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[SOLVED] What's the purpose of these diodes in this driver IC?

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tajiknomi

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Whats the purpose of these diodes in this driver IC

I would like to know the function of these 3 diodes in ULN2803A driver
Capture.JPG

A is used as a suppression diode but i didn't get the purpose of B & C.
 
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Re: Whats the purpose of these diodes in this driver IC

Hi,

* open the datasheet with a PDF reader
* start a search for "diode"
* Read the information related to "diode". Complete chapter.

If there is still something unclear please ask a more detailed question.

Klaus
 

Re: Whats the purpose of these diodes in this driver IC

A = protects the input from reverse polarity.
B = protects the output from reverse polarity.
C = clamps the output pin voltage from rising above COM voltage.

Explaining B & C a bit more: these devices are often used to drive inductive loads. When the current through the load turns off, it generates a voltage in the opposite polarity so diode B prevents it damaging the output transistor. Diode C is present on all the outputs but they join together at the COM pin, usually it is tied to the supply line of the load to present a clamp to back-EMF from the load. It can also be used as a 'turn on' for all the output stages simultaneously by taking it to ground potential.


Brian.
 

Re: Whats the purpose of these diodes in this driver IC

B and C are the unavoidable IC substrate diodes rather than added on purpose, shown by dashed lines in most datasheets. They don't have nominal or maximum ratings.

A is the dedicated free-wheeling or clamp diode, e.g. used to limit the voltage across inductive loads.

It should be mentioned that forward biasing of the substrate diodes can have unexpected side effects like sinking an additional current in adjacent channels by the working of a parasitic transistor.
 
Re: Whats the purpose of these diodes in this driver IC

My apologies, looking on a small screen the letters weren't clear. I crossed A and C in my explanation.
As FvM states, B & C are part of the substrate structure and not added as components, there just happens to be a PN junction there.

Brian.
 

Re: Whats the purpose of these diodes in this driver IC

If the output is reversed biased then the B would short circuit and it will blows any way. Same is the case with C when input polarity is reverse bias. Correct if i am wrong.
When the current through the load turns off, it generates a voltage in the opposite polarity so diode B prevents it damaging the output transistor
Click to expand...
You means to say diode A prevents the output transistor because the diode B will be in OFF condition when the voltage source is off. The path for back Emf will be via A.
 
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Re: Whats the purpose of these diodes in this driver IC

tajiknomi said:
If the output is reversed biased then the B would short circuit and it will blows any way. Same is the case with C when input polarity is reverse bias. Correct if i am wrong.
Click to expand...
Accidental or incidental reverse voltages will often have enough resistance in series to limit the current and prevent damage, but the diode will still clamp the reverse voltage to a level well below the reverse breakdown voltage of the transistors' b-e junctions. This prevents weird hard-to-predict behavior that may occur without the diode. An external diode is often added for this purpose to parts that don't have a substrate diode.
 
Re: Whats the purpose of these diodes in this driver IC

The COM pin is normally connected to the positive supply for the inductive load, but it can be connected to a higher voltage if it is desirable that the magnetic field decays faster.
 

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