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The Soundwerx Designs DoodleBug

Technical Highlights

Analog Devices ADuM3160 -
The Soundwerx Designs DoodleBug is based on 1) the Analog Devices ADuM3160 USB Isolator chip and 2) the venerable National Semiconductor (now Texas Instruments) LM317 voltage regulator. The ADuM3160 was developed for industrial and medical applications where USB devices must be isolated for safety reasons. It can protect devices from up to 2500 volts for 1 minute. (The ADuM4160 can protect up to 5000V!) Of course, we don’t need that kind of voltage protection for our purposes. Nevertheless, the chip provides useful isolation from extraneous and/or static voltages of both USB signal and power

that we desire in audiophile applications. The ADuM3160 requires a minimum of surrounding components to function and has a built-in voltage regulator for its own 3.3V supply. However, the DoodleBug's designer, Head-Fi user Avro_Arrow decided to include entirely separate linear-regulation on the PCB to provide ultra-low-noise USB output power for audiophile applications.
LM317 Voltage Regulation -
We decided to use a power supply based on plug in AC/AC transformer for convenience and safety reasons. Any plug in transformer rated between 6 volts AC and 9 volts AC at 1 amp of current should do. The AC from the transformer is then rectified by full bridge rectifier based on the common 1n400x series diodes. (They are also used for the two protection diodes shown in the generic LM317 application diagram at right.) Voltage is then smoothed by electrolytic capacitor C5.

The fun part is what we did with the regulator. In the typical LM317 circuit, the output voltage is set by two resistors that form a voltage divider (as shown at right). The top resistor (usually between 120 and 220 ohms) sets the current through the bottom resistor. The voltage drop across the bottom resistor plus 1.25 volts equals the output voltage. In the case of the DoodleBug, the bottom resistor was replaced with something that has a more “fixed” voltage drop, namely a diode(s). (LEDs were used, to be more specific.) This provides for much greater noise rejection by the regulator.

(TI/National Semi LM317)

(DoodleBug Power Schematic)

By varying the current through the diode, a small amount of trim can be provided for the output voltage. As with the typical circuit, the voltage drop of the diode/resistor plus 1.25 volts equals the output voltage. The LED’s we have chosen have a rated voltage drop of 2 volts each at 20ma of current. The series combination of R1 and VR1 set the current through the LED’s and can vary it from about 8ma to 25ma. So, 2 plus 2 plus 1.25 equals an expected output of 5.25 volts. In practice, component values vary and the maximum output may be slightly more or slightly less.

Output from the regulator is then decoupled from ground by C6. The power output of the Doodlebug is protected by an auto-reset, 500mA PTC fuse. Current loops are minimized by use of ground planes and multiple vias at strategic points.

We think you will find that the use of the DoodleBug will improve almost every USB-powered audiophile device.
ADuM3160 specs -
The ADuM3160 is a USB port isolator based on Analog Devices, Inc., iCoupler® technology. Combining high speed CMOS and monolithic air core transformer technology, this isolation component provides outstanding performance characteristics and is easily integrated with low and full speed USB-compatible peripheral devices. Some of the primary features of the ADuM3160 chip are listed below:
  • USB 2.0 compatible
  • Bidirectional communication
  • 4.5V to 5.5V VBUS operation
  • Class 3A contact ESD performance per ANSI/ESD STM5.1-2007
  • High common-mode transient immunity: >25 kV/μs
  • UL recognition: 2500V rms for 1 minute per UL 1577
  • CSA Component Acceptance Notice #5A
    IEC 60950-1: 600V rms (basic)
  • VDE certificate of conformity DIN V VDE V 0884-10
    (VDE V 0884-10):2006-12 VIORM = 560V peak

(ADuM3160 Pin Connections)
LM317 specs -
Some of the primary features of the LM317 voltage regulator are listed below:
  • Specified 1% Output Voltage Tolerance
  • Specified max. 0.01%/V Line Regulation
  • Specified 1.5A Output Current
  • Adjustable Output Down to 1.2V
  • 80 dB Ripple Rejection
  • Output is Short-Circuit Protected

(LM317 Schematic)

Manufacturer data sheets:
file last changed:Monday, November 10, 2014 10:13:43 AM
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