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NCV887700D1R2G Datasheet(PDF) 11 Page - ON Semiconductor |
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NCV887700D1R2G Datasheet(HTML) 11 Page - ON Semiconductor |
11 / 17 page NCV8877 www.onsemi.com 11 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. 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. 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. VDRV uses an internal linear regulator to charge the VDRV bypass capacitor. VOUT must be decoupled at the IC by a capacitor that is equal or larger in value than the VDRV decoupling capacitor. APPLICATION INFORMATION Design Methodology This section details an overview of the component selection process for the NCV8877 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 Operating Frequency 3. Select Current Sense Resistor 4. Select Output Inductor 5. Select Output Capacitors 6. Select Input Capacitors 7. Select Compensator Components 8. Select MOSFET(s) 9. Select Diode 10. Design Notes 11. 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: D min + 1 * V IN(max) V OUT Dmax + 1 * V IN(min) V OUT 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 NCV8877, the conversion will not be possible. It is important for a boost converter to have a restricted Dmax, because while the ideal conversion ration of a boost converter goes up to infinity as D approaches 1, a real converter’s conversion ratio starts to decrease as losses overtake the increased power transfer. If the converter is in this range it will not be able to regulate properly. If the following equation is not satisfied, the device will skip pulses at high VIN: D min fs w t on(min) Where: fs: switching frequency [Hz] ton(min): minimum on time [s] 2. Select Operating Frequency The default setting is an open ROSC pin, allowing the oscillator to operate at the default frequency Fs. Adding a resistor to GND increases the switching frequency. The graph in Figure 17, below, shows the required resistance to program the frequency. From 200 kHz to 500 kHz, the following formula is accurate to within 3% of the expected Figure 17. ROSC vs. FSW 100 90 80 70 60 50 40 30 20 10 0 150 200 250 500 450 400 350 300 FSW (kHz) 550 |
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