M5450 Display driver

Buzby

Buzby

Senior Member
My first thought was to cool it to a cryogenic temperature, and hope it goes into super conduction !.

But the datasheet says min operating temperature is -25'C, not cold enough.

Which raises my question, ' Why do chips have minimum operating temperatures ? '
 
J

john thorne

New Member
Hi thanks for the reply.
Each of the 34 pins has 4 or five leds.
Only 1 output is on at a time.
I am using a 18m2 to read a keypad and pass the number 1 to 136 on to a 20x2 which then send to a casscade of M5450 display drivers.
It works fine for 1 led per ouput but I would like to be able to sink a larger load.
I was thinking of using a P channel mosfet but not sure how it would connect.
I wondered if anyone had a solution.
 
Buzby

Buzby

Senior Member
If you are definitely only driving 1 of 34 pins at any one time, then you can sink up to 25mA from the active pin.

If you then put 5 LEDs in series, with say 1.7v drop over each, the supply for the LEDs needs to be at least 8.5v, ( 5 x 1.7 ), but up to 15v will work.

The 5 LEDs will all have the same 25mA current, because they are in series.
There is no need for MOSFETs etc., unless you want huge currents, in which case you will need 136 MOSFETS !.
 
Buzby

Buzby

Senior Member
What kind of LEDs are they ?. Little panel display LEDS running at 10mA, or big LEDs for illuminating a room, running at 500mA?. Do you have a datasheet ?
 
J

john thorne

New Member
They could be any type probably high brightness 30 Millamps each, they are needed to illuminate objects.
I won't know how many I will need but it will be more than the M5450 can sink.
If I can find a way to up the output the whole project is future proof.
 
W

westaust55

Moderator
Buzby said:
If you then put 5 LEDs in series, with say 1.7v drop over each, the supply for the LEDs needs to be at least 8.5v, ( 5 x 1.7 ), but up to 15v will work.
Click to expand...

Yes the outputs are current sinking so with several LEDs in series, as long as the supply to the LEDs is not greater than 15 Volts Absolute Max, but 13.2 Volts is the recommended Max working voltage at the outputs, for which the M5450 outputs can handle the off state voltage on the outputs.
 
R

REB100

New Member
john thorne said:
Any suggestions on how to increase the output from the M5450?
I would like to sink about 160 Milliamps.
Data sheet:
http://www.datasheetcatalog.org/datasheet2/e/0zqj6og5wcoq8y1zyxt7r79056py.pdf

Thanks
john
Click to expand...
My first thought would be to use something similar to a TIP30 PNP power tansistor. A quick check of suppliers shows a price range of $0.50 to $1.00 each depending on exact part and quanity. 160ma should be no problem, some parts show max current in the 15 Amp range. That would require heatsinks and a lot of space.

MOSFETs should also work but I haven't done anything with them in years so I can't quote any part numbers.
 
W

westaust55

Moderator
Should be okay and from this (5 V supply indicated) you are going to put the LEDs in parallel.
Don't forget to put a resistor in series with each LED - not one resistor for the lot.
Semiconductors do not always share the current equally when connected in parallel which could lead to one LED drawing a higher current than another with the prospect of non uniform brightness and possible overcurrent and failure of some LEDs.

This can be a reason for using a higher voltage and having the LEDs in series where a Single resistor per LED string and uniform current is achieved.
 
B

BeanieBots

Moderator
Buzby said:
' Why do chips have minimum operating temperatures ? '
Click to expand...
Never used a diode as a temperature sensor?
All semiconductor junctions have characteristics which change with temperature. Unlike most other materials, semiconductors tend to conduct LESS as temperature decreases. (insufficient energy for the electrons to 'jump' the gap).
 
Buzby

Buzby

Senior Member
BeanieBots said:
.... Unlike most other materials, semiconductors tend to conduct LESS as temperature decreases. (insufficient energy for the electrons to 'jump' the gap).
Click to expand...
I've just finished reading a Brian Cox book, one section of which explained the operation of a diode by using quantum thingies.
I must admit, I'm going to have to re-read it again, I think my brain has too many holes.
 
J

john thorne

New Member
Thanks for the help.
I used a P channel mosfet and it worked fine.
I tried it on 5 and 12 volts With ST M5450 chips.
The same chips made by Micrel seem to have a lower maximum voltage (11 volts).
 

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