555 CMOS timer for aux supplies..!

This humble timer IC is a highly under-rated chip for a wide variety of aux power supplies and low to medium power DC/DC converters, at very low cost.

Sometimes simpler really is better, and there are few chips more simple or ubiquitous than the cmos 555 timer. ( the figure shows the bjt version, cmos has 100k resistors )

For cmos you have to keep the Vcc to 18V or less (operational down to 3V actually) but this little beauty can control a buck ( P-fet ), forward (single ended), boost, buck-boost (aka flyback), Cuk, or Sepic topology with ease.

For driving larger fets a bjt emitter follower on pin 3 suffices to supply all the gate drive you need - max Ton can be limited by resistor ( & diode) from pin3 to pins 2&6, with the Toff set by resistor to pin 7 - the dedicated discharge pin.

The dynamics of the control ( and a free built in Soft Start ) can be set by a cap on pin 5 - the Control pin, any where from 47nF to 470nF (typ) - this is also the point that is controlled to regulate the output ( pull it below 1/3 Vcc for 0% pwm - and let go high to 2/3 Vcc for full pwm).

Vout control can be a simple opto-coupler and resistor to pin 5 for an isolated topology, or more directly via npn collector on pin5 for non-isolated. Admittedly there is some variance in freq (lower freq at lighter loads) and the impulse response is not telecom grade - but often this is really not a requirement for an aux supply ( fan supply, raw Vcc, for e.g ).

The freq can be set by the cap on pins 2&6 to gnd, and if run at 5-15V the chip is pleasantly immune to RFI from nearby high power converters. Operating freq to 350kHz is quite feasible - great for compact Sepics where you need step up & down.

Pin 4 provides a handy ON/OFF enable line ( hi = go ) and can be used for cycle by cycle current limiting (with soft re-start). Average current limiting can also be enabled along side Vcontrol on pin5 with a simple RC ckt and npn xtor looking at a source resistor on the main fet.

So, under-rated and often overlooked - the cmos 555 timer - a cheap & cheerful & surprisingly effective choice for aux power and even main power control in all sorts of designs ...