Following on from Super Duper 1.7: SmartPower yesterday, we thought you may find interesting the underlying power supply solution/s we finally solved and the various sub-aspects developed for the micro iDSD.
Power Supply section special (i)
The Meaty Monster is Power Mad!
The power supply is the heart of every audio device. It provides the power which is modulated into audio signals. One just cannot take too much care to the power supply. Within the breast of the beast that is the "Meaty Monster" beats two hearts. And they beat very, very fast. And they draw their power from one of the biggest batteries found in any portable audio device.
A. The poor old standard 500mA USB port …
In the micro iDSD, we need a lot of power, to produce one of the most powerful headphone amplifiers fitted to ANY DAC/Headamp, never mind a portable one. Our 18Watt/Hour Lithium-polymer can deliver 18 Watt for one hour, or 1.8 Watt for 10 Hours. That is a lot of power.
But standard USB ports are limited to 500mA! This issue not something unique to us, to deliver 1W per channel and power the DAC, clocks and the XMOS USB interface, 500mA are wholly insufficient.
"Intelligent Power-Path" (IPP):
A standard USB port offering 500mA is only sufficient to operate the iDSD micro in “xxx” mode (you have to wait for upcoming OTW#2), without leaving any current to charge the battery.
Hence for the micro iDSD, when on USB power, we have developed the special "Intelligent Power-Path" (IPP) battery charger will use battery power to "backup" USB power if more current is needed. So, the micro iDSD will dip into the battery backup to sustain power peaks. This is somewhat like a hybrid car that uses the petrol engine if and when required.
B. “With such a humongous battery, it will take ages to recharge!”
This is why we integrated a special circuit to detect the attachment of dedicated charges and the special high power USB ports found on many modern computers. The micro iDSD can detect these ports and then can take advantage of the so-called BCP-1.2 standard, able to play audio AND draw 1,500mA from the computer, not just a measly 500mA. This way in normal operation the battery can recharge in only around 5 hours while simultaneously playing music.
C. High Power output needs High-Voltage
In order to deliver a powerful headphone output, a powerful and high-voltage power supply is needed. With Lithium-ion batteries limited to around 4.2V maximum, it would mean to either use 8 batteries in series (which requires a lot of space and very complex charging systems) or use some form of power conversion to step-up a single battery voltage to the voltages needed.
D. Can I have both Positive and Negative voltages too please?
For uncompromised audio performance, a dual supply with positive and negative voltage is a must. Hence we use power converters to create a symmetrical PSU adjustable from ±4.5V@1,000mA transient to a maximum ±13.5V@500mA.
Our first take producing this analogue dual-rail power, using a theoretically very neat design just did not work right, never reaching design efficiency and had to be abandoned. This was the problem we highlighted a month ago.
Our next take was to cascade two converters, first one to make an adjustable positive voltage at twice the current required and then a second to invert this power line to provide the negative voltage.
This circuit alone is quite challenging (in fact, very challenging), while many power IC's are on offer, very few are fast enough for our needs and even fewer allow the output voltage to be adjusted in real-time! We had to find a chip (we went through more than 100 different ICs) that is sufficiently flexible to allow us to use a rather neat design trick – so we can adjust the output voltage of this converter from our 32Bit ARM Cortex CPU, which has a 12-Bit DAC built-in.
This way we can adjust the output voltage between 4.5V and 13.5V in over 4,000 precise steps. This chip operates at around 95% efficiency at 0.64MHz and uses a very small PCB area with a solid ground plane and a shielded choke, so very low radiated noise.
But this leaves us with only a positive supply, no negative line. The next step was to find a way to invert this positive power line.
Again, there is no simple off-the-shelf option; we had to use another "undocumented feature" to create a servo-controlled power converter that precisely inverts the positive rail, so by adjusting the positive rail the negative rail follows precisely to within 1-2mV!
Note: Servo-controlled power converter normally only used in $$$$ equipment like the AMR PH-77 (Euro12k/US$12k).
To be cont'd Part (ii) tomorrow.
