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FSDM0265RNB Datasheet(PDF) 9 Page - Fairchild Semiconductor |
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FSDM0265RNB Datasheet(HTML) 9 Page - Fairchild Semiconductor |
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9 / 20 page  FSDM0265RNB 9 Functional Description 1. Startup : In previous generations of Fairchild Power Switches (FPSTM) the Vstr pin had an external resistor to the DC input voltage line. In this generation the startup resistor is replaced by an internal high voltage current source and a switch that shuts off when 15ms goes by after the supply voltage, Vcc, gets above 12V. The source turns back on if Vcc drops below 8V. Figure 4. High Voltage Current Source 2. Feedback Control : The FSDM0265RNB employs cur- rent mode control, as shown in Figure 5. An opto-coupler (such as the H11A817A) and shunt regulator (such as the KA431) are typically used to implement the feedback net- work. Comparing the feedback voltage with the voltage across the Rsense resistor plus an offset voltage makes it possible to control the switching duty cycle. When the KA431 reference pin voltage exceeds the internal reference voltage of 2.5V, the optocoupler LED current increases, the feedback voltage Vfb is pulled down and it reduces the duty cycle. This event typically happens when the input voltage is increased or the output load is decreased. Figure 5. Pulse Width Modulation (PWM) Circuit 3. Leading Edge Blanking (LEB) : At the instant the inter- nal Sense FET is turned on, the primary side capacitance and secondary side rectifier diode reverse recovery typically cause a high current spike through the Sense FET. Excessive voltage across the Rsense resistor leads to incorrect feedback operation in the current mode PWM control. To counter this effect, the FPS employs a leading edge blanking (LEB) cir- cuit. This circuit inhibits the PWM comparator for a short time (tLEB) after the Sense FET is turned on. 4. Protection Circuits : The FPS has several protective functions such as over load protection (OLP), over voltage protection (OVP), under voltage lock out (UVLO) and ther- mal shutdown (TSD). Because these protection circuits are fully integrated inside the IC without external components, the reliability is improved without increasing cost. Once a fault condition occurs, switching is terminated and the Sense FET remains off. This causes Vcc to fall. When Vcc reaches the UVLO stop voltage VSTOP (8V), the protection is reset and the internal high voltage current source charges the Vcc capacitor via the Vstr pin. When Vcc reaches the UVLO start voltage VSTART (12V), the FPS resumes its normal operation. In this manner, the auto-restart can alternately enable and disable the switching of the power Sense FET until the fault condition is eliminated. 4.1 Over Load Protection (OLP) : Overload is defined as the load current exceeding a pre-set level due to an unex- pected event. In this situation, the protection circuit should be activated in order to protect the SMPS. However, even when the SMPS is operating normally, the over load protec- tion (OLP) circuit can be activated during the load transition. In order to avoid this undesired operation, the OLP circuit is designed to be activated after a specified time to determine whether it is a transient situation or an overload situation. In conjunction with the Ipk current limit pin (if used) the cur- rent mode feedback path would limit the current in the Sense FET when the maximum PWM duty cycle is attained. If the output consumes more than this maximum power, the output voltage (Vo) decreases below its rating voltage. This reduces the current through the opto-coupler LED, which also reduces the opto-coupler transistor current, thus increasing the feedback voltage (VFB). If VFB exceeds 3V, the feed- back input diode is blocked and the 5uA current source (IDE- LAY) starts to charge Cfb slowly up to Vcc. In this condition, VFB increases until it reaches 6V, when the switching opera- tion is terminated as shown in Figure 6. The shutdown delay time is the time required to charge Cfb from 3V to 6V with 5uA current source. Vin,dc Vstr Vcc 15ms after Vcc≥12V UVLO off Vcc<8V UVLO on I STR J-FET I CH 3 OSC Vcc Vcc 5uA 0.9mA V SD R 2.5R Gate driver OLP D1 D2 V FB Vfb 431 C FB Vo + - V FB,in |
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Similar Description - FSDM0265RNB |
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