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ML4435CS Datasheet(PDF) 5 Page - Micro Linear Corporation |
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ML4435CS Datasheet(HTML) 5 Page - Micro Linear Corporation |
5 / 14 page May, 2000 PRELIMINARY DATASHEET 5 PRELIMINARY ML4435 COMPONENT SELECTION Selecting external components for the ML4435 requires calculations based on the motor’s electrical and mechani- cal parameters. The following is a list of the motor param- eters needed to for these calculations: The maximum DC motor supply voltage V MOTOR (V) The maximum operating current I MAX (A) The winding resistance measured line to line Rl-l Ω The number of magnetic poles N (Unitless) The Back EMF constant Ke (V s/RAD) The torque constant K τ (N m/A) of the motor (Kτ = Ke in SI units) The maximum speed of operation RPMMAX (RPM) The moment of inertia J (Kg m2 ) of the motor and its load The viscous damping factor ζ (Unitless) of the motor and its load If one or more of the above values is not known, it is still possible to pick components for the ML4435, but some experimentation may be necessary to determine the optimal values. All quantities are in SI units unless other- wise specified. The following formulas and component selection graphs should be considered as a starting point from which to optimize the application. All calculations for capacitors and resistors should be used as the first approximation for selecting the closest standard value. SUPPLY VOLTAGE AND ON-CHIP VOLTAGE REFERENCE VCC The supply voltage at VCC (pin 10) is nominally 12V ± 10%. A bypass capacitor of 0.1µF to ground as close as possible to VCC (pin 10) is recommended. RT An internal 6V reference is generated inside the ML4435. The reference appears on RT (pin 6). A resistor to ground on RT sets the PWM frequency. This resistor can be replace with a potentiomenter for use in setting the speed command. This topic is discussed under the PWM SPEED CONTROL section. Note: Buffer this pin with an op amp with at least a 1M Ω input impedance if external circuits are necessary. OUTPUT DRIVERS The output drivers LA, LB, LC, HA, HB, and HC provide totem pole output drive signals for a 3 phase bridge power stage. All control functions in the ML4435 translate to outputs at these pins. LA, LB, LC provide the low side drive signals for phases A, B, and C of the 3 phase power stage and are 12V active high signals. HA, HB, and HC provide the high side signals for phases A, B, and C of the 3 phase power stage and are 12V active low signals. CURRENT LIMITING IN THE 3 PHASE BRIDGE POWER STAGE A current sense resistor RSENSE shown in Figure 1 is installed in the 3 phase power stage to regulate the maxi- mum current in the power stage and the BLDC motor. Current regulation is accomplished by shutting off the output drivers LA, LB, and LC for the remainder of the PWM period if the voltage across RSENSE exceeds the current limit threshold set by the SOFT START (pin 19). The maximum power dissipated in RSENSE is shown in Figure 2. Figure 1. Current Limit with RSENSE Figure 2. RSENSE Power vs. Motor Current RSENSE ISENSE R C IMAX [MOTOR] A 02 4 60 2 3 4 5 6 0 FUNCTIONAL DESCRIPTION |
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