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NCV887000D1R2G Datasheet(PDF) 8 Page - ON Semiconductor |
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NCV887000D1R2G Datasheet(HTML) 8 Page - ON Semiconductor |
8 / 15 page NCV8870 www.onsemi.com 8 Current Limit The NCV8870 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. EN/SYNC The Enable/Synchronization pin has three modes. When a dc logic high (CMOS/TTL compatible) voltage is applied to this pin the NCV8870 operates at the programmed frequency. When a dc logic low voltage is applied to this pin the NCV8870 enters a low quiescent current sleep mode. When a square wave of at least %fsync,min of the free running switching frequency is applied to this pin, the switcher operates at the same frequency as the square wave. If the signal is slower than this, it will be interpreted as enabling and disabling the part. The falling edge of the square wave corresponds to the start of the switching cycle. If device is disabled, it must be disabled for 7 clock cycles before being re-enabled. 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. Internal Soft-Start To insure moderate inrush current and reduce output overshoot, the NCV8870 features a soft start which charges a capacitor with a fixed current to ramp up the reference voltage. This fixed current is based on the switching frequency, so that if the NCV8870 is synchronized to twice the default switching frequency the soft start will last half as long. VDRV An internal regulator provides the drive voltage for the gate driver. Bypass with a ceramic capacitor to ground to ensure fast turn on times. The capacitor should be between 0.1 mF and 1 mF, depending on switching speed and charge requirements of the external MOSFET. GDRV An RGND = 15 kW (typical) GDRV−GND resistor is strongly recommended. APPLICATION INFORMATION Design Methodology This section details an overview of the component selection process for the NCV8870 in continuous conduction mode boost. It is intended to assist with the design process but does not remove all engineering design work. Many of the equations make heavy use of the small ripple approximation. This process entails the following steps: 1. Define Operational Parameters 2. Select Current Sense Resistor 3. Select Output Inductor 4. Select Output Capacitors 5. Select Input Capacitors 6. Select Feedback Resistors 7. Select Compensator Components 8. Select MOSFET(s) 9. Select Diode 10. Determine Feedback Loop Compensation Network 1. Define Operational Parameters Before beginning the design, define the operating parameters of the application. These include: VIN(min): minimum input voltage [V] VIN(max): maximum input voltage [V] VOUT: output voltage [V] IOUT(max): maximum output current [A] ICL: desired typical cycle-by-cycle current limit [A] From this the ideal minimum and maximum duty cycles can be calculated as follows: Dmin + 1 * VIN(max) VOUT Dmax + 1 * VIN(min) VOUT Both duty cycles will actually be higher due to power loss in the conversion. The exact duty cycles will depend on conduction and switching losses. If the maximum input voltage is higher than the output voltage, the minimum duty cycle will be negative. This is because a boost converter cannot have an output lower than the input. In situations where the input is higher than the output, the output will follow the input, minus the diode drop of the output diode and the converter will not attempt to switch. If the calculated Dmax is higher the Dmax of the NCV8870, the conversion will not be possible. It is important for a boost |
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