I'd like to properly power a gadget off USB (2.4A USB powerbank https://www.amazon.com/Anker-20100mAh-Portable-Charger-PowerCore/dp/B00X5RV14Y/ref=sr_1_3?ie=UTF8&qid=1512261941&sr=8-3) consisting of:
- A cheap power only USB Y-splitter
- Raspberry Pi Zero
- A boost converter from 5V USB to 12V
- 4 LEDs in series
- An ULN2003A used to control a unipolar stepper motor
- Some sort of graphical output (OLED/Character LCD)
Adding all the respective current values together, the max. current draw of my gadget should be:
180mA RPi Zero + 500mA LEDs (384mA for perfect transformation, I assumed 75% efficiency) + 165mA Stepper Motor + 165mA Character LCD = 1010mA total power draw.
What I know so far:
Having delved into (and having been overwhelmed by) the USB specification, I have found that USB can under some circumstances supply 5V@1.5A (or even more).
Back in the day, there apparently used to be a kind-of handshake process (enumeration/negotiation/...) which was necessary to allow >100mA. I found an article explaining some aspects of USB charging: http://www.electronicdesign.com/interconnects/introduction-usb-power-delivery
It’s important to note that BC-1.1 was released as an Engineering Change Notice (ECN) to USB 2.0 and it significantly deviated from the sanctions of USB 2.0. As per USB 2.0, any USB device could be classified as either low power (5 V @ 100 mA) or high power (5 V @ 500 mA). On connection, a USB device was allowed to draw 100-mA current initially while enumerating and negotiating its power budget with the host. Based on the enumeration, the host would either raise the power delivery to 2.5 W or continue at 0.5 W.
The battery-charging spec went on to define more power sources than what was recommended above:
Standard downstream port (SDP): power source compliant with USB 2.0 Spec.
Charging downstream port (CDP): power source not compliant to USB 2.0. CDP can supply up to 7.5 W (5 V, 1.5 A) and the 1.5-A current can be supplied before enumeration.
Dedicated charging port (DCP): There’s no enumeration here, and charging occurs without any digital connection. DCP supplies up to 1.5 A and 5 V.
There has been a similar question before ("Dumb" power from USB), but it does not mention SDP and CDP. Apparently, you can achieve CDP-mode when you short the data lines together.
- Can I get the full 5V 1.5A (or even 1.8A) power from an ideal (100% USB standard-conforming, protocol-following) port without any fancy enumeration/registration/...
- Can I get the full 5V 1.5A (or even 1.8A) power from a real world (a decently beefy power bank like the Anker in the Amazon link) port without any fancy enumeration/registration/...
- In case I need negotiation, couldn't the Pi just handle it in the name of both sides of the Y-splitter (they terminate in one Micro-USB port, so the power bank shouldn't even know that the cable splits somewhere downstream...