high-side current sensing chips

gearhead · 2008-04-08T16:40:29+00:00

Hi all, I'd like to help out some friends on an antique motorcycleforum that have a 6 volt regulator project. One of them built aregulator on perfboard and posted the schematic. It has a couple ofthings I'd tweak, but it works.The main challenge: his regulator design doesn't incorporate currentlimiting. Even the original relay-style mechanical regulatorsincorporated current limiting, because the generators required it.Somebody on the forum suggested the zxct1009, but it only comes insurface mount which means we can't rejigger the existing perfboardproject to include the new chip. It would mean having a custom-printed circuit board, and soldering techniques perhaps a little toodemanding for somebody building his first circuit -- which probablydescribes a lot of the guys on the forum.pdf of the circuit:'ve tried to find a through-hole component that fits the bill, butcan't seem to come up with anything just right. I saw the micrelMIC5021, but it supposedly operates on 12 volts and up. perhaps they make that statement on the datasheet because thechip actually also has a voltage doubler and the high side drive needsthat kind of voltage to turn on a mosfet, and it's possible the chipitself will actually run on a much lower voltage. But the datasheetdoesn't say anything to this effect. Has anybody used the MIC5021 andknow if it might actually turn on at say 5 volts or perhaps know ofany other through hole chips for current sensing?

Apr 08, 2008 26 Replies

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gearhead 18 years ago

- snipped-for-privacy@c65g2000hsa.googlegroups.com...

(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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JosephKK 18 years ago

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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gearhead 18 years ago

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At 300K and Vf=3D.6, a change of 0.1 degree would result in delta Vf =3D

200 uV. At a current decade for every 60 mV, that introduces an error (added to existing offset) of 10^.003333 =3D 1.0077 or about eight tenths of one percent. At Vf =3D .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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JosephKK 18 years ago

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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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gearhead 18 years ago

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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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JosephKK 18 years ago

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Just use "matched transistor pair" with any search engine.

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Tim Wescott 18 years ago

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.

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