Make 5 V using the LM2825N-5.0

SRS Home | Front Page | Monthly Issue | Index

Make 5 V using the LM2825N-5.0

This article looks at the LM2825N-5.0 which is an very easy to use 5 V regulator based on switching. It's efficiency is around 80 %, and it accepts from 7 V and up to 40 V on input.

As with the 7805, there is actual only 3 connections : Input, output and ground, where they share the connection over several pins.

In my first robot I used a 7.2 V battery. Because the LM7805 demands at least 7 V to work, I designed a "Low dropout regulator". In my new robot I use a 14.4 V battery and if I used a linear regulator like the 7805 or my own LDO regulator, the efficiency would be around 36 %. My electronic uses around 350 mA, which is around 1.8 W. Using a linear regulator, the power use would be around 5,1 W, giving the 36 %. Therefore I searched the Net, for a better solution and found the LM2825N-5.0 at National Semiconductors. It's an "Integrated Power Supply" using switching frequency around 150 KHz. Using this devise, the current use is only 135 mA at 14 V (= 1.9 W) input voltage and still it delivers 300 mA out at 4.95 V (= 1.5 W) which gives an efficiency around 80 %. The advantage is that a heatsink is obsolete and that I save around 165 mA of current use from my batteries.

The LM2825 and other switching regulators is based on the same principle. The regulator is either full open or total closed. Then the output is open there isn't any voltage loss and then the output is closed there is no current flow, which in theory gives zero loss. The PWM signal is then filtered by an coil and an capacitor. The output ripple is around 40 mVpp.

As with the aging 7805, the LM2825 needs at least 6.5 V extra to work fine. It produces around 5 V with only 5.23 V in, but this is without a load. If there is drawn 50 mA from the output the minimum input voltage rises to 5.83 V and drawing 700 mA from output, the demand for input voltage rises to around 6.3 V. It's also visible that the idle current use, is pretty high with input voltage under 7 V.

For some reason the LM2825 is using a lot of power when the input voltage is between 4 V and 7 V. That might be seen as a hint for not using it with under 7 V on input. The LM2825 contains also an On/Off pin. When the pin is "floating" there is 5 V on output and when the pin is drawn to ground there is 0 V on output and the idle current is under 200 µA.

The LM2825 is rather big, but that because it contains all components internally, including coils, Schottky diode and capacitors. This makes the LM2825 as easy to use as the LM7805. The devise also contains an ON/OFF pin and soft-start. The LM2825 works with inputs from 8 V and up to 40 V, even through I only have tested it up to 24 V. It comes in 3.3 V, 5.0 V, 12 V and two adjustable versions and more important it's available as a "free sample".