high-side current sensing chips



(snip)
You mean the ZDS1009, petrus. Check out that circuit in the datasheet -- if it doesn't leak, it won't turn on. The circuit below shouldn't have any problem starting up. Below Ilimit, the collector output will not source any current. Just as load current passes Ilimit the output collector should start sourcing a small current. I can use this collector current in a feedback loop with the voltage regulator and make the load current settle at Ilimit.
V+----+-----Rs-------+-----+---load | | | Re | | | | | | | | \ PN2907 / | >| x2 |< | |---+----| | /| | |\ | / | \ | | | | | +------' | / | | |< | ,-------+--| | | | | \ | / \ \| | |/ \ |---+----| '---out /| |\ < PN2222x2 > | | '------+-------' | current sink I = 2 Ilimit Rs / Re | | gnd
For example, Isink = 200 uA, Re = 1k, Rs = .01, Ilimit = 10 A The current mirrors reach an equilibrium at Ilimit, with 100 uA on each side. Below Ilimit, the right side of the current mirror pulls up and the left side pulls down. The output transistor stays turned off. Above Ilimit, the left side pulls up and the ride side pulls down, drawing some current from the base of the output transistor.
Whaddya think, it might work?
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Correction: instead of a mirror it needs separate current sinks on the ground in order to work.

And another transistor on the output for more gain, if necessary.
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wrote:

Correction: instead of a mirror it needs separate current sinks on the ground in order to work.

And another transistor on the output for more gain, if necessary.
Yes, I meant the ZDS1009.
I gave a quick glance and I doubt this circuit will work. The way the outputtransistor is connected will ruin whatever the others might do. For instance, most of the current through the right current source will come through the BE-junction of that transistor. So the balancing for the PN2907 will be gone. Besides, you will have to match both current sources which brings you directly to the original schematic. Sure, it will not start without leakage. As a matter of fact, that leakage is specified in the datasheet. All bipolar transistors I met so far, had some leakage. It's inherent to to physics of the transistor. Once amplified, it will become serious current which brings the circuit to live. I ever build a current sensor like that and it wordked though not very accurate. I used no matched or twins transistors and did not bother to make them thermal connected. At the time, it was good enough for my application.
petrus bitbyter
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wrote:

5% accuracy would be lots in a project like this.
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On Apr 11, 3:56 pm, "petrus bitbyter"

(snip)
Fred Bloggs turned me on to this circuit patented by Analog Devices:
. . -Is-> . . ----+------[Rs]------+----- . | | . [R] [R] . | | . .----+ | . | | | . | >| |< . | Q1 |-----+------| Q2 . | /| | |\ . | | |< | . | +-----| Q3 | . | | |\ | . >| | | | . Q4 |-------------------+ . /| | | | . | / \ | / \ . | | I | | | I | . Io \ / | \ / . | | | . | | | . '-------+--------' . | . --- . Yair, the ground currents gotta be fixed, not a mirror. If I use my circuit I'll replace the output transistor with a p- channel mosfet. gotta go.
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On Tue, 8 Apr 2008 09:40:29 -0700 (PDT), gearhead

AFAIK, the LM723 requires a minimum supply voltage of 9.5V. The internal reference zener is 7.15V, typical.
- Franc Zabkar
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I'd pick up a surfboard and give up on the thruhole idea.
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Scott
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