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NCV898031 Datasheet(PDF) 7 Page - ON Semiconductor |
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NCV898031 Datasheet(HTML) 7 Page - ON Semiconductor |
7 / 13 page NCV898031 http://onsemi.com 7 TYPICAL PERFORMANCE CHARACTERISTICS Figure 8. Enable Pulldown Current vs. Voltage TJ, JUNCTION TEMPERATURE (°C) Figure 9. Enable Pulldown Current vs. Temperature 012 3 4 Venable, VOLTAGE (V) TJ = 25°C 56 −40 10 60 110 160 0 1 2 3 4 5 7 6 5.0 5.5 6.0 6.5 7.0 7.5 8.0 Current Mode Control The NCV898031 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 directly from the inductor, the ramp signal responds immediately to line voltage changes. This eliminates the delay caused by the output filter and the 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 NCV898031 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. Current Limit The NCV898031 features two current limit protections, peak current mode and over current latch off. When the current sense amplifier detects a voltage above the peak current limit between ISNS and GND 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 ISNS to GND, with R = VCL / Ilimit. If the voltage across the current sense resistor exceeds the over current threshold voltage, the device enters over current hiccup mode. The device will remain off for the hiccup time and then go through the soft−start procedure. Short Circuit Protection If the short circuit enable bit is set (SCE = Y), the device will attempt to protect the power MOSFET from damage. When the output voltage falls below the short circuit trip voltage, after the initial short circuit blanking time, the device enters short circuit latch−off. The device will remain off for the hiccup time and then go through the soft−start. Enable The Enable pin has two modes. When a DC logic high (CMOS/TTL compatible) voltage is applied to this pin, the NCV898031 operates at the programmed frequency. When a DC logic low voltage is applied, the NCV898031 enters a low quiescent current sleep mode. The NCV898031 requires 2 clock cycles after the falling edge of the Enable signal to stop switching. UVLO Input Undervoltage Lockout (UVLO) is provided to ensure that unexpected behavior does not occur when VIN is too low to support the internal rails and power the controller. The IC will start up when enabled and VIN surpasses the UVLO threshold plus the UVLO hysteresis and will shut down when VIN drops below the UVLO threshold or the part is disabled. |
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