Posted on Oct 21, 2012

This circuit uses a tachometer to generate a feedback signal which is compared to a reference supplied by the auxiliary output. When power is applied, the tachometer output is zero and the regulator output comes on, forcing current into the motor. As motor rotation increases, the negative tachometer output pulls the enable pin toward ground. When the enaole pin`s threshold voltage is reached, the regulator output decreases and the motor slows.

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Cl provides positive feedback, ensuring clean transitions. In this fashion, the motor"s speed is servo-controlled at a point determined by the 2-KO potentiometer setting. The regulator free-runs at whatever frequency and duty cycle are required to maintain the enable pin at its threshold. The loop bandwidth and stability are set by C2 and C3. The 1N914 diode prevents the negative output tachometer from pulling the enable pin below ground, and the 1N4002 commutates the motor"s negative flyback pulse.

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