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VLF10045 Datasheet(PDF) 10 Page - Richtek Technology Corporation |
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VLF10045 Datasheet(HTML) 10 Page - Richtek Technology Corporation |
10 / 15 page RT7272B 10 DS7272B-01 January 2013 www.richtek.com Copyright 2013 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation. © Under Voltage Protection Hiccup Mode The RT7272B provides Hiccup Mode Under Voltage Protection (UVP). When the VFB voltage drops below 0.4V, the UVP function will be triggered to shut down switching operation. If the UVP condition remains for a period, the RT7272B will retry automatically. When the UVP condition is removed, the converter will resume operation. The UVP is disabled during soft-start period. 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 and SW pins. 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 RT7272B. 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 shutdown the device. During shutdown mode, the RT7272B quiescent current drops to lower than 3 μA. Driving the EN pin high (>2.5V, < 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). 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 RT7272B GND FB R1 R2 VOUT SW BOOT 5V RT7272B 100nF RT7272B EN GND VIN REN CEN EN RT7272B EN GND 100k VIN REN Q1 EN |
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