Pump Express requires the wall adapter to vary its output voltage in response to commands from the phone (see below).
Dialog, which bought power chip company iWatt last year, has produced iW1680, an ac-dc wall adapter chip that reads Pump Express protocol through the adapter's isolation transformer.
"MediaTek sends information back via the V+ by modulating the load current," Dialog marketing v-p Scott Brown told Electronics Weekly. "The IC, despite being on the primary side, is able to extract that modulation."
"All I can say is that it is load current modulation, MediaTek invented the protocol, and we will be first to market with a chip," said Brown.
MediaTek has yet to reveal all. Electronics Weekly speculates that 'up a bit' and 'down a bit' commands are sent back on the V+ rail by modulating load current at two different frequencies.
The chip is largely digital inside - iWatt's trademark is the use of sophisticated state-machines for digital power control.
It comes in a 6pin SOT23, supports power adapters up to 7.5W, and can consume under 30mW with no load.
Mains voltage switching is via an external bipolar transistor, and protection features include output short-circuit, output over-voltage, output over-current and over-temperature - there is an iW1680 application note here.
Why drop the USB voltage?
MediaTek is initially using Pump Express to knock cost and size out of mobile phones by replacing the phone's internal switching battery charger with a linear regulator.
It argues that there is already a switching regulator in the wall adapter, so why also have a buck regulator at the front end of the phone?
If the internal linear regulator can dissipate say 1W, and the battery needs to be charged at say 4V, charging has to be limited to 1A maximum with a 5V adapter.
If the wall adapter can be throttle back to 4.5V, 2A can now be delivered to the battery without exceeding the dissipation limit.
With 200mV steps at the adapter, the adapter voltage can be set so close to the battery voltage that system efficiency is similar to that of an all switching solution, said Dialog's Brown.
An enabled phone will still function with a standard charger, although accelerated charging will not be available.
Increasing USB voltage
Voltage scaling up from 5V is proposed to solve another issue in phones - the 1.5-1.8A limit imposed by the contacts inside Micro-USB connectors.
Increasing the charger voltage allows more power to be sent through the connector - although a buck front-end within the phone becomes essential.
Qualcomm's scheme for increasing supply voltage while remaining compatible with bog-standard charging is called Quick Charge 2.0, for which Dialog also has a chipset. This protocol is slightly less elegant - requiring a secondary-side interface chip in enabled wall adapters because it signals on the USB data pins.
MediaTek is expected to extend Pump Express to voltages above 5V in the near future.