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ML4425CH Datasheet(PDF) 9 Page - Micro Linear Corporation |
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ML4425CH Datasheet(HTML) 9 Page - Micro Linear Corporation |
9 / 16 page 9 ML4425 The maximum frequency on the VCO pin is found by: fN RPM MAX MAX = 005 . (3) The voltage at the VCO/TACH pin is equal to the rotor speed. The voltage at SPEED FB is controlled by the back EMF sampler. BACK EMF SAMPLER The input to the voltage controlled oscillator is the back EMF sampler. The back EMF sense pins FB A, FB B, and FB C inputs to the back EMF sampler require a signal from the motor phase leads that is below the VDD of the ML4425. The phase sense input impedance is 8k W. This requires a series resistor RES1 from the motor phase lead as shown in Figure 6 based on the following equation: RES V V V MOTOR 1 670 10 = - W / 16 (4) The back EMF sampler takes the motor phase voltages divided down to signals that are less than VDD (12V nominal) and calculates the neutral point of the motor by the following equation: Neutral PH PH PH = ++ 12 3 3 (5) This allows the ML4425 to compare the back EMF signal to the motor's neutral point without the need for bringing out an extra wire on a WYE wound motor. For DELTA wound motors there is no physical neutral to bring out, so this reference point must be calculated in any case. The back EMF sampler measures the motor phase that is not driven (i.e. if LA and HB are on, then phase A is driven low, phase B is driven high, and phase C is FUNCTIONAL DESCRIPTION (Continued) Figure 5. External VCO Component Connections CVCO CVCO 4.3V 0V 5V 2.3V RVCO SPEED FB CVCO RVCO VOLTAGE CONTROLLED OSCILLATOR VCO/TACH VCO/TACH RESET (FROM CAT) FROM BACK EMF SAMPLER & RAMP GENERATOR NEUTRAL SIMULATOR ΦA + ΦB + ΦC 6 MULTIPLEXER COMMUTATION STATE MACHINE gm = 1 8k Ω 4k Ω 4k Ω 4k Ω 4k Ω 4k Ω 4k Ω + – SIGN CHANGER TO SPEED FB FB A RES1 RES2 RES3 FB B FB C F/R MOTOR ΦC F/R MOTOR ΦB MOTOR ΦA Figure 6. Back EMF Sampler Detailed Block Diagram sampled). The sampled phase provides a back EMF signal that is compared against the neutral of the motor. The sampler is controlled by the commutation state machine. The sampled back EMF is compared to the neutral through an error amplifier. The output of the error amplifier outputs a charging or discharging current to SPEED FB, which provides the control voltage to the VCO. |
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