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EMK325BJ476MM Datasheet(PDF) 8 Page - Richtek Technology Corporation |
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EMK325BJ476MM Datasheet(HTML) 8 Page - Richtek Technology Corporation |
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8 / 12 page  RT8272 8 DS8272-02 March 2011 www.richtek.com Application Information The RT8272 is an asynchronous high voltage buck converter that can support the input voltage range from 4.75V to 24V and the output current can be up to 3A. Output Voltage Setting The resistive divider allows the FB pin to sense the output voltage as shown in Figure 1. Figure 1. Output Voltage Setting The output voltage is set by an external resistive divider according to the following equation : ⎛⎞ + ⎜⎟ ⎝⎠ OUT FB R1 V = V 1 R2 Where VFB is the feedback reference voltage (0.92V typ.). Figure 2. External Bootstrap Diode Soft-Start The RT8272 contains an external soft-start clamp that gradually raises the output voltage. The soft-start timming can be programed by the external capacitor between SS pin and GND. The chip provides a 10 μA charge current for the external capacitor. If 0.1 μF capacitor is used to set the soft-start and it ’ s period will be 10ms(typ.). Inductor Selection The inductor value and operating frequency determine the ripple current according to a specific input and output voltage. The ripple current ΔIL increases with higher VIN and decreases with higher inductance. OUT OUT L IN VV I = 1 fL V ⎡⎤ ⎡ ⎤ Δ× − ⎢⎥ ⎢ ⎥ × ⎣⎦ ⎣ ⎦ Having a lower ripple current reduces not only the ESR losses in the output capacitors but also the output voltage ripple. High frequency with small ripple current can achieve highest efficiency operation. However, it requires a large inductor to achieve this goal. For the ripple current selection, the value of ΔIL= 0.4(IMAX) will be a reasonable starting point. The largest ripple current occurs at the highest VIN. To guarantee that the ripple current stays below the specified maximum, the inductor value should be chosen according to the following equation : OUT OUT L(MAX) IN(MAX) VV L = 1 fI V ⎡⎤ ⎡ ⎤ ×− ⎢⎥ ⎢ ⎥ ×Δ ⎣⎦ ⎣ ⎦ Inductor Core Selection The inductor type must be selected once the value for L is known. Generally speaking, high efficiency converters can not afford the core loss found in low cost powdered iron cores. So, the more expensive ferrite or mollypermalloy cores will be a better choice. The selected inductance rather than the core size for a fixed inductor value is the key for actual core loss. As the inductance increases, core losses decrease. Unfortunately, increase of the inductance requires more turns of wire and therefore the copper losses will increase. Ferrite designs are preferred at high switching frequency due to the characteristics of very low core losses. So, design goals can focus on the reduction of copper loss and the saturation prevention. RT8272 GND FB R1 R2 VOUT SW BOOT 5V RT8272 10nF External Bootstrap Diode Connect a 10nF low ESR ceramic capacitor between the BOOT pin and SW pin. This capacitor provides the gate driver voltage for the high side MOSFET. It is recommended to add an external bootstrap diode between an external 5V and the BOOT pin for efficiency improvement when input voltage is lower than 5.5V or duty ratio is higher than 65%. The bootstrap diode can be a low cost one such as 1N4148 or BAT54. The external 5V can be a 5V fixed input from system or a 5V output of the RT8272. |
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