In Reply to: Tracking Voltage Regulator- Comments on Design Invited posted by Steve Harvey on 02/01/00 at 2:10 AM:
: I need a design for a protected tracking voltage regulator which will give a 5V [0-50mA] output. It needs to be protected against the output
: shorting to ground or any voltage up to +30V.
: The output voltage needs to track the logic supply (which is 4.75 to 5.25V) so that the regulator output is always within +/-20mV of it at all times.
: I have designed the circuit as shown on the referenced diagram [acrobat format] but it does suffer from instability as there is a small'sawtooth' AC signal superimposed on the output.
: The diode on the output is required to protect against accidental shorting to a supply voltage (typ 24V) as this circuit is to be used on vehicles with nominal 24V DC supplies.
: Does anyone know if a better circuit which is of a similarly low cost and complexity OR could suggest a modification that would make the current circuit more stable.
: The circuit is to go into a product which may be built in large volumes so it needs to be cheap !
: Steve Harvey. AMIEE
: steveharvey AT lineone DOT net
The LM217 might have instability problems if the input and output bypass capacitors are ceramic. Their relatively low equivalent series resistance (ESR) can cause oscillations. Bob Pease of National Semiconductor has written about this. If you've used ceramic capacitors, try replacing them with sold tantalum capacitors instead. Also, you might want to check the back issues of Electronic Design or get in touch with National Semiconductor for Mr. Pease's article on the subject.
John Dunn - President