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RT8294 Datasheet(PDF) 9 Page - Richtek Technology Corporation |
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RT8294 Datasheet(HTML) 9 Page - Richtek Technology Corporation |
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9 / 15 page  RT8294 9 DS8294-02 March 2011 www.richtek.com Application Information The RT8294 is a synchronous high voltage buck converter that can support the input voltage range from 4.5V to 23V and the output current can be up to 2A. Output Voltage Setting The resistive voltage divider allows the FB pin to sense the output voltage as shown in Figure 1. + OUT FB R1 V = V1 R2 where VFB is the feedback reference voltage 0.923V (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 and 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 IN4148 or BAT54. The external 5V can be a 5V fixed input from system or a 5V output of the RT8294. Note that the external boot voltage must be lower than 5.5V Figure 2. External Bootstrap Diode Figure 1. Output Voltage Setting Soft-Start The RT8294 contains an external soft-start clamp that gradually raises the output voltage. The soft-start timing can be programmed by the external capacitor between SS pin and GND. The chip provides a 6 µA charge current for the external capacitor. If 0.1 µF capacitor is used to set the soft-start, the period will be 15.5ms (typ.). Chip Enable Operation The EN pin is the chip enable input. Pulling the EN pin low (<0.4V) will shut down the device. During shutdown mode, the RT8294 quiescent current drops to lower than 3 µA. Driving the EN pin high ( > 2.7V, < 5.5V) will turn on the device again. For external timing control (e.g.RC), the EN pin can also be externally pulled high by adding a REN* resistor and CEN* capacitor from the VIN pin (see Figure 5). An external MOSFET can be added to implement digital control on the EN pin when no system voltage above 2.5V is available, as shown in Figure 3. In this case, a 100k Ω pull-up resistor, REN, is connected between VIN and the EN pin. MOSFET Q1 will be under logic control to pull down the EN pin. The output voltage is set by an external resistive voltage divider according to the following equation : Figure 3. Enable Control Circuit for Logic Control with Low Voltage To prevent enabling circuit when VIN is smaller than the VOUT target value, a resistive voltage divider can be placed between the input voltage and ground and connected to the EN pin to adjust IC lockout threshold, as shown in Figure 4. For example, if an 8V output voltage is regulated from a 12V input voltage, the resistor, REN2, can be selected to set input lockout threshold larger than 8V. RT8294 GND FB R1 R2 VOUT VIN EN GND BOOT FB SW 7 5 2 3 1 L R1 R2 VOUT Chip Enable VIN RT8294 SS 8 CSS COMP CC RC CP 6 4, 9 (Exposed Pad) CBOOT COUT CIN REN Q1 100k SW BOOT 5V RT8294 10nF |
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Similar Description - RT8294 |
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