Clarify requirments of USB Vregin. Does USB really work with Vregin down to 2.7V?

[davidburton]

There are quite a few posts on Vregin and Vout33 and USB, but none that I have found completely clarify the issues.

The K20P48M50SF0 data sheet says Vregin can be 2.7 to 5.5V

Vout33 supplies the internal USB transceivers.

Vout33 regulator is internally powered from Vregin.

Vout33 can be connected to an external 3.3V supply, which I assume would then be the power for the transceivers and the internal Vout33 regulator would do nothing.

The Vout33 internal regulator is specified to be 3-3.6V or 2.1-3.6V(standby) output.

It's stated in other posts that Vregin must be connected for USB to work, even if Vout33 is connected to an external regulator. 

Is this true if I am providing 3.3V to Vout33 from an external regulator?

If I provide 3.3V to Vout33, then will USB still work if Vregin is also connected to 3.3V?  Does it need to be higher than 3.3V?  If so, why is that, if my external 3.3V is powering the USB transceivers?

I have 3.3V on board, but my host controller does not want to provide me 5V over the USB cable. (dedicated application).

Thanks.

[kerryzhou]

Hi David,

May be you have misunderstood the Vout33 pin.

I don't know where you get this information:"Vout33 can be connected to an external 3.3V supply, which I assume would then be the power for the transceivers and the internal Vout33 regulator would do nothing."

Actually, we don't connect Vout33 pin direct to the external LDO, Vout33 pin in the MCU is the output pin, not the input pin.

In some application, just use the Vout33 as the power supplier, instead of the external regulator.

If you don't want to use it, you just need to add a capacitor to the Vout33, then leave it floating, but the VREGIN you still need to connect, from the datasheet, the VREGIN can powered from 2.7V to 5.5V.

So, if you don't have the 5V, you can connect your 3.3v to the VREGIN pin.

Wish it helps you!

If you still have question, please contact with me!

Have a great day,

Jingjing

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[davidburton]

Connecting an external 3.3V to Vout33 When there is no Vregin>3.6V was implied in another post on this forum somewhere.So also is a statement that Vregin needs to be >=3.6V for the Vout33 regulator and USB to work.  From your last sentence I am still wondering if are you saying USB works if Vregin is in the range of 2.7-3.3V?  I already know I can go as low as 2.7V on Vregin, but I hope someone can just come right out and directly say the answer to this question: Does USB work with Vregin in the range of 2.7-3.3V?

Thanks.

[kerryzhou · Accepted solution]

Hi David,

    Could you give me that post link in this forum?

  From our datasheet, the VREGIN input supply voltage is from 2.7V to 5.5V.

  When the VREGIN is lower than 3.6V, the regulator will enter in the pass-through mode, the Vreg33out will be 2.1V- 3.6V,  the USB still can work, so you can use 3.3V in the Vregin, but don't connect the outside 3.3V to the Vreg33out pin, just add a capacitor is ok. Now we already have a customer use the 3.3V Vregin and make the host works.

  But take care, even you can use the VREGIN as 3.3V, you still should give the USB Vbus 5V, otherwise, it may cases the device can't work.

Wish it helps you!

Have a great day,
Jingjing

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[davidburton]

Thanks for clarifying this.  Makes sense the way you have stated it.

So USB OK with Vregin down to 2.7V, but do not allow any external loads on Vreg33out if Vregin is <3.6V.

Here is the link to the discussion that talks about connecting Vreg33out to external Vdd:

K20 USB host power circuit configuration

Thanks.