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ISL6731BFBZ Datasheet(PDF) 11 Page - Intersil Corporation |
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ISL6731BFBZ Datasheet(HTML) 11 Page - Intersil Corporation |
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11 / 20 page  ISL6731A, ISL6731B 11 FN8582.1 February 13, 2015 Submit Document Feedback Functional Description VCC Undervoltage Lockout (UVLO) The ISL6731A and ISL6731B start automatically once the voltage at VCC exceeds the UVLO threshold. Shutdown When the VFB pin is below 0.2V, the controller is disabled and the PWM output driver is tri-stated. When disabled, the IC power will be reduced. During shutdown, the COMP pin is discharged to GND and the controller is disabled. The Over-Temperature Protection (OTP) is still alive to prevent the controller from starting up in a high temperature ambient condition. In the event that the FB pin is disconnected from the feedback resistors, the FB pin is pulled to ground by an internal current source IFB. When the FB pin voltage drops below 0.2V, the gate driver is disabled. The ISL6731A or ISL6731B enters shutdown mode. Soft-Start The COMP pin is released once the soft-start operation begins. A 13µA current sources out to the RC network connected from the COMP pin until the FB pin voltage reaches 90% of the reference voltage. Switching is inhibited when the COMP pin voltage is below 1V. When the COMP pin reaches 1V, the current error amplifier and the gate driver are activated and the converter starts switching. During UVLO, brownout and shutdown, the COMP is pulled to the ground. Input Voltage Sensing The VIN pin is needed to sense the rectified input voltage. The sensed semi-sinusoidal waveform is needed to shape the inductor current, which helps achieves unity power factor. At the same time, the voltage on the VIN pin is used to generate the negative capacitive element at the input. This will cancel the input filter capacitor, CF. Canceling the effect of CF will increase the displacement power factor and alleviate the zero crossing distortion, which is related to the distortion power factor. The BO pin also utilizes the VIN resistor divider for voltage sensing. Set the resistor divider ratio to satisfy the brownout requirement. First, calculate the resistor divider ratio, KBO. Where VF is the forward voltage drop of the bridge rectifier and the voltage drop of DF1; DF2. Then, select the RIN2 based on the highest reasonable resistance value. Then select the RIN1 based upon the desirable minimum RMS value of the line voltage for the PFC operation. Inductor Current Sensing The current sensing of the converter has two purposes. One is to force the inductor current to track the input semi-sinusoidal waveform. The other purpose is for overcurrent protection. Refer to Figure 11 for the current sensing scheme. The sensed current ICS is in proportion to the inductor current, IL as described in Equation 3: where: RCS is the current sensing resistor with low value in the return path to the bridge rectifier. RSEN is the current scaling resistor connected between ISEN to the RCS. A high value RCS renders more accurate current sensing. It is recommended to use the RCS to render 120mV peak voltage at the maximum line voltage during full load condition. Where is the efficiency of the converter at the maximum line input with full load. FIGURE 10. INPUT VOLTAGE SENSING SCHEMATIC BO RIN1 CBO VIN CF2 VLINE CF3 EMI CHOKE Lm DF1 DF2 RIN2 KBO VBORMAX VRMSmin 2VF – ------------------------------------------- = (EQ. 1) RIN1 KBO 1K – BO --------------------- RIN2 = (EQ. 2) ICS 1 2 --- RCS RSEN ---------------- IL = (EQ. 3) FIGURE 11. INDUCTOR CURRENT SENSING SCHEME Q1 COUT VOUT L CF1 VI RCS ISEN RSEN CURRENT MIRROR 2:1 ICS 0.5 IOC > ICS RCS 120mV VRMSMAX 2POmax ------------------------------------------------------------- (EQ. 4) |
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