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C3225X5R0J476M Datasheet(PDF) 9 Page - Richtek Technology Corporation |
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C3225X5R0J476M Datasheet(HTML) 9 Page - Richtek Technology Corporation |
9 / 16 page RT8278 9 DS8278-02 March 2011 www.richtek.com Application Information The RT8278 is an asynchronous high voltage buck converter that supports an input voltage range from 4.5V to 24V with output current up to 2A. Output Voltage Setting The resistive voltage divider allows the FB pin to sense the output voltage as shown in Figure 1. RT8278 GND FB R1 R2 VOUT Figure 1. Output Voltage Setting Operating Frequency Selection of the operating frequency is a trade off between efficiency and component size. High frequency operation allows the use of smaller inductor and capacitor values. Operation at lower frequency improves efficiency by reducing internal gate charge and switching losses, but requires larger inductance and/or capacitance to maintain low output ripple voltage. The operating frequency of the RT8278 is determined by an external resistor that is connected between the RT pin and ground. The value of the resistor sets the ramp current that is used to charge and discharge an internal timing capacitor within the oscillator. Selection of the RT resistor value can be determined by examining the curve below in Figure3. Although frequencies as high as 3MHz are available, the minimum on-time of the RT8278 imposes a limit on the operating duty cycle. Figure 4 shows the examples of minimum on-time constraint for output voltages 3.3V and 1.8V. It is recommended to operate the RT8278 in the region under the corresponding Vout curve. Except the minimum on-time constraint, the limit of maximum duty also needs to be considered. In ideal case, the duty cycle of the RT8278 can be calculated by below equation, But in practical case it will be higher than the calculation result since all the components in a converter circuit are not ideal. Figure 5 shows an example for the limit of maximum duty. With 5V input voltage, the 3.3V output voltage of the RT8278 becomes out of regulation when the output current is increased. However, when the input voltage is changed to 12V, the 3.3V output voltage of the RT8278 remains in regulation even with 2A output current. According to equation below, the duty cycle is 0.67 for the RT8278 operated with 5V input voltage and 3.3V output voltage in 2.2MHz switching frequency. The ideal case duty cycle calculation is already over the limit of maximum duty(65%). Thus, it is obvious that the RT8278 can't support 2A output current in such conditions : SW Duty Cycle 1 0.15 x f (MHz) =− The output voltage is set by an external resistive voltage divider according to the following equation : ⎛⎞ + ⎜⎟ ⎝⎠ OUT FB R1 V = V 1 R2 where VFB is the feedback reference voltage (0.8V typ.). 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 voltage source and the BOOT pin for efficiency improvement when input voltage is lower than 5.5V or duty cycle is higher than 65% .The bootstrap diode can be a low cost one such as IN4148 or BAT54. The external voltage source must be fixed at 5V and can be provided from the system or the output of the RT8278. SW BOOT 5V RT8278 10nF Figure 2. External Bootstrap Diode |
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