ULN2803a any need to ground unused pins?

[bigal_scorpio]

Hi to all,

I am making a circuit using a PIC to drive a ULN2803a octal darlington IC and after looking at the datasheet I am still left with a couple of little queries.

1. Should I tie any unused input pins to GND as is done with many ICs to stop floating and reduce power usage?

2. The sheet says that the inputs are buffered with 2.7k resistors internally for 5v TTL and CMOS inputs. Does this mean I can directly connect to the PIC and the Darlington pairs will be driven at saturation by the 5v present on the PICs pins?

I am puzzled as in the sheets Electrical Characteristics section it seems to show 3v as the absolute max input voltage! baffled.

Thanks for looking.......Al

 

[ronv]

1- No need it has a built in pull down.
2- Yes.
3- I think this is the maximum voltage required to get the stated collector current not the maximum you can apply.

 

[bigal_scorpio]

1- No need it has a built in pull down.
2- Yes.
3- I think this is the maximum voltage required to get the stated collector current not the maximum you can apply.

Hi Ronv,

Cheers mate, its good to get confirmation on things before the magic smoke has chance to escape.

Datasheets are often a little lacking in plain explanations and only seem to cater for the people who actually wrote them I think!

Anyway thanks again mate, now on with the build...........Al

 

[Boncuk]

I don't see any pull down resistor in the ULN2803/2804.

 

[dougy83]

I don't see any pull down resistor in the ULN2803/2804.

Look inside the circle marked "PULL DOWN?" that I assume you wrote.

 

[Boncuk]

Look inside the circle marked "PULL DOWN?" that I assume you wrote.

Would you use a 12K9 pull down resistor for a supply voltage of 5V?

The base resistor of T1 adds to the circled values.

 

[dougy83]

Would you use a 12K9 pull down resistor for a supply voltage of 5V?

Sure, why not?

The base resistor of T1 adds to the circled values.

Oh, right.. you are talking about pull downs on the input pins of the uC? The ULN likely will be connected to the uC outputs. If the OP was asking about floating uC inputs, if connected to a ULN input, they will have sufficient pulldown effected through the ULN resistors. I assumed the OP was asking about whether unused inputs of the ULN require pulldowns - which they don't (also, the base resistor of T1 has no effect with unconnected input).

 

[Boncuk]

Sure, why not?


Oh, right.. you are talking about pull downs on the input pins of the uC? The ULN likely will be connected to the uC outputs. If the OP was asking about floating uC inputs, if connected to a ULN input, they will have sufficient pulldown effected through the ULN resistors. I assumed the OP was asking about whether unused inputs of the ULN require pulldowns - which they don't (also, the base resistor of T1 has no effect with unconnected input).

You're right. I won't care to connect an unused base pin of an NPN transistor anywhere.

However it's good practice to connect floating inputs somewhere (in that case to ground). It won't really hurt.

I'd rather pull the input pins of the MCU high to make sure they are not floating.

Boncuk