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전자부품 데이터시트 검색엔진 |
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전자부품 데이터시트 검색엔진 |
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ML4425 데이터시트(PDF) 7 Page - Fairchild Semiconductor |
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ML4425 데이터시트(HTML) 7 Page - Fairchild Semiconductor |
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7 / 16 page  PRODUCT SPECIFICATION ML4425 REV. 1.0.2 7/2/01 7 Current Limiting in the Power Stage The current sense resistor (RSENSE) shown in Figure 1 regu- lates the maximum current in the power stage and the BLDC motor. Current regulation is accomplished by shutting off the output drivers LA, LB, and LC for a fixed amount of time if the voltage across RSENSE exceeds the current limit thresh- old. ILIMIT The voltage on the ILIMIT pin sets the current limit threshold. The ML4425 has an internal voltage divider from VREF that sets a default current limit threshold of 2.3V (see Figure 2). An external voltage divider referenced to VREF can be used to override the default ILIMIT setting. The external divider should have at least 10 times the current flow of the internal divider. RSENSE The function of RSENSE is to provide a voltage proportional to the motor current to set the current limit trip point. The default trip voltage across RSENSE is 460mV, set by the internal ILIMIT divider ratio. The current sense resistor should be a low inductance resistor such as a carbon compo- sition. For resistors in the milliohms range, wire-wound resistors tend to have low values of inductance. RSENSE should be sized to handle the power dissipation (IMAX 2 ↔ RSENSE). ISENSE Filter The ISENSE RC lowpass filter is placed in series with the cur- rent sense signal as shown in Figure 2. The purpose of this filter is to remove the diode reverse recovery shootthrough current. This current causes a voltage spike on the leading edge of the current sense signal which may falsely trigger the current limit. The current sense voltage waveform is shown before and after filtering in Figure 3. The recommended starting values for this circuit are R = 1k Ω and C = 330pF. This gives a time constant of 330ns, and will filter out spikes of shorter duration. C can be increased to as much as 2.2nF, but should not exceed a time constant of more than a few microseconds. CIOS When ISENSE exceeds 0.2 ↔ ILIMIT, the current limit one- shot is activated, turning off LA, LB, and LC for a fixed amount of time (tOFF). tOFF is set by the amount of capaci- tance connected to CIOS. CIOS is usually set for a fixed off time equal to or less than the PWM period. For a 25kHz PWM frequency, the PWM period is 40µs; tOFF should be between 20µs and 40µs. The lower limit of tOFF is dictated by the minimum on time of the power stage; a safe approxi- mation is 5µs or less. The equation for finding the CIOS capacitance value is as follows: Commutation Control A 3-phase BLDC motor requires electronic commutation to achieve rotational motion. Electronic commutation requires the switching on and off of the power switches of a 3-phase half bridge. For torque production to be achieved in one direction, the commutation is dictated by the rotor position. Electronic commutation in the ML4425 is achieved by turn- ing on and off, in the proper sequence, one N output from one phase and one P output from another phase. There are six combinations of N and P outputs (six switching states) that constitute a full commutation cycle. These combinations are illustrated in Table 1 and Figure 4, and are labeled states A through F. This sequence is programmed into the commu- tation state machine. Clocking of the commutation state machine is provided by a voltage controlled oscillator (VCO). C OS t OFF 50 µA × 2.4V --------------------------------- = (1) × 5 ILIMIT ISENSE FROM RSENSE PWM ON/OFF – + VREF CIOS 16k Ω 2.9V 0V STOP 30 µA START 8k Ω VREF S R Q Q (a) 0V 460mV (b) Figure 2. Current Sense Circuitry Figure 3. Current Sense Resistor Waveforms (a) Without Filtering, and (b) With Filtering |
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