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NCV887740D1R2G Datasheet(PDF) 10 Page  ON Semiconductor 

NCV887740D1R2G Datasheet(HTML) 10 Page  ON Semiconductor 
10 / 15 page NCV8877 www.onsemi.com 10 R OSC + 2859 (Fsw * 170) Where: fsw: switching frequency [kHz] ROSC: resistor from ROSC pin to GND [k] Note: The ROSC resistor ground return to the NCV8877 pin 3 must be independent of power grounds. 3. Select Current Sense Resistor Current sensing for peak current mode control and current limit relies on the MOSFET current signal, which is measured with a ground referenced amplifier. The easiest method of generating this signal is to use a current sense resistor from the source of the MOSFET to device ground. The sense resistor should be selected as follows: R S + V CL I CL Where: RS: sense resistor [ W] VCL: current limit threshold voltage [V] ICL: desire current limit [A] 4. Select Output Inductor The output inductor controls the current ripple that occurs over a switching period. A high current ripple will result in excessive power loss and ripple current requirements. A low current ripple will result in a poor control signal and a slow current slew rate in case of load steps. A good starting point for peak to peak ripple is around 20−40% of the inductor current at the maximum load at the worst case VIN, but operation should be verified empirically. The worst case VIN is half of VOUT, or whatever VIN is closest to half of VOUT. After choosing a peak current ripple value, calculate the inductor value as follows: L + V IN(WC) DWC DI L,max fs Where: VIN(WC): VIN value as close as possible to half of VOUT [V] DWC: duty cycle at VIN(WC) DIL,max: maximum peak to peak ripple [A] The maximum average inductor current can be calculated as follows: I L,AVG + V OUTIOUT(max) V IN(min)h The Peak Inductor current can be calculated as follows: I L,peak + IL,avg ) DI L,max 2 Where: IL,peak: Peak inductor current value [A] 5. Select Output Capacitors The output capacitors smooth the output voltage and reduce the overshoot and undershoot associated with line transients. The steady state output ripple associated with the output capacitors can be calculated as follows: V OUT(ripple) + DI OUT(max) fC OUT ) I OUT(max) 1 * D ) V IN(min)D 2fL R ESR The capacitors need to survive an RMS ripple current as follows: I Cout(RMS) + IOUT D WC D WC ) D WC 12 D WC L R OUT T SW 2 The use of parallel ceramic bypass capacitors is strongly encouraged to help with the transient response. 6. Select Input Capacitors The input capacitor reduces voltage ripple on the input to the module associated with the ac component of the input current. I Cin(RMS) + V IN(WC) 2 D WC LfsVOUT23 7. Select Compensator Components Current Mode control method employed by the NCV8877 allows the use of a simple, Type II compensation to optimize the dynamic response according to system requirements. 8. Select MOSFET(s) In order to ensure the gate drive voltage does not drop out the MOSFET(s) chosen must not violate the following inequality: Q g(total) v I drv fs Where: Qg(total): Total Gate Charge of MOSFET(s) [C] Idrv: Drive voltage current [A] fs: Switching Frequency [Hz] The maximum RMS Current can be calculated as follows: I Q(max) + Iout D D The maximum voltage across the MOSFET will be the maximum output voltage, which is the higher of the maximum input voltage and the regulated output voltaged: V Q(max) + VOUT(max) 9. Select Diode The output diode rectifies the output current. The average current through diode will be equal to the output current: I D(avg) + IOUT(max) Additionally, the diode must block voltage equal to the higher of the output voltage and the maximum input voltage: V D(max) + VOUT(max) The maximum power dissipation in the diode can be calculated as follows: P D + Vf (max) IOUT(max) 
Similar Part No.  NCV887740D1R2G 

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