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ML4435CP Datasheet(PDF) 6 Page - Micro Linear Corporation |
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ML4435CP Datasheet(HTML) 6 Page - Micro Linear Corporation |
6 / 14 page PRELIMINARY DATASHEET May, 2000 6 PRELIMINARY ML4435 SOFT START The voltage at SOFT START (pin 19) sets the current limit threshold. The ML4435 has an internal voltage divider with a 1.1V supply voltage. This circuit is shown in Figure 3. The divider consists of two 225k Ω resistors setting the current limit threshold to approximately 0.55V. An external voltage divider off of VCC or an external reference can be used to override the default setting of SOFT START by using a divider with 10 times the current draw of the internal divider. RSENSE The function of RSENSE is to provide a voltage propor- tional to the motor current, for current limiting. The default trip voltage across RSENSE is 0.6V as set by the SOFT START (pin 19). The current sense resistor should be a low inductance resistor such as a carbon composition. For resistors in the milli ohms range wire wound resistors tend to have low values of inductance. RSENSE can be selected using Figure 4. The power rating of RSENSE should be sized to handle the power dissipation (I MAX squared times RSENSE) seen at maximum current. ISENSE FILTER An RC lowpass filter is required at the ISENSE input pin to remove the voltage spike on the leading edge of the current sense signal caused by the diode reverse recovery shoot through current. Absent the filter, false triggering of the current limit could occur. The recommended starting values for this circuit are R = 1K Ω and C = 1000pF a configuration that will filter out spikes less than 1 µs long. It is recommended that the capacitor value not be increased beyond 330pF. Pulse-By-Pulse Current Limiting When current limit is activated by the voltage on ISENSE exceeding the voltage on SOFT START the current limit is tripped, turning off LA, LB, and LC for the remainder of the PWM period. COMMUTATION CONTROL A 3 phase Brushless DC motor requires electronic com- Figure 3. SOFT START Function 0.5V .V CSOFT START SOFT START 225k Ω 225k Ω 9 A capacitor to ground on the SOFT START pin can be used to provide a soft ramping of the current limit on power up. The ramp time can be selected using Figure 4. Figure 4. SOFT START Ramp Time vs. CSOFTSTART C SOFT START F 050 00 30 5 0 Figure 5. ISENSE Filter Wave Forms BEFORE FILTERIN AFTER FILTERIN FUNCTIONAL DESCRIPTION |
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