Audio Out to Power
Jeremy O’Driscoll and Jonathan Rubens
Advisor: Robert Morley
We designed an ASIC CMOS circuit to use a smartphone’s audio
output to power an integrated circuit at 1.8 V and 2 mA. The design
works on a range of smartphones using iOS and Android operating
systems taking into account their varying power outputs at an
efficiency of 37.3%. The design can fit on a 1mm x 1mm die with
two 2µF and two 1µF capacitors off chip.
Block Diagram
Ripple Rejection
The overall block diagram shows how all the parts connect to form the
final product. In addition to the five parts described, there are three
inverters which act as amplifiers for clock signals to the voltage
doubler. The majority of current will flow from the power source
through the rectifier to the voltage doubler and into the voltage
Charge Pump
The input to the system will be two square waves 180º out-ofphase. A square wave was selected because it transfers the most
power. Our design consists of 5 main parts: rectifier, voltage
doubler, charge pump, voltage regulator, and voltage reference.
Each part was designed and tested separately and then added to
the complete schematic. The design process for each part started
with reading literature related to the function needed and then
choosing which design we would adapt to fit our needs. Next, we
built a working schematic of the paper’s model in Cadence Virtuoso
in order to decide whether we made the correct choice for design,
and then finally adapted it to fit our specific needs.
The charge pump provides the necessary increase in voltage for the
clocks of the voltage doubler. To operate correctly, the clocks into the
voltage doubler had to be boosted to a higher voltage than the output
in order for the transistors to function. This was done with a charge
pump design that pushes charge through eight capacitors, multiplying
the voltage by four. The small capacitors used cause the component to
only function under low current draw, but this works for our design
because ideally zero current would flow through the clocks.
Input Range
The rectifier converts the square wave input into two DC waves.
This is achieved by selecting which wave is high at any given time
and using that as the high output and the same for the low output.
We accomplish this with only 4 transistors. The variation on the
output at the switching frequency is negligible.
Voltage Reference
This part generates a stable 0.9 V output used in the voltage regulator.
The characteristic drop across a MOSFET is used along with a current
mirror to provide a stable reference for output. The output changes
slightly with temperature, but not enough to make a significant
difference at normal operating points.
Voltage Regulator
Voltage Doubler
In order to reach the desired output of 1.8 V, the main line voltage
had to be raised from its input level. The voltage doubler ideally
outputs double the input voltage. This is accomplished by using a
switch capacitor, which allows us to have relatively high current
output as well as increased voltage. The circuit uses two clocks to
operate the switching.
The output of the voltage regulator (in blue) rejects the square
wave at the input (in red) with very little ripple. We have achieved
66.3 dB of ripple rejection
This part takes in a wave always greater than 2.2 V and returns a stable 1.8 V DC wave. It is the final part in the main current flow. The circuit
works by comparing the output to a reference voltage of 0.9 V and adjusting a pass transistor accordingly until the output is at the right level.
The desired output voltage is changed by tweaking the two output resistors. The regulator works by feedback to a comparator and then small
changes until 1.8 V is achieved. Stability was a large concern so we had to implement compensation circuitry to slowdown the feedback process.
Using this method we achieved 66.3 dB of ripple reduction so that the output is incredibly stable.
The blue curve shows the output voltage of our voltage regulator as
a function of the input voltage. It shows that when the input
reaches about 2.2 V the output will correctly be 1.8 V. This
constraint set the output level of our voltage doubler to be at least
2.2 V.
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