high-side current sensing chips

Hi all, I'd like to help out some friends on an antique motorcycle forum that have a 6 volt regulator project. One of them built a
regulator on perfboard and posted the schematic. It has a couple of things I'd tweak, but it works. The main challenge: his regulator design doesn't incorporate current limiting. Even the original relay-style mechanical regulators incorporated current limiting, because the generators required it. Somebody on the forum suggested the zxct1009, but it only comes in surface mount which means we can't rejigger the existing perfboard project to include the new chip. It would mean having a custom- printed circuit board, and soldering techniques perhaps a little too demanding for somebody building his first circuit -- which probably describes a lot of the guys on the forum. pdf of the circuit: http://www.hydra-glide.com/phpBB2/download.php?id 73 I've tried to find a through-hole component that fits the bill, but can't seem to come up with anything just right. I saw the micrel MIC5021, but it supposedly operates on 12 volts and up. http://www.micrel.com/_PDF/mic5021.pdf Now, perhaps they make that statement on the datasheet because the chip actually also has a voltage doubler and the high side drive needs that kind of voltage to turn on a mosfet, and it's possible the chip itself will actually run on a much lower voltage. But the datasheet doesn't say anything to this effect. Has anybody used the MIC5021 and know if it might actually turn on at say 5 volts or perhaps know of any other through hole chips for current sensing?
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gearhead wrote:

This is just a suggestion, but consider the circuit below. If the transistors are well matched Vchg should come pretty close to Vgen/10 + 47 * 0.02 * Ibatt -- and you can jigger your component values around to change the numbers. You'll _probably_ be able to make this work OK with any two unmatched (but like part-number) PNP transistors, like 2N3904's, but you'd do better to get a matched pair (DigiKey has some Zetex parts that would do, if there are any through-hole ones).
Try this out with SPICE before you run with it: I just threw it down off the top of my head, it's not exactly like anything I've done before nor is it tested at all.
Vgen ___ Vbatt o-----o---|___|---o-----o | | -----> | 0.02 | Ibatt .-. .-. | |100 | |100 | | | | '-' '-' | | | | >| |< |---o---| /| | |\ | | | | | | Vchg | '-----o o-----o | | | .-. .-. | |4.7K | |47K | | | | '-' '-' | | | | === == GND GND (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)
--

Tim Wescott
Wescott Design Services
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I've done plenty of scheming about how to do it with discretes. But like you said, I'd still want to use a chip in the end, to get matched transistors. Pairs and arrays, I see ony surface mount, including zetex. If I have to go that way, might as well use the ZXCT1009F in SOT-23 with pins 1.9 mm apart according to the diagram. Almost a tenth of an inch, could probably solder it right onto the pads of a perfboard, eh?
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Hello,
When you buy about 10 general purpose PNP transistors from one batch, you will probably find several transistors that are within some mV with same Ic. When you design the 2 transistor current sensor at low bias current, self heating can be neglected and it saves you from soldering SMD devices like NXP's BCM857. When matching is impossible, you might add a trimmer potentiometer for nulling.
best regards,
Wim PA3DJS www.tetech.nl please remove abc from the address
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Doesn't gain determine the balance in a current mirror? If so, one would have to match for gain. I did match for gain last time I experimented with current mirrors. I'd like to hear from the experts about the need for this.

About temperature matching. I calculate that one tenth of a degree centigrade difference between the transistors in a mirror would add about one percent error to the current mirror. I wouldn't want temperature vagaries causing any more error than that. I'm wondering if you could count on discrete transistors on a board to stay within a tenth of a degree to each other. More fodder for the experts.
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Hello,
For such circuits matching on Vbe with constant collector current is sufficient. I normally do it by connecting B to C, bias at required current and measure the voltage across the diode. Of course for high volume applications I use a matched pair.
When 1% additional tolerance is unacceptable, you also should evaluate all other components. I think of current sensing resistor, other resistors, voltage variatons, offsets in control loop etc. Regarding temperature, put them close together, take some epoxy resin and some piece of coper/alu, and temperature difference due to environment will be negligible.
I can hardly imagine that your original application requires that high accuracy. Last year I designed several current limiting circuits for hot swap applications and inrush current limiting and less then 5% accuracy without adjustment and cheap components will be very difficult.
Best regards,
Wim PA3DJS www.tetech.nl please remove abc from the address.
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My first dog was named Wimpie. And there was a Wimpie character in the old Popeye comic that used to say, "I will gladly pay you Thursday for a hamburger today!" Ah, Saturday mornings in front of the TV.
Thanks for the tip about testing Vbe, it makes sense. I guess putting a bunch of diode-connected transistors in series would really be the best way, you could go down the line testing voltage and sort them out right quick.
73 de KF2HI
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wrote:

Doesn't gain determine the balance in a current mirror? If so, one would have to match for gain. I did match for gain last time I experimented with current mirrors. I'd like to hear from the experts about the need for this.

About temperature matching. I calculate that one tenth of a degree centigrade difference between the transistors in a mirror would add about one percent error to the current mirror. I wouldn't want temperature vagaries causing any more error than that.
For C> sakes! You are talking about a motorcycle here. 1% up or down will not make an ioata of difference to the battery charging on the bike.
I'm wondering if you could count on discrete transistors on a board to stay within a tenth of a degree to each other. More fodder for the experts.
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It is not 1 percent, it is 1 percent per degree C. if you have a 10 degree C temperature difference change you get a 10 percent unbalance.
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At 300K and Vf=.6, a change of 0.1 degree would result in delta Vf 200 uV. At a current decade for every 60 mV, that introduces an error (added to existing offset) of 10^.003333 = 1.0077 or about eight tenths of one percent. At Vf = .7, it comes out closer to 0.9 percent. So I get a round figure of about one percent error in the current mirror for each tenth of a degree temp mismatch centigrade in the transistors.
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On Fri, 18 Apr 2008 09:18:41 -0700 (PDT), gearhead

So you are paying attention after all. Good. Transistors not on the same die are difficult to keep within a few degrees of each other. In this case thermal design matters.
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Pickings for current mirrors are slim. I found BCV62, but it is not what you would call well matched at all.
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On Sun, 20 Apr 2008 11:28:27 -0700 (PDT), gearhead

Just use "matched transistor pair" with any search engine.
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gearhead wrote:

The 100 ohm base-loading resistors are there to take care of gain and temperature match. You may still want to hand-select transistors, but that would be gilding the lily for your application.
--

Tim Wescott
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That Micrel chip is not a current sensor, it is a mosfet driver. Looking for a highside current sensor, try the MAX4374. A ZXCT1008 or ZXCT1009 may be even better. But I doubt you to need that special (and expensive) chips.
Can't be sure what current you want to limit, but usually the field current is limited to limit the output voltage of the generator. Which in turn prevents the battery from being overloaded. This is exactly what the circuit of your schematic is supposed to do. Just use R1 to set the correct voltage. For a 6V lead-accid battery this will be about 6,9V. So even for fully loaded battery the voltage should never exceed that value. You may have to take some voltage loss accross D6 into account which is about 0.2V for this particular diode.
petrus bitbyter
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On Apr 8, 1:30pm, "petrus bitbyter"

Current limiting protects the generator, not the battery.
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On Apr 8, 1:30pm, "petrus bitbyter"

Don't say the micrel chip is not a current sensor. It does current sensing. And current limiting is for the generator, not the battery.
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| Don't say the micrel chip is not a current sensor. It does current | sensing. | And current limiting is for the generator, not the battery.
Hmm... Did you check the high-side current sensors I mentioned? As you know the datasheet of the mic5021 can you explain how you get a signal from it that is proportional to the measured current? I don't see it, but I like to learn.
I am not aware of an automotive generator with a current limiter other then the fields. Normaly the generator is big enough to provide the required power for the vehicles appliances with a wide margin but yes, you can damage it by overloading it. Most of the times however you will blow one or more fuses when you try.
petrus bitbyter
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On Apr 8, 5:59pm, "petrus bitbyter"
s.com...

The MIC5021 does not put out a proportional signal, it trips at 50mV. Yes, maybe it would make the lights flicker if it were to toggle on and off too slowly. And I looked at the sensors you mentioned, okay?

You need to know, this isn't an alernator. Are you familiar with the old-fashioned generators with the long, skinny armatures with segmented commutators? They are different from modern alternators in that they will burn themselves up if you overload them. Is English a second language for you?
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| | The MIC5021 does not put out a proportional signal, it trips at 50mV. | Yes, maybe it would make the lights flicker if it were to toggle on | and off too slowly. And I looked at the sensors you mentioned, okay?

| | You need to know, this isn't an alernator. Are you familiar with the | old-fashioned generators with the long, skinny armatures with | segmented commutators? They are different from modern alternators in | that they will burn themselves up if you overload them. | Is English a second language for you?
I remember those old things and the electromecanical circuits that controlled them, though I never got into their inner workings. So I do not know about current control and how it was done.
English is not my native tongue.
Consider a ZDX1009. It is SMD but large enough to be mounted on perf board. The datasheet shows the internal schematic which is very simple so you can also build it using discretes. You have to use matched transistors as others stated already.
petrus bitbyter
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