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I'm drawing a LiPo 3.7V charger circuit. This circuit contains a power path to continue delivering power to the circuit, or by battery or usb. In the final of the circuit I've an LDO to 3v3.

Now, I want to use CP2102 as an RS232 converter and my question is: Should I supply CP2102 with power from the BUS pin?

enter image description here Or should I feed it with the 3v3 that come from the LDO?

enter image description here The second options seems to be better to me, becouse:

  1. I don't need the 3v3 from voltage regulator (REG0)
  2. Using Bus as supply it could affect the behaviour of my power path?!

I've a few more questions about it.

  1. Whenever I disconnect VBus the CP2102 automatically enters on standby mode?
  2. Should I use any pull-up resistor on D+ and D-? Datasheet doesn't make any reference to that.

Best Regards

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Should I supply CP2102 with power from the BUS pin?

You MUST always connect VBUS to the +5V feed in some way (either directly or through a divider). The IC will not operate otherwise, it uses this signal as an indication that USB is connected.

Or should I feed it with the 3v3 that come from the LDO?

Per the datasheet,

Alternatively, if 3 V power is supplied to the VDD pin, the CP2102/9 can function as a USB self-powered device withthe voltage regulator disabled. For this configuration, it is recommended that the REGIN input be tied to the 3 V net to disable the voltage regulator. In addition, if VDD or REGIN may be unpowered while VBUS is 5 V, a resistor divider (or functionally-equivalent circuit) shown in Note 1 of Figure 8 and Figure 10 is required to meet theabsolute maximum voltage on VBUS specification in Table 2.

No matter what, all of the configurations show at least some connection to VBUS - either directly or through a resisor-divider.

Whenever I disconnect VBus the CP2102 automatically enters on standby mode?

I'm not sure about that, but most likely the USB transceiver would go idle since the chip would recognize that the USB connection is not present.

Should I use any pull-up resistor on D+ and D-? Datasheet doesn't make any reference to that.

This is fundamental USB knowledge, so the datasheet author most likely skipped this in the datasheet. Pullups - no. Depending on your PCB layout, it sometimes does not hurt to put zero-ohm resistors in series with D+ and D- in case you need to tweak the impedances slightly (i.e. if you get data errors with long cables and whatnot, you can change to a few ohms to dampen things). The CP2102 only runs at full speed (12MHz) so it's fairly forgiving in this regard.

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  • \$\begingroup\$ Hi, I really appreciate your answer. But my question is: There is any advantage about connecting VDD to my LDO output? Or I can use VBus to supply the CP2102? What's the better approach? I understand your point about left VRegin disconnected if I'm not using the regulator, but in datasheet they never left this pin disconneted. Do your really think there is no problem? \$\endgroup\$ – Miguel Andrade May 27 '20 at 15:01
  • \$\begingroup\$ The datasheet for the CP2102 shows in Fig.9 on page 21 that Vregin should not be left disconnected, but should be connected to the 3V power net when the internal regulator is not being used. \$\endgroup\$ – brhans May 27 '20 at 15:38
  • \$\begingroup\$ Yes, that's right. \$\endgroup\$ – Miguel Andrade May 27 '20 at 16:18
  • \$\begingroup\$ Fixed. Seems a little weird but I trust the datasheet. \$\endgroup\$ – Adam Lawrence May 27 '20 at 22:28

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