Power Supply

Apart from using the player in the car, I wanted the option of being able to run it as a mobile player with a small battery. So I needed a high efficiency power supply. A 7805 wouldn't cut it.
As these players all requires a 5V supply, I've designed a switch mode supply around a MAX887 chip. It can deliver about 600 mA (we're running it at the max (no pun intended) here), from an input voltage of 6-20 Volts.
It's a high efficiency circuit (about 90-95%), so no cooling is necessary.

Unfortunately, it seems that it's virtually impossible to get switch regulators from Maxim. When I found that it was impossible to get the MAX887, I checked up on the MAX1626, which would also make a nice regulator.
No luck. All MAX regulators are in backorder until February 2001 !!!

But a power supply I needed, so I had to use another chip. Enter National Semiconductor LM2575. It's not as effective as the MAX chips, 80% compared to 90-95%, and requires a slightly higher input voltage, but it'll do the job.
It accepts an input voltage from about 7-40 volts, and can deliver 1 Amp continously.
Here's the supply I made with that :


Here's the supply with a 7.2V 1300mA video camera battery.


A closer look. 5 components only. At 80 % efficiency it gives about 3 hours of music with the above battery !

I actually made this power supply 15 minutes ago ! Don't say I'm not updating the pages quick enough !

A quick measurement on it shows (music playing) :

Input: 7.75 V @ 0.38 A => 2.95 W
Output: 5.0 V @ 0.47 A => 2.35 W
Efficiency: 2.35*100/2.95 => 80 %

I've been running this setup for a while now to verify battery times.
The 7.2V 1250mA video camera battery, gives 3 hours of playing time. It's a Li-Ion battery, which stays pretty much at 1.2 V per cell, and then dies VERY suddenly after about 3 hours. A freshly charged pack delivers 8.0 V under load. 5 minutes before the player stopped, the pack voltage was 7.13 V, and when it stopped the pack was down at 6 volts, which quite correctly are the ending voltage for a 6 cell pack.

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