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Issue #216 July 2008
Second Place Microchip 2007 Design Contest
’Net-Enabled Alarm Clock
by DJ Delorie
Start | System Overview | Network | Display | MP3 | ADC | Memory | Power | Software | Time | Alarms | GUI | Remote Protocols | Construction & Packaging | Smart Combinations | Sources & PDF
POWER
Most people plug in a wall wart and forget about power. However, this project requires a range of power sources, each with varying requirements for efficiency and quality.
There are three separate regulators. The input is an unregulated 12 V DC from a wall wart. What most people don’t realize is that “unregulated” means “could be anything.” In reality, the voltage ranges from 17 V at no load to 12 V at its rated current. Mine can provide up to 1 A, so it is normally providing about 15 V when the clock is running. A Schottky diode prevents damage from incorrect wiring, and a 220-µF capacitor guards against problems due to contact glitches in the power jack. This raw voltage is used for the amplifiers and feeding the other regulators.
The main power bus is the 3.3 V. Because this is a large voltage drop and it is heavily used, a high-efficiency switching regulator is used to produce it. L400/C402 and L402/C405 form low-pass filters that help keep the high-frequency switching noise away from the other circuits, and D401 guards against switching transients leaking to the other regulators. D401 probably isn’t needed, but inductors can generate wild spikes in unusual circumstances, so I kept it to satisfy my paranoia.
The 5-V supply for the DACs must be as ripple-free as possible, so it is produced using a standard linear regulator with a large output filter capacitor. The OLED needs a 12- to 15-V supply, so there’s an optional 12-V low-dropout regulator for when the input voltage exceeds 15 V. It can be bypassed if the input voltage is regulated to 12 V regardless of load.
Because the unregulated input can drop below 12 V when the amplifiers are running, the carefully selected 12-V regulator has a low drop-out voltage. In addition, it acts like a shunt if the input voltage drops below 12 V. As a result, the display continues to operate, although dimmer, rather than blinking off when the music is loud.
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