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PCMB065T-2R2MS Datasheet(PDF) 13 Page - International Rectifier |
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PCMB065T-2R2MS Datasheet(HTML) 13 Page - International Rectifier |
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13 / 21 page  13 IR3863MPBF 8/8/2012 Rev3.2 CIRCUIT DESCRIPTION Selection of components for the converter is an iterative process which involves meeting the specifications and tradeoffs between performance and cost. The following sections will guide one through the process. Inductor Selection Inductor selection involves meeting the steady state output ripple requirement, minimizing the switching loss of the upper MOSFET, meeting transient response specifications and minimizing the output capacitance. The output voltage includes a DC voltage and a small AC ripple component due to the low pass filter which has incomplete attenuation of the switching harmonics. Neglecting the inductance in series with the output capacitor, the magnitude of the AC voltage ripple is determined by the total inductor ripple current flowing through the total equivalent series resistance (ESR) of the output capacitor bank. (5) L 2 V V T ΔI OUT IN ON One can use equation 5 to find the required inductance. ΔI is defined as shown in Figure 24. The main advantage of small inductance is increased inductor current slew rate during a load transient, which leads to a smaller output capacitance requirement as discussed in the Output Capacitor Selection section. The draw back of using smaller inductances is increased switching power loss in the upper MOSFET, which reduces the system efficiency and increases the thermal dissipation. COMPONENT SELECTION Input Current ΔI IOUT TS Figure 24. Typical Input Current Waveform OVER TEMPERATURE PROTECTION When the IR3863 exceeds its over temperature threshold, the MOSFET gates are tri-state and PGOOD is pulled low. Switching resumes once temperature drops below the over temperature hysteresis level. GATE DRIVE LOGIC The gate drive logic features adaptive dead time, diode emulation, and a minimum lower gate interval. An adaptive dead time prevents the simultaneous conduction of the upper and lower MOSFETs. The lower gate voltage must be below approximately 1V after PWM goes HIGH before the upper MOSFET can be gated on. Also, the differential voltage between the upper gate and PHASE must be below approximately 1V after PWM goes LOW before the lower MOSFET can be gated on. The upper MOSFET is gated on after the adaptive delay for PWM = HIGH and the lower MOSFET is gated on after the adaptive delay for PWM = LOW. When FCCM = LOW, the lower MOSFET is driven ‘off’ when the ZCROSS signal indicates that the inductor current is about to reverse direction. The ZCROSS comparator monitors the PHASE voltage to determine when to turn off the lower MOSFET. The lower MOSFET stays ‘off’ until the next PWM falling edge. When the lower peak of the inductor current is above zero, IR3863 operates in continuous conduction mode. The continuous conduction mode can also be selected for all load current levels by pulling FCCM to HIGH. Whenever the upper MOSFET is turned ‘off’, it stays ‘off’ for the Min Off Time denoted in the Electrical Specifications. This minimum duration allows time to recharge the bootstrap capacitor and allows the over current monitor to sample the PHASE voltage. |
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