Re: Current source design (tricky?)
From: Fred Bloggs (nospam_at_nospam.com)
Date: 03/12/05
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Date: Sat, 12 Mar 2005 22:11:23 GMT
Larry Brasfield wrote:
> "Fred Bloggs" <nospam@nospam.com> wrote in message
> news:42334EFD.1060207@nospam.com...
>
>>
>>Larry Brasfield wrote:
>>
>>>"Fred Bloggs" <nospam@nospam.com> wrote in message
>>> news:4233017B.3010007@nospam.com...
>>>
>>>
>>>>Fred Bloggs wrote:
>>>>[..rearrange that pull-up which may not even be necessary with these
>>>>power PMOS with VGS,th > 2V minimum...]
>>>>
>>>> View in a fixed-width font such as Courier.
>>>>
>>>>.
>>>>. +--------------------+-----+
>>>>. | | | PMOS
>>>>. | I-> | | +-------+
>>>>.VBATT>---+------+---/\/\-----+---------------------|s d|---->
>>>>. | | Rs 100m | | | | |
>>>>. / / / | | | g |
>>>>. 680 10k 10k | / +-------+
>>>>. / / / | 4.7k |
>>>>. \ \ \ LM741| / |
>>>>. | | | |\ | \ |
>>>>. | +------------|----|+\ | 330 |
>>>>. | | | | >-----+---/\/\----+
>>>>. | | +----|-/ |
>>>>. | | | |/ | |
>>>>. | | | | |
>>>>. | | +--||---------+
>>>>. | | | 1.5n |
>>>>. | / / |
>>>>. | 10k 10k |
>>>>. | / / |
>>>>. | \ \ |
>>>>. | | 330 | |
>>>>. Vd +-----------/\/\----+ |
>>>>. | | | |
>>>>. --- | / |
>>>>. \ / | 100 |
>>>>. --- | / |
>>>>. 1n4148 | \ |
>>>>. | | | |
>>>>. | | | |
>>>>. +------+------------+-------+
>>>>.
>>>
>>>
>>>
>>>Here's your chance to show how smart you really are,
>>>Fred. Beware, these are real questions and blowing
>>>them off with a bunch of insults will merely reveal (to
>>>the newcomers) that you are an electronics idiot.
>>>
>>>Do you believe that your diode forward drop used as
>>>a reference will meet the OP's stated 5% accuracy
>>>requirement over the voltage and temperature range
>>>to be expected in an automotive application?
>>>
>>>Do you believe that the current output of your circuit
>>>will remain within the OP's stated transient response
>>>requirement in the presence of load-dump transients?
>>>
>>
>>Yeah- it's all good enough for what the OP needs- an output current in the range 2-3 amps and compliance within 0.5V of the rail.
>>The OP has no other requirements, is probably charging an accessory battery, and really doesn't give a damn about any transient
>>response other than it doesn't blow his components.
>
>
>
> I take it, then, that you believe the OP to be mistaken
> about his claim to need the current controlled within
> a +/-5% range, after being specifically asked about the
> transient response requirement.
Yeah- that's right- he's trying to pretend he's something other than
Harry-hobby-hacker, he has no real requirements.
>
> I also take it that you believe your circuit will itself
> survive in the automotive environment, with nothing
> in the way of protection between it and the battery.
I stated that the protection was omitted, the w/o means "without". It
will just clutter the essential details of the current clamp.
>
> I have some advice to the OP: Take Fred's advice
> with a large grain of salt, and be sure not to choke
> on that "grain".
>
Eh- you don't like it because you don't understand how it works. You can
make it more of a textbook ideal by going to the LM385 or a zener for
the voltage offset reference and adding one more resistor in the (-)
input leg of the bridge to balance the impedances- making S(Io,
Vbatt)=0- but like who cares. Go spend $30 on a bunch of unnecessary
fluff instead of $3 on something of equal and indistinguishable
performance modulo the application.
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