The start-up circuit is a small piece of circuitry that is needed in almost all designs, is often taken- for-granted, and maybe because of that, is responsible for many design iterations.
So what is a start-up circuit? Many circuits have more than one stable operating mode and to make sure that the whole system will work correctly, they need one or more of their inputs to be properly initialized. Examples of circuits that need this are flip-flops, oscillators and current references. By forcing a voltage on a node, or a current into a branch, the start-up circuit brings the recipient of the start-up signal in the proper initial state after which normal operation can begin.
Ideally, the start-up circuit reliably and predictably provides its kick-start signal, but then has no impact on the system afterwards. So that means among other things, that the start-up signal will be generated independent of the speed of the supply ramp, that there will be no false triggers on negative supply transients and that static supply current will be near zero.
Start-up circuits are closely related, but not necessarily identical to under-voltage-lockout (UVLO) circuits and power-on reset (POR) circuits. A UVLO turns off a circuit when the supply drops below a preset value, and turns it on again once it rises above this value, often with some hysteresis. A POR generates a reset signal, often to a bi-stable circuit like a flip-flop, every time the power turns-on to bring the flip-flop to a known state.
To come to the aid of system engineers, many IC suppliers offer a wide variety of supply monitors, reset circuits or whatever name they choose to use for the function. The supply voltage for which the reset or start-up kicks in can be chosen to match the need of the application, and in general, this value has come down to lower and lower numbers. Our company, for instance, now has one, the TS12001, where the reset voltage can be as low as 800mV. Similarly, the supply currents have come down too, and sub-one micro amp currents are now quite common.
But what if you’re an IC circuit design engineer and you need to design a start up circuit for your IC project? Based on my own experience, there is no one solution that fits all needs and if the number of entries on forums like “EDAboard” and “The Designers Guide” is an indication, the start-up circuit causes problems for many design engineers. At my company, Touchstone Semiconductor, too, I find that we spend a disproportional amount of time during design reviews on these start-up circuits, in an effort to avoid the iterations I mentioned above. And even then it is not easy to prove that the start-up circuit works as intended under all conditions.
So what is your experience with start-up circuits? Do you believe you have the ideal one, or know which to avoid at all cost? As always, I’m very interested to hear your inputs.
Jeroen Fonderie, Vice President of Engineering