The ‘Dual diode’ approach is one of the most used on-chip and off-chip concept for ESD protection of IO interfaces. It is simple to implement, smaller than any other IO/ESD concept, has a low parasitic capacitance and low leakage.
Dual diode based ESD protection concept for IOs.
However, especially the ‘diode up’, from IO-pad to VDD can create a lot of problems in the functional operation of Internet of Things (IoT) circuits.
There are basically 3 main reasons why the ‘dual diode’ concept is causing trouble.
1.) Signal voltage for sensor interfaces differ from the IC supply voltage
Several IoT systems include sensor or actuator interfaces that come with distinctive signal conditions. If the signal voltage from the sensor is higher than the supply voltage the diode from IO-pad to VDD causes an unwanted current flow from the sensor to VDD.
The top diode is forward biased when the IO voltage goes beyond the Vdd supply voltage
2.) Communication between ICs – open drain concept
In many IoT systems different ICs share a common communication channel. The I²C concept for instance is used a lot in sensor communication. It is a multi-master, multi-slave, single-ended, serial computer bus invented by Philips Semiconductor (now NXP Semiconductors). It is a cheap approach because it only requires 2 pins per IC. The system can operate even when some of the components on the bus are powered down or when ICs use different supply voltage levels (3.3V, 5V or 1.8V).
I²C communication channel:
Two buses are shared between different ICs: SDA for data and SCL for clock.
It is not possible to include a diode from SDA or SCL pads to Vdd unless all connected devices remain powered and use the same supply voltage.
3.) Sub-systems of the SoC are powered down
To reduce the total standby leakage of IoT systems, typically only a small part of the system is ‘awake’ all the time. All the other circuits are turned off most of the time. The entire system is only running when there is new data available.
For example, in the Silicon Labs MCUs the power consumption is reduced because the entire system is only active for short periods.
Traditional ESD protection like ‘dual diode’ can ruin these power-saving efforts because signals applied on the I/O circuits can power-up a functional block that is not required at that time. The same issue can be seen inside the SoC. IC designers frequently use dual diode protection when different domains across the chip communicate. Without such protection the sensitive receivers can be damaged. But when sub-systems are powered down another protection approach is needed.
Conclusion
It is clear that the traditional diode-based ESD protection for IOs creates problems in IoT applications. Fortunately there are many different concepts that IC designers can use. Feel free to connect by emailto discuss your product.
Next Generation of Systems Design at Siemens