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Author Topic: High Current, 5KHz Switching............. Darlington Array or MOSFET?  (Read 1072 times)
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Dear All,
Following is my project brief:
 1.MCU controlled switching is used for High current (250mA).
 2.Load 1 to 8 will be switching ON/OFF in chain-sequence one by one.
 3.Load 1 will be ON for 200uS while the rest of the loads will be OFF. then Load 2 will be ON for 200uS while rest of the loads will be OFF and so on...
 4. Instead of the switches shown in schematic, a chip is required to perform high current, high speed switching (5.1KHz).
Schematic is as follow;


Questions:
A. What kind of chip/chip-array is best suitable for this type of applications? Darling array, P channel MOSFET etc..??
B. Can following ICs be used for this application?
 UDN2981A (Would this ICs work with 5V supply voltage (Vs) ?)
 DMP2240UDM


 Please advice...
« Last Edit: November 01, 2012, 07:29:26 am by DirtyBits » Logged

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 4. Instead of the switches shown in schematic, a chip is required to perform high current, high speed switching (5.1KHz).
By no stretch of the imagination is 5KHz high speed.

Darlington drivers loose voltage across them when switching, look it up on the data sheet. Use a MOSFET.
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Darlington drivers loose voltage across them when switching, look it up on the data sheet. Use a MOSFET.

Is there a MOSFET which comes in Array? Like 8 Channel?
« Last Edit: November 01, 2012, 04:07:04 am by DirtyBits » Logged

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I normally advice against using Darlingtons because of their high voltage drop and consequent power dissipation. However, in this case you are only turning on one channel at a time, and the current is only 250mA, so power dissipation would not be a problem.

You could use the UDN2981A if you increase its supply voltage to allow for its voltage drop (1.7V typical, 1.9V maximum @ 225mA). Or use low-side switching with the ULN2803A, which has a slightly lower voltage drop (1.3V max @ 200mA).

Unfortunately, nobody seems to make octal mosfet driver chips, otherwise I would probably be recommending them.
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Schematic is as follow;
http://forum.allaboutcircuits.com/image_cache/httpi49.tinypic.com2589v0h.png

Questions:
A. What kind of chip/chip-array is best suitable for this type of applications? Darling array, P channel MOSFET etc..??
B. Can following ICs be used for this application?
 UDN2981A (Would this ICs work with 5V supply voltage (Vs) ?)
 DMP2240UDM


 Please advice...

Firstly that schematic is confused and wrong - the switches seem backwards, a current _sink_ driver is apparently common to all the loads yet is connected to +5V rather than ground(*).  We have no information on the type of load (inductive?, incandescent bulb?, resistive?) nor how much voltage drop can be tolerated. 

So what are the loads?  Do you want high or low-side switching (or doesn't it matter)?  Is there really a current sink driver?

(*)  A sink driver connects its load to ground (sinks current to ground), a source driver sources current from the supply into the load (high side).
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Following is my project brief:

You should try to use the mcu to control the "current sink driver", not supplying current to the current sink driver through the load.
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250ma current load is nothing for even a puny transistor. and 5Khz can be excessively high for a relay.
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Following is my project brief:

You should try to use the mcu to control the "current sink driver", not supplying current to the current sink driver through the load.


The UDN2981A is a current source driver, so it doesn't meet the brief. The ULN2803A is a current sink driver, and will do this job if you can tolerate the voltage drop of that device. Otherwise, use 8 medium-current NPN transistors such as BC337, or 8 medium-power mosfets.
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