USB Charger Circuit - Initial Consumption Before Power Negotiation - Timing?

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Joined Oct 21, 2014
Hello, I have a voltage regulator (switching & boost regulator) who's output voltage is being set/controlled by a microcontroller. I can set the output voltage to anything between 4.5 volts and 6.5 volts. Under normal operation, the voltage is always going to be between 5.2 volts and 6.5 volts - this is used to power an external circuit. I want to be able to double the device as a USB charger and leverage this voltage regulator I already have.

The USB standard voltage is 5V, plus or minus 0.5 volts (e.g. 4.5 volts to 5.5 volts). Some places I have seen it mentioned to be 4.75 to 5.25 volts. Because of this, I cannot simply have the above mentioned voltage regulator connected directly to the VBUS on the USB, because the voltage is likely to be above 5.25 or 5.5 volts.

I was hoping there would be a way for me to provide a temporary 5V on the VBUS at all times by using an inexpensive extra voltage regulator... then detect when a device is plugged into the USB, then have the microcontroller bring the voltage down of the main voltage regulator to 5V, and then connect this up to VBUS to begin charging. I was thinking if this was fast enough, the device that is being plugged in wouldn't even know, and I am assuming there would be a delay before the device tries to pull a higher amperage to charge (I have the D+ and D- connected together to signal to external devices that this USB is a dedicated charging port and you can pull higher amperage off it). I could never really find definitive info on the timing of USB charging, in regards to initial current draw, and power negotiation.

I could always use a LDO voltage regulator, such as the "NCP59301", and have this connected up to the VBUS at all times. This would provide the high current capabilities as well... its just additional components and additional decreased battery efficiency, and I already have the main voltage regulator onboard, so I am trying to figure out a way to leverage that instead of adding a complete 2nd voltage regulator.

Do you think there is any way to accomplish this, or a different approach? Thanks and any help or advice is greatly appreciated!


Joined Aug 12, 2014
Does it really need to be automatic and risk race-condition issues with not getting down to an acceptable voltage fast enough? Why not just manually adjust the voltage to an appropriate level before plugging into the USB output?

If you wanted a little more safety, you could add a window comparator and light an LED that indicates when the voltage is between 4.75-5.25, so it's obvious when it's safe to use.

If you wanted still more safety, you could put a MOSFET (or some other switch) between regulator and USB plug, controlled by the comparator such that there's no power at all on the USB plug unless the voltage is in the compliant range. That way you never get an unacceptable voltage - your only choices are correct voltage or none at all.

For that matter, you mentioned a microcontroller, so forget what I said about extra comparators. The micro can presumably tell what voltage the regulator is at, so it could directly control an indicator LED and/or MOSFET disconnect.

Anyway, sorry for sidestepping your question. I can't think of any good way to detect that something has been plugged in and adjust the voltage regulator fast enough to be confident in the system. If it were me, I'd find a different approach.