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FS6X1220R Datasheet(PDF) 8 Page - Fairchild Semiconductor |
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FS6X1220R Datasheet(HTML) 8 Page - Fairchild Semiconductor |
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8 / 16 page  FS6X1220R 8 Functional Description 1. Startup : To guarantee stable operation of the control IC, Vcc has under voltage lockout (UVLO) with 6V hysteresis. Figure 1 shows the relation between the supply current (Icc) and the supply voltage (Vcc). Before Vcc reaches 15V, the start-up current is 60 µA, which is usually provided by the DC link through start-up resistor. When Vcc reaches 15V, the control IC begins operation and the operating current increases to 10mA as shown. Once the control IC starts operation, it continues its normal operation unless Vcc goes below the stop voltage of 9V. Figure 1. Relation between supply current and voltage 2. Feedback Control : The FS6X1220R employs current mode control. The voltage of the feedback pin is compared with the current sense voltage for pulse width modulation (PWM). Figure 2 illustrates the simplified PWM block. The feedback voltage determines the peak drain current of the SenseFET. Usually opto-coupler along with TL431 are used to implement feedback network. The collector of the opto- coupler transistor is connected to feedback pin and the emitter is connected to the ground pin. When the voltage of the reference pin of TL431 exceeds the internal reference voltage of 2.5V, the opto-coupler diode current increases, pulling down the feedback voltage. Figure 2. Pulse width modulation (PWM) circuit 3. Protection Circuit : Besides pulse-by-pulse current limit, the FS6X1220R has 3 self protection functions; over load protection (OLP), over voltage protection (OVP) and thermal shutdown (TSD). Because these protection circuits are fully integrated into the IC without external components, the reliability can be improved. In the event of these fault conditions, FS6X1220R enters into auto-restart operation. Once the fault condition occurs, switching operation is terminated and MOSFET remains off, which causes Vcc to be reduced. When Vcc reaches 9V, the protection is reset and the supply current reduces to 60uA. Then, Vcc begin to increase with the current provided through the start-up resistor. When Vcc reaches 15V, FS6X1220R resumes its normal operation if the fault condition is removed. In this manner, the auto-restart alternately enables and disables the switching of the power MOSFET until the fault condition is eliminated as illustrated in figure 3. Figure 3. Auto restart operation after protection 3.1 Pulse-by-pulse current limit : As shown in figure 2, the drain current of the power MOSFET is limited by the inverting input of PWM comparator (Vfb*). Assuming that the 0.9mA current source flows only through the internal resistor (28R +R= 2.9k), the cathode voltage of diode D2 is about 2.6V. Since D1 is blocked when the feedback voltage (Vfb) exceeds 2.6V, the maximum voltage of the cathode of D2 is 2.6V. Therefore, the maximum value of Vfb* is about 0.1V, which limits the peak value of the power MOSFET drain current. Icc Vcc Vstop=9V 60uA 10mA Vstart=15V Vz 4 OSC Vcc Vref 5uA 0.9mA V SD R 28R FB Gate driver OLP D1 D2 Vfb* Vfb 431 Cfb Vo Fault situation 9V 15V Vcc Vds Icc 10mA 60uA t Fault occurs Fault removed Normal operation Normal operation Power on |
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Similar Description - FS6X1220R |
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