74act high output curve (voltage vs current)
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Posted by Mook Johnson on December 13, 2005, 1:33 am
Please log in for more thread options I'm looking to power an array of 32 small 5V 4 - 6mA devices with the outputs of a 74 ACT bus driver or similar. I'm doing ths because I only power one device at a time and only for a short period of time (say 200mS then off for 1 second). I know the 74AC(T) line is rated for 24mA output current but I would like to know the output voltage drop as the current increases from 0 through 24mA (a curve). Or more specifically what the voltage is at 5mA draw. I need the voltage to be fairly stiff as the output of these devices are ratiometric with the supply voltage. The datasheets only specify Voh at a few uA and at 24mA output. I don't think voltage drop would be linear (might be). I know the proper way is to use a bank of low RDS p-mos but I'm trying to reduce parts count. | |||||||||||||
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Posted by Jim Thompson on December 12, 2005, 6:38 pm
Please log in for more thread options wrote: I would expect it to look pretty much resistive, until you start to run out of IDSS. ...Jim Thompson -- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | | http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food. | |||||||||||||
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Posted by Fred Bloggs on December 13, 2005, 1:59 am
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> I'm looking to power an array of 32 small 5V 4 - 6mA devices with the
> outputs of a 74 ACT bus driver or similar. I'm doing ths because I only > power one device at a time and only for a short period of time (say 200mS > then off for 1 second). > > I know the 74AC(T) line is rated for 24mA output current but I would like to > know the output voltage drop as the current increases from 0 through 24mA (a > curve). Or more specifically what the voltage is at 5mA draw. I need the > voltage to be fairly stiff as the output of these devices are ratiometric > with the supply voltage. > > The datasheets only specify Voh at a few uA and at 24mA output. I don't > think voltage drop would be linear (might be). > > I know the proper way is to use a bank of low RDS p-mos but I'm trying to > reduce parts count. Even if there was an IV curve it would just be "typical." From the data sheet of the TI 74AC244 you can infer that RDS,ON of the pull-up PMOS is no more than 27 ohms from the minimum Voh specifications, and this seems to be the limit in the Vcc=4.5 to 5.5 volt range. At your 6mA load, this makes for a 0.006 x 27=0.16V drop which amounts to 0.16/5= -3.2% just due to the driver. The typical might only be -2.5%, which may or not be considered "stiff" depending on the load tolerance. | |||||||||||||
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Posted by Joerg on December 13, 2005, 2:36 am
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Hello Mook, >
> The datasheets only specify Voh at a few uA and at 24mA output. I don't > think voltage drop would be linear (might be). > It'll be different from device to device as Fred had hinted. So if you have 32 loads driven by four chips and one of the chips happens to orginate from a different batch or date code you may have a discrepancy in RDSon and thus voltage. Why not replace them with some stiff ULN driver? These come in SO packages. or do you need the speed? Regards, Joerg http://www.analogconsultants.com | |||||||||||||
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Posted by mook Johnson on December 13, 2005, 4:48 am
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Looks interesting but the forward drop looks like its 0.6V even at low currents. > Hello Mook,
>>
>> The datasheets only specify Voh at a few uA and at 24mA output. I don't >> think voltage drop would be linear (might be). >> >
> It'll be different from device to device as Fred had hinted. So if you > have 32 loads driven by four chips and one of the chips happens to > orginate from a different batch or date code you may have a discrepancy in > RDSon and thus voltage. > > Why not replace them with some stiff ULN driver? These come in SO > packages. or do you need the speed? > > Regards, Joerg > > http://www.analogconsultants.com | |||||||||||||
>outputs of a 74 ACT bus driver or similar. I'm doing ths because I only
>power one device at a time and only for a short period of time (say 200mS
>then off for 1 second).
>
>I know the 74AC(T) line is rated for 24mA output current but I would like to
>know the output voltage drop as the current increases from 0 through 24mA (a
>curve). Or more specifically what the voltage is at 5mA draw. I need the
>voltage to be fairly stiff as the output of these devices are ratiometric
>with the supply voltage.
>
>The datasheets only specify Voh at a few uA and at 24mA output. I don't
>think voltage drop would be linear (might be).
>
>I know the proper way is to use a bank of low RDS p-mos but I'm trying to
>reduce parts count.
>
>
>