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Title says it: I'm working on building a high current USB charger for my devices so that I can integrate USB ports into my headboard. However, I'm having a few glitches.

  1. When I plug in a device, the voltage drops ~0.5V. Each device after drops it ~0.1V further per device. I have it up to 5.5V at the moment, and plugging in my devices will drop it to 5V. Yet it's charging my tablet just fine. Very fast actually even without declaring the current capability.
  2. My iPhone will not charge through this. I have a MAX14667 chip I am integrating, but was hoping I could get it to charge without it for testing purposes as they are small chips and pain in the butt to work with. Is this because I haven't declared the current capability with a resistor divider yet? The most I got is a "not supported" message from the iPhone.
  3. For the purpose of easy testing, I'm using a TI LMZ23605 module. However, for some strange reason the resistor values calculated by the TI site are not giving me 5V on either of the two modules I have prepped for a breadboard. Here is the schematic:
    lmz23605

So my values are Rfbb = 1K, Rfbt = 5.6K + 352. This has given me 5.5V. Plugged in my tablet, dropped to 5V.

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  • \$\begingroup\$ What is your input voltage? You don't show your power ground connected anywhere - why not? \$\endgroup\$ – WhatRoughBeast Feb 1 '16 at 4:56
  • \$\begingroup\$ Input is 12V @ 6A. This schematic is straight off their website, so you'd have to ask them why; I couldn't tell you. \$\endgroup\$ – Dominic Luciano Feb 1 '16 at 5:11
  • \$\begingroup\$ If this is for a "dedicated charger" the USB specification calls for data+ and data- to be shorted together to signal attached device(s) to enter "rapid charge" mode. As for the voltage, voltage variation from 5.5v down to 4.6v have been reported by my devices' internal DACs while connected to mainstream chargers/PC usb ports. \$\endgroup\$ – Robherc KV5ROB Feb 1 '16 at 5:13
  • \$\begingroup\$ That much, eh? Wow! I've read these devices charge down to 4.5V. Is that the case? Is the regulator being at 5.5V safe? \$\endgroup\$ – Dominic Luciano Feb 1 '16 at 6:46
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1 and 3 are the same. You likely don't have a load when you are measuring it at first. Most regulators have a minimum load for proper regulation. Once you place a load, it starts regulating in the stated range. The LMZ23605 requires a minimum load of 1 mA. Throw an LED + Resistor on there and that should be enough.

ΔVO/ΔVIN Line regulation VIN = 12V to 36V, IO = 0.001A ±0.02% mVPP

And depending on the the wire length, size, and resistance, that would explain a drop as well. Undersized or crappy USB cables will drop too much voltage at a higher current load. This explains the 0.1V drop per extra device. A 0.2 Ω cable resistance (round trip), at 500mA, would result in a 0.1V drop.

You already know 2. Yes, you need the signaling resistor pairs on the D+ and D- lines, or an iOS device will not charge. You need to use the Apple Standard for it to work with your iPhone.

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    \$\begingroup\$ Well actually I do have a load. I've been measuring it with LED's, as well as a phone\tablet plugged into it. It's at the specific voltage when it starts up, even with a small load, and then the minute I plug in a phone\tablet, the voltage drops like ~0.5V \$\endgroup\$ – Dominic Luciano Feb 1 '16 at 6:50

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