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BD63621MUV Scheda tecnica(PDF) 7 Page - Rohm |
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BD63621MUV Scheda tecnica(HTML) 7 Page - Rohm |
7 / 29 page 7/24 BD63621MUV TSZ02201-0P2P0B701290-1-2 © 2017 ROHM Co., Ltd. All rights reserved. 21.Dec.2020 Rev.002 www.rohm.com TSZ22111・15・001 ○PWM Constant current control 1) Current control operation When the output transistor is turned on, the output current increases, raising the voltage over the current sense resistor. When the voltage on the RNF pin reaches the voltage value set by the internal 2-bit/4-bit DAC and the VREF input voltage, the current limit comparator engages and enters current decay mode. The output is then held off for a period of time determined by the RC time constant connected to the CR pin. The process repeats itself constantly for PWM operation. 2) Noise-masking function In order to avoid misdetection of output current due to RNF spikes that may occur when the output turns ON, the IC employs an automatic current detection-masking period during charging of CR (tONMIN 0.7µs Typ) and during change in internal PHASE (tONMIN2 1.5µs Typ: fixed in internal circuit), during which current detection is disabled immediately after the output transistor is turned on. This allows for constant-current drive without the need for an external filter. This noise-masking period defines the minimum ON-time for the motor output transistor. 3) CR Timer The CR filter connected to the CR pin is repeatedly charged and discharged between the VCRH and VCRL levels. The output of the internal comparator is masked while charging from VCRL to VCRH in order to cancel noise. (As mentioned above, this period defines the minimum ON-time of the motor output transistor.) The CR terminal begins discharging once the voltage reaches VCRH. When the output current reaches the current limit during this period (i.e. RNF voltage reaches the decay trigger voltage), then the IC enters decay mode. The CR continues to discharge during this period until it reaches VCRL, at which point the IC output is switched back ON. The current output and CR pin begin charging simultaneously. The CR charge time (tONMIN) and discharge time (tdischarge) are set by external components, according to the following formulas. The total of tONMIN and tdischarge yield the chopping period, tchop. tONMIN[s]≒C・R'・R / (R'+R)・ln[(VCR-0.4)/(VCR-1.0)] VCR=V・R/(R'+R) V: internal regulator voltage 5V(Typ) R': CR terminal internal impedance 5k Ω(Typ) tdischarge[s]≒C・R・ln[(1+ α)/0.4] α:See the right graph. tCHOP[s]≒tONMIN + tdischarge Figure 4 Timing chart of CR voltage, RNF voltage and output current Attach a resistor of at least 10 k Ω to the CR terminal (10 kΩ to 200 kΩ recommended) as lower values may keep the RC from reaching the VCRH voltage level. A capacitor in the range of 470 pF – 3300 pF is also recommended. As the capacitance value is increased, however, the noise-masking period (tonmin) also increases, and there is a risk that the output current may exceed the current limit threshold due to the internal L and R components of the output motor coil. Also, ensure that the chopping period (tchop) is not set longer than necessary, as doing so will increase the output ripple, thereby decreasing the average output current and yielding lower output rotation efficiency. The optimal value should reduce the motor drive noise while keeping distortion of the output current waveform to a minimum. 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0 500 1000 1500 2000 C [pF] CR Voltage RNF Voltage Output current Spike noise Current limit Value GND Minimum ON Time tONMIN VCRH(1.0V Typ) VCRL(0.4V Typ) GND 0mA Current limit Value Chopping Period tCHOP Discharge time tdischarge |
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