please help on this Zener diode

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will_fung

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Hi Everyone,
Please refer to the attached pic, I add a Zener diode D27 on the +VBATT trace to protect the ICs after it, I just want to know is it OK?
The +VBATT was provided by a external power supply, if the +VBATT is higher than the break voltage of the Zener diode, the diode will clamp the voltage. But in this situation, the current would be very high since there is no resistor, and the diode would be damaged.
Do you have any other comment or some good solution, I need a very low leakage current(less than 300uA) and the total impedance of the trace should be less than 0.1ohm, so I cannot add many components.

Thanks..
 

You have three choices:

1) Add a resistor in series with the battery to limit the zener current.

2) Add a voltage regulator in series with the battery to limit the voltage.

3) Use a zener with enough power handling to absorb any excess current the battery can deliver.

What's your preference?
 

Providing the Zener has a higher breakdown voltage than the battery should produce, the leakage should be quite small. When used this way, the Zener is designed to be sacrificed to protect the remaining circuitry or is power rated to withstand the current that might flow through it. Bear in mind that it would be a very rare thing for a battery to produce more voltage than it should. I use this method on power and signal lines when equipment is connected along long cables and prone to interference or static discharges and I have never had a problem.

Brian.
 

u have to add a current limiting resistor between diode and ground if u want to limit the current to zener and prevent overheating. Put the value of resistor which will not cross your zener max reverse current specification.
If this zener diode is used for power supply, i suggest using switching regulator for very small impedance and leakage current. If used for voltage reference, provide output of switching regulator and use a small current limit resistor.
for eg. for Imax of 300uA from zener with zener voltage of 6V and Vbat=10V, R-series=(10-6)/0.0003=13.333kOhm

hope that helps.
 

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