You don't actually control current. Current, measured in amps, is PULLED by the device. So you really only have control over the voltage.

Take the example of a 50-watt light bulb and a 100-watt light bulb.
Both run on 120-volts, but the 50-watt pulls less power, as the filament inside has more resistance to the flow of electricity (and generates less light)
The 100-watt however pulls twice as much current, as it has half the resistance, and converts twice as much current into light.

The 770 wall adapter CAN put out 890mA, but your 770 may not pull 890mA of current. But if you doubled the voltage, without increasing the resistance, then the circuit WILL pull twice as much current and then something fries.

The 770 would pull the most current if you were charging a dead battery and running the unit at full power (playing a movie over wifi for example)
But if it is just sitting on the desk, then it may consume only draw 100mA or so to keep the battery charged and the processor idle.

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If the basics of voltage/amps/power/resistance were never explained to you here is a dirty crash course, it's off topic but hey it's educational.

Think of electricity as water.
Think of a water tower as a battery
The water tower works by raising the water level above the ground.
That is Voltage. The potential amount of energy you can use.
So a 100-foot water tower could equate to 100-volts of potential energy.
A 5-foot water tower would be 5-volts.
A dead battery is like a empty water tower.

If you want to know what voltage really is: It is a measure of the distance the electrons flying around a atom are from their normal state (or distance from the center of the atom if you want)
The reason that high-voltages like lightning can travel a great distance is because the electrons move far enough away from the center of the atom that they can mix with the next atom nearby.

Voltage can also be equated to pressure. If you take a fire hose flow of water, and connect it to a garden hose...sure the water WILL flow through faster, but it will probably rip the garden hose to shreds doing it. (Like frying a wire)

Current is easier to understand, it is just like the flow of water. The more current, the more actual electrons that are flying by at the speed of light.
Cut you can't control how fast the water moves by, you can either slow it down by putting something in the way or you can reduce the amount of water that could enter the flow.

Resistance is like a waterwheel, or a tube. A water wheel takes a stored supply of water (voltage) and does something with it (grind grain in a mill)
A fire hose can carry a higher flow of water then a garden hose. That is why you need a thicker wire to carry a higher flow of electrical current. And that is why stupidly overpowered subwoofers need wire as thick as a pencil to work.

Power is a bit harder to understand for some people. Power is the amount of work being done, and the amount of energy wasted while doing that work. Electrically power is Voltage multiplied times the Current. Or the amount of current needed for 'x' amount of potential energy.

Google: Ohms Law for more.