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NCV8853 Datasheet(PDF) 8 Page - ON Semiconductor |
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NCV8853 Datasheet(HTML) 8 Page - ON Semiconductor |
8 / 12 page NCV8853 www.onsemi.com 8 THEORY OF OPERATION Oscillator S R Q + Slope Compensation NCV885300 VIN ISNS Current Information Voltage Error Vout GDRV Compensation CO L CSA VEA PWM COMPARATOR GATE DRIVE - + Figure 13. Current Mode Control Schematic COMP FB − + RSNS RLOAD VREF Current Mode Control The NCV8853 SMPS incorporates a current mode control scheme, in which the PWM ramp signal is derived from the power switch current. This ramp signal is compared to the output of the error amplifier to control the on−time of the power switch. The oscillator is used as a fixed−frequency clock to ensure a constant operational frequency. The resulting control scheme features several advantages over conventional voltage mode control. First, derived from the resistor in the power path, the signal responds immediately to line voltage changes. This eliminates the delay caused by the output filter and error amplifier, which is commonly found in voltage mode controllers. The second benefit comes from inherent pulse−by−pulse current limiting by merely clamping the peak switching current. Finally, since current mode commands an output current rather than voltage, the filter offers only a single pole to the feedback loop. This allows for a simpler compensation. The NCV8853 also includes a slope compensation scheme in which a fixed ramp generated by the oscillator is added to the current ramp. A proper slope rate is provided to improve circuit stability without sacrificing the advantages of current mode control. Overcurrent Protection The NCV8853 features two current limit protections: peak current mode and overcurrent hiccup mode. When the current sense amplifier detects a voltage above the peak current limit between VIN and ISNS after the current limit leading edge blanking time, the peak current limit causes the power switch to turn off for the remainder of the cycle. Set the current limit with a resistor from VIN to ISNS, with R = 0.100 / Ilimit. If the voltage across the current sense resistor exceeds the overcurrent threshold voltage the part enters overcurrent hiccup mode. The part will remain off for the hiccup time and then go through the power on reset procedure. Short Circuit Hiccup Protection When the output voltage falls below the short circuit trip voltage the part enters short circuit latch off. When a short is detected the NCV8853 disables the outputs and attempts to re−enable the outputs after the short circuit hiccup time. The part remains off for the hiccup time and then goes through the power on reset procedure. If the short has been removed then the output stage re−enables and operates normally; however, if the short is still present the cycle begins again. Internal heat dissipation is kept to a minimum as current will only flow during the reset time of the protection circuitry. The hiccup mode is continuous until the short is removed. |
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