum Gitnik.

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