NCP551 Voltage Regulator Issue

[bobledoux]

I've been using the 5.0 volt version of this TSOP-5 chip for sometime without problems, until now. The regulator is driving a PIC 16F690. The PIC A/D reads a KTY-81 resistance temperature sensor as one side of a voltage divider and displays the result on a graphic LCD.

The temperature output is showing 5 degree F swings, at room temperature, when sampled at half second intervals. Bypassing the NCP551 with 5 volts eliminates these swings.

With a power source of 4 series alkaline "AA" cells the scope shows the Vreg output swinging from 4.92 to about 5.04 volts at a consistent frequency.

I prefer the NCP551 because the exterior capacitors are supposed to be non-critical. I'm using 0805 caps; a 0.1uf to bypass the input to the Vreg and a 1.0uf for Vreg output. Both caps are run of the mill chip units.

Have I underestimated the cap quality required? The NCP551 data sheet doesn't suggest this is so.

 

[Hero999]

You seem surprised.

I don't see why you should be surprised.

As the battery discharges, its voltage will drop below 6V and its output impedance will increase.

You circuit is not working properly because the input impedance is too high and the voltage is too low.

The NCP551 has a dropout voltage of 220mV which means that it stops regulating when the input voltage drops below 5.2V.

Use a fresh set of batteries and the problem should go away.

Unfortunately the voltage will drop below 5.22V when the batteries still has lots of charge left., as a general rule of thumb, you should design circuits to work until the battery voltage drops below 1V per cell., for longer battery life, use five or six AA cells in series.