Dying Gasp Circuit Design on PCBA Level

The topic of this article is how to design a Dying Gasp circuit on PCBA level.

First,we should know why we need this function on our product. Literally,this function is used to send a message to others when the system power source is disconnected or broken.Apparently,message should be a specific type of electrical signal.

Second,there are two blocks of dying gasp circuit as below :

1. Backup Power Source : It depends on what power consumption which system used to send this message to others is.Mostly,a electrolytic capacitor or a solid capacitor is enough.Certainly,you can use a battery as backup power source if system needs more power to finish this process.
2. Limitation of Current Direction : This block is used to prevent stored power from draining out through power input circuit.Diode is a common solution of this block but you can also choose another way to design it.For example,you can use MOSFETs.

I often design the dying gasp circuit shown in Figure 1.

Figure 1 Dying Gasp Circuit

D1 is a Schottky Diode which is used to limit the current direction.To avoid inrush current,I add R1 to limit the current when first power on.D2 is also a Schottky Diode used to limit the current direction.D1 and D2 are necessary in this circuit because they are used to block different way of current direction.

And then we should figure out what value of C1 is.There are some parameters we must know.

1. Time Requirement of Dying Gasp (T) : Our product needs how many time to send message to others.Its unit is usually millisecond.
2. Power Consumption of Dying Gasp Block (P) : When dying gasp function is triggered,other function of our product should be turned off or not be supplied from backup power source.So,we just have to determine what power consumption of dying gasp is.
3. Triggered Voltage of Dying Gasp (V1) : This function won’t be triggered before system voltage is below a specific voltage.It depends on the internal design of IC and it is normally a voltage comparator.
4. Minimum Operation Voltage of Dying Gasp (V2) : Because we often use a DC-DC to convert corresponding voltages to supply system,these DC-DC won’t work when input voltage is below their specification even backup power source still have energy.

When we get these parameters,we can use a derived formula to calculate C1’s value shown in Figure 2.

Figure 2 Formula used to calculate backup capacitor value

Basic concept is the potential energy of C1 between V1 and V2 is equal to the energy consumption of dying gasp block.You can derive it by yourself.

Please feel free to contact me if you find any mistake of my article.