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.
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.
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
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.
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.
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.