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I need to protect my GPIO from Electrostatic discharge as my inputs are coming from outside.I am using LPC2132 Micro controller which uses +3.3V Supply.Bidirectional ESD diodes that I have seen so far has maximum clamping voltage 0f 7V. Since the micro controller does not withstand more than Vdd+0.3V,I am confused how to protect the micro controller.Is there any way to protect from Electrostatic discharge other than TVS diodes

Attached the screenshotenter image description here

Added the circuit to protect the Output of Microcontrollerenter image description here

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Digital input pins will normally have something stated in the data sheet about the maximum current that can be fed into an input. Normally, the input current is nano amps but when an input voltage rises above the positive rail this current can sky-rocket. Ditto when an input falls below the negative rail.

The DS might say that the maximum current is 1 mA - this gives you something to work with because, an input pin can have a resistor placed in series with it. For example, if an input pin rose by 0.3 volts above the 3V3 rail then there is a danger of too much current however, if there was a 1 kohm resistor in series, you could raise that input voltage to 4.3 volts at the risk of only 1 mA flowing.

So, by adding a series resistor you are giving yourself an easier job of input protection. If 5 mA is allowed, a 1 kohm resistor will give you 5V extra protection beyond the 3V3 +0.3 volts you specify in the question.

Protection offered by a 7 V device is now clearly feasible.

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  • \$\begingroup\$ This is good, but I'd underline that there is no need to protect for ESD. The Vdd+.3V rating is for the continuous current of the protection diodes, but of course the device can withstand ESD discharges: see p26 here \$\endgroup\$ – Vladimir Cravero Aug 15 '16 at 8:26
  • \$\begingroup\$ @VladimirCravero The DS is based on the 4 kV human body model. "ESD" could imply 8 kV or greater. \$\endgroup\$ – Andy aka Aug 15 '16 at 8:30
  • \$\begingroup\$ Yeah well of course the used HBM should fit the spec, which is not given. Plus I re read the question (all the words this time), and it is quite confusing. A R divider is necessary, or a level shifter for bidir comms, but then again OP goes back to ESD... \$\endgroup\$ – Vladimir Cravero Aug 15 '16 at 8:32
  • \$\begingroup\$ @Andyaka:I have checked the datasheet and found the High level input current as 3uA.In that case,my resistance value increases to 1Mohm.I have attached the screen shot for your reference.Am I considering the right value? \$\endgroup\$ – ANONYMOUS Aug 15 '16 at 9:01
  • \$\begingroup\$ No, that is the normal worst case current that might be taken/delivered by an input when driving it normally. Luckily you also show the "latch-up current" on that screen shot and that is 100 mA. Some MCUs don't allow more than 10 mA so you are going to be safe assuming that an over-driven input pin can work with 10 mA but I would play as safe as you can and if you can tolerate 1 kohm in series then a 5mA limit will take you 5V above the Vdd threshold. When unpowered remember that your Vdd is zero volts so, in cases of ESD, the absolute clamping limit is 5V assuming 1 kohm and 5 mA. \$\endgroup\$ – Andy aka Aug 15 '16 at 9:18

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