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C3225X5R0J476M Datasheet(PDF) 10 Page - Richtek Technology Corporation |
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C3225X5R0J476M Datasheet(HTML) 10 Page - Richtek Technology Corporation |
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10 / 15 page  RT7237J 10 DS7237J-00 September 2012 www.richtek.com © Copyright 2012 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 reference voltage (0.8V typ.). External Bootstrap Diode Connect a 0.1 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 RT7237J. Note that the external boot voltage must be lower than 5.5V Figure 2. External Bootstrap Diode 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 RT7237J quiescent current drops to lower than 3 μA. Driving the EN pin high (>2V, <18V) will turn on 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). Soft-Start The RT7237J provides soft-start function. The soft-start function is used to prevent large inrush current while converter is being powered-up. The soft-start timing can be programmed by the external capacitor between SS and GND. An internal current source ISS (6 μA) charges an external capacitor to build a soft-start ramp voltage. The VFB voltage will track the internal ramp voltage during soft- start interval. The typical soft start time is calculated as follows : SS SS SS SS 0.8 C Soft-Start time t = , if C capacitor I 0.8 0.1 is 0.1 F, then soft-start time = 13.5ms 6 μ μ μ × × ≒ 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 4. 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. Figure 3. Enable Timing Control Figure 4. Digital Enable Control Circuit RT7237J GND FB R1 R2 VOUT RT7237J EN GND VIN REN CEN EN RT7237J EN GND 100k VIN REN Q1 EN SW BOOT 5V RT7237J 0.1µF |
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