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FDS6670AS датащи(PDF) 11 Page - Richtek Technology Corporation |
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FDS6670AS датащи(HTML) 11 Page - Richtek Technology Corporation |
11 / 18 page 11 DS8298-01 November 2011 www.richtek.com RT8298 Copyright 2011 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation. © Application Information 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 voltage divider according to the following equation : OUT REF R1 V = V 1 R2 ⎛⎞ + ⎜⎟ ⎝⎠ Where VREF is the feedback reference voltage (0.808V typ.). External Bootstrap Diode Connect a 1 μF 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 RT8298. Note that the external boot voltage must be lower than 5.5V. Figure 2. External Bootstrap Diode RT8298 GND FB R1 R2 VOUT SW BOOT 5V RT8298 1µF Chip Enable Operation The EN pin is the chip enable input. Pulling the EN pin low (<0.4V) will shutdown the device. During shutdown mode, the RT8298 quiescent current drops to lower than 3 μA. Driving the EN pin high (2V < EN < 5.5V) will turn on An external MOSFET can be added to implement digital control on the EN pin, as shown in Figure 4. In this case, a 100k Ω pull-up resistor, REN, is connected between VIN pin and the EN pin. MOSFET Q2 will be under logic control to pull down the EN pin. Figure 3. Enable Timing Control Figure 4. Digital Enable Control Circuit RT8298 EN GND 100k VIN REN Q2 EN the device again. For external timing control, the EN pin can also be externally pulled high by adding a REN resistor and CEN capacitor from the VIN pin (see Figure 3). The chip starts to operate when VIN rises to 4.2V (UVLO threshold). During the VIN rising period, if an 8V output voltage is set, VIN is lower than the VOUT target value and it may cause the chip to shut down. To prevent this situation, a resistive voltage divider can be placed between the input voltage and ground and connected to the EN pin to adjust enable threshold, as shown in Figure 5. For example, the setting VOUT is 8V and VIN is from 0V to 12V, when VIN is higher than 10V, the chip is triggered to enable the converter. Assume REN1 = 50k Ω. Then, Figure 5. Resistor Divider for Lockout Threshold Setting RT8298 EN GND VIN REN1 REN2 RT8298 EN GND VIN REN CEN EN EN1 EN_T EN2 IN_S EN_T (R x V ) R = (V V ) − where VEN_T is the enable comparator's logic-high reference threshold voltage (1.7V) and VIN_S is the target turn on input voltage (10V in this example). According to the equation, the suggested resistor R EN2 is 10.2k Ω. |
Аналогичный номер детали - FDS6670AS |
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Аналогичное описание - FDS6670AS |
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