Low-dropout regulators circuits using a IC

Low Drop-Out Regulators with IC MIC29152

A major drawback of the 317T and similar standard regulators is that 1.7–2 volts are lost from the 12V supply, so the fan can only run up to about 85% of its full speed and flow rating. Although this site is aimed at keeping noise down, there may be occasions when you want to turn the wick right up.

You can get much nearer the 12V input with the newer low-dropout regulators, and with the controller shown below my voltage loss running a 200mA (2.4W) fan was only 130mV (12V in, 11.87V out, 99% full speed!).

They cost a fair bit more than the humble 317T – Rapid Electronics price for a MIC29152 is £3.10 compared to 38p for a 317T – but when you factor in the other parts, especially with a fancy control knob, it’s not such a big difference overall, and well worth it IMHO.

I’ve used Micrel’s MIC29152BT regulator, which is rated at 1.5A and comes in a 5-pin TO-220 case. If you need more current, the MIC29302BT is identical but rated to 3A, or the similar MIC29502BT will carry 5A. You may need a bigger heat-sink with the bigger boys. See the heat-sink page for the details.

Unlike the 317, the Reference Voltage is across R2, not R1, and the calculated voltage output is
Vout = 1.24(1 + [R1 + VR1] / R2)

With the resistors shown, the calculated output range works out at 6.9V to 12.5V, giving a bit of headroom if component tolerances run the wrong way. (The ±20% tolerance band on pots can play havoc with calculations – if your “10k” pot is below about 9k you won’t get the full potential. The ratio of (R1+VR1):R2 must be over 8.7:1 – increase R1 to 12k or reduce R2 to 2k)

The two capacitors ensure stability, particularly useful with long supply leads. Some alternative resistors are given below to suit different minimum voltages.

Low Dropout Positive Regulators 5A with LM1084


* Available in 3.3V, 5.0V, 12V and Adjustable Versions
* Current Limiting and Thermal Protection

The LM1084 is a series of low dropout voltage positive regulators with a maximum dropout of 1.5V at 5A of load current.

Low Dropout Positive Regulators  5A with LM1084

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Read More Source:http://www.cpemma.co.uk/reg.html
Thank you.

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