Help with Power to D+ and D- on USB

[joeygemma]

I'm building a switching voltage regulator using an LM2675. I am converting 28VDC to 5VDC @ 1A. The 5V end is USB and needs to be able to charge an iPhone. The entire circuit works and I have voltage going to D+ and D- (about 1.5 and 2.0 respectively).

I used the resistor layout from Ladyada (MintyBoost - The mysteries of Apple device charging) to adjust the voltage for D+ and D-. Her voltage comes out differently, which I can't figure out. She has one resistor from each D pin to 5V and one to ground, which I also did. My question is can I just use a resistor to go from 5V to 2V and 2.8V without another resistor going to the ground?

Thanks for your input.

 

[RCinFLA]

The D+ and D- line are only for informational control of the +5vdc Vbus sourcing current capability.

The arrangment of setting a fixed voltage between D+ and D- is unique to Apple and appearently what they use to determine charger current capability. I don't know the exact voltage settings but they do allow for different charge current rates based on charger's maximum current output capability.

 

[joeygemma]

I understand what they're used for, and I'm aware that different voltages give different current rates (ie: 2.0V and 2.0 volts results in 500mA and 2.8 and 2.0 results in about 1A).

I just need to know if it is possible to reduce the 5V input to 2.0V and/or 2.8V safely with one resistor on each lead (D+/D-).

 

[RCinFLA]

Probably not.

One thing you might want to check out is that the differential voltage level determines what the charger pack's current limit is set to. (based on charger pack power capability). This current I.D. reading may be used to determine how the Apple device charges its battery.

To avoid excessive heating within phone it is easier to just hard switch on and off the series pass regulator MOSFET in the phone. The phone relies on the charger pack to limit the current, suitable to the phone's battery AH size.

If the I.D. says the power pack is capable of a higher current then the phone's battery maximum charge current based on battery A-H size then the phone's series pass MOSFET must operating in linear mode subjecting it to greater heating. Under this condition the charge current may be less then the matching charger power pack, even though the power pack is capable of higher current then phone's matching power pack. It is forced to be less because it cannot operating in ON-OFF mode so regulator in phone must reduce charge current to avoid overheating the series pass MOSFET.

 

[joeygemma]

The charger is set to 1A, but the phone will also accept 500mA. Ideally I want 1A and that is what this circuit is set up for, but I will settle for 500mA if that's all I can get.

 

[RCinFLA]

The max allowed current is based battery size. Just because power pack can supply 1 amp it doesn't mean that charge rate can be used.

Normal USB spec is no more then 500 mA loading on any single USB port. A desktop may be capable of greater then this but Apple can not count on it and must limit its charge rate to 500 mA and assume there is not an accurate current limiter on that Vbus supply.

Overdraw on USB Vbus from a computer can have unpredictable results. Typical result is shut down of that ports Vbus by host computer. Some tablets and small laptops only allow 100 mA draw on Vbus.

 

[joeygemma]

I'm not asking if the iPhone 4 accepts 1A. I'm telling you that it does. Thank you for your input, but that is not the information I'm looking for. I'm simply trying to figure out the best/easiest way to get the correct voltages to the D+ and D- pins on the USB port.