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NR10050 Datasheet(PDF) 10 Page - Richtek Technology Corporation |
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NR10050 Datasheet(HTML) 10 Page - Richtek Technology Corporation |
10 / 15 page RT2805A 10 DS2805A-00 November 2012 www.richtek.com © Copyright 2012 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation. Application Information The RT2805A is an asynchronous high voltage buck converter that can support the input voltage range from 5.5V to 32V and the output current can be up to 5A. 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 divider according to the following equation : OUT REF R1 V = V 1 R2 ⎛⎞ + ⎜⎟ ⎝⎠ Where VREF is the reference voltage (1.222V typ.). Where R1 = 100k Ω. 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 RT2805A. Figure 2. External Bootstrap Diode Soft-Start The RT2805A contains an internal soft-start clamp that gradually raises the output voltage. The typical soft-start time is 5ms. Chip Enable Operation The EN pin is the chip enable input. Pull the EN pin low (<0.4V) will shutdown the device. During shutdown mode, the RT2805A quiescent current drops to lower than 25 μA. Drive the EN pin to high (>1.4V, <5.5V) will turn on the device again. If the EN pin is open, it will be pulled to high by internal circuit. For external timing control (e.g.RC), the EN pin can also be externally pulled to High by adding a 100k Ω or greater resistor from the VIN pin (see Figure 3). Inductor Selection The inductor value and operating frequency determine the ripple current according to a specific input and output voltage. The ripple current ΔIL increases with higher VIN and decreases with higher inductance. OUT OUT L IN VV I = 1 fL V ⎡⎤ ⎡ ⎤ Δ× − ⎢⎥ ⎢ ⎥ × ⎣⎦ ⎣ ⎦ Having a lower ripple current reduces not only the ESR losses in the output capacitors but also the output voltage ripple. High frequency with small ripple current can achieve highest efficiency operation. However, it requires a large inductor to achieve this goal. For the ripple current selection, the value of ΔIL= 0.2(IMAX) will be a reasonable starting point. The largest ripple current occurs at the highest VIN. To guarantee that the ripple current stays below the specified maximum, the inductor value should be chosen according to the following equation : OUT OUT L(MAX) IN(MAX) VV L = 1 fI V ⎡⎤ ⎡ ⎤ ×− ⎢⎥ ⎢ ⎥ ×Δ ⎣⎦ ⎣ ⎦ The inductor's current rating (caused a 40 °C temperature rising from 25 °C ambient) should be greater than the maximum load current and its saturation current should be greater than the short circuit peak current limit. Please see Table 2 for the inductor selection reference. RT2805A GND FB R1 R2 VOUT SW BOOT 5V RT2805A 10nF |
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