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Author Topic: Could you please help me to understand the operating principle of this circuit?  (Read 1356 times)
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hardworking
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« on: May 28, 2008, 05:31:12 17:31 »







The circuit above is used to control DC Motor. IC MC33486 is capable of measuring the current of the motor (to build a servo control loop) and my question is How can it do that?
The datasheet of the IC give some brief information on my question, such as "
This feature provides a current mirror with the ratio of 1/3700 of the sum of the high-side output current. An external resistor must be connected to the Cur R terminal, then tied to a microcontroller A  /  D input for analog voltage measurement This current recopy uses the well-known Wheatstone bridge principle with the Sense, the Power, and the load as the three known resistances.Owing to the internal zener clamp in the gate of the M1 transistor, the Cur R max voltage is typically 11 V."

"In case a ground shift occurs between the MCU and the 33486A, the amplifier A (Figure 6, page 10) will adapt its output to keep the same ICOPY. Of course the shift has to keep between ±1.0 V. "

but what does the bold sentence mean? could you help me to explain it?
Thanks in advance!
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DTiziano
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« Reply #1 on: May 29, 2008, 12:17:28 00:17 »

Refer to Fig. 6 the sense mosfet work in parallel to the power one (really there are hundred or thousand of them in parallel). The Wheatstone bridge is made by the sense and power, on the high side , M1 and Load for low side. The OP is connected in the middle of the bridge, and balance (zero) it by driving M1 (sense and power drain at the same voltage level).
In this condition the copy current is a simple function of the number of mosfet used in Power versus the Sense ones.

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zokij
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Nice time :)


« Reply #2 on: May 29, 2008, 06:43:41 06:43 »

Hi try read this:

Quote
SOFTWARE VS. HARDWARE

The latest generation of microcontrollers makes closed-loop servo control an easy task, especially when the micro’s brains are teamed with the nerves and muscle of power ASIC’s (e.g., the MC33887, MC33886, MC33486, MC33186, and MC33880). Using the MCU’s A/D inputs to read the position sensors and its PWM or parallel/serial output to communicate with the power ASIC makes servo control more of a programming exercise than a hardware design exercise.

more at link: http://www.circuitcellar.com/library/print/0403/Mays153-s/2.htm

but i mean that you will found all answer in this PDF

http://www.powersystemsdesign.com/automotive_2_sept2004.pdf

nice time
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hardworking
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« Reply #3 on: May 29, 2008, 01:50:31 13:50 »

Thank you two but i've still not understand the problem yet.
Let's reconsider the circuit with some points added:



Is the mission of the Opamp to keep the bridge balance? And when the bridge is balance, we have U CA = U CB. So for the recopy current to be 1/3700 of the load current, the resistance of the sense mosfet has to be 3700 lager than that of the power mosfet?
Another question is What does the numbers '1' and '5000' in the circuit mean?
« Last Edit: May 29, 2008, 04:45:16 16:45 by hardworking » Logged
zokij
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« Reply #4 on: May 29, 2008, 03:57:30 15:57 »

sorry, but this  can take lot time on this way...
try to read application notes for this ic . sch and software ...

http://www.freescale.com/files/microcontrollers/doc/app_note/AN2960.pdf
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DTiziano
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« Reply #5 on: May 29, 2008, 10:57:44 22:57 »

You are right. What is the problem?
Yes the OP keep the bridge balanced.
Yes the voltage CA = CB.
Yes the Rsense resistance is 3700 times.
Keep in mind that power mosfet are typically built by thousands paralleled small mosfets so is not so difficult to get that values.
'1 and 5000' I think is an example.
 
Of couse all this apply within the manufacturer specifications.
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