Understanding Short circuit protection in regulators - Part 1

Have you ever experienced your tracks getting burnt, ic getting damaged because of excessive current. It is because you don't have a short circuit protection for your circuit. During power on, validation engineers do check for shorts but during the course of validation improper reworks causing shorting due to lead balls falling between ground and power pins may cause over current to flow and damage your circuitry.

This kind of shorts are random and deterministic. So, care must be taken before hand. In a design, it always adds up a additional circuitry to implement such short protection mechanisms. For taking care of such cases, modern day regulators are coming up with short circuit protection within the ic itself. This causes the regulator to shutdown (or restrict the current ti Imax automatically) in case there is a short circuit. As the regulators are the main sources of current, if they are controlled first hand, then, it always provides a shield to the entire circuit. So, applying short circuit protection at the source is a common design trend. Check the old regulators, you may not find these kind of protection mechanisms. The modern day regulators have such protection and be careful to choose such regulators only in your design.

The common protection mechanisms employed in regulators are:

1. Constant current limiting circuit
2. Fold back limiting current or passive current limiting (fold back current is the terminology used to indicate maximum current condition before which protection is applied)
3. If short circuit protection not present in regulators, a high side low resistance MOSFET switch is used to control in case of short circuit. In such circuits the configuration of the circuit can be changed to control the current.

Check out the below graph which shows the variation of output voltage and current in case of current reaching fold back current:

Check out the below circuit from Linear technologies employing a high side switch (LTC1477):

The above circuit employs a control path at the output where the boost regulator connection to load can be cut-off from load using LTC1477. So, a enable pin on LTC1477 will take care of such scenario. There are scenarios where it is desirable to limit the current rather than disconnecting the power. This is from the user convenience prospective because regular switching off rather than limiting may cause annoyance to the end user. For such cases, we use a discrete circuitry.