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C3225X5R1C22M Datasheet(PDF) 11 Page - Richtek Technology Corporation |
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C3225X5R1C22M Datasheet(HTML) 11 Page - Richtek Technology Corporation |
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11 / 15 page  RT7272B 11 DS7272B-01 January 2013 www.richtek.com Copyright 2013 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation. © 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 the highest efficiency operation. However, it requires a large inductor to achieve this goal. For the ripple current selection, the value of ΔIL= 0.24(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 : Table 2. Suggested Inductors for Typical Application Circuit Component Supplier Series Dimensions (mm) TDK VLF10045 10 x 9.7 x 4.5 TDK SLF12565 12.5 x 12.5 x 6.5 TAIYO YUDEN NR8040 8 x 8 x 4 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. Figure 5. Hiccup Mode Under Voltage Protection Time (50ms/Div) Hiccup Mode VOUT (2V/Div) ILX (2A/Div) IOUT = Short Over Temperature Protection The RT7272B features an Over Temperature Protection (OTP) circuitry to prevent from overheating due to excessive power dissipation. The OTP will shut down switching operation when junction temperature exceeds 150 °C. Once the junction temperature cools down by approximately 20 °C, the converter will resume operation. To maintain continuous operation, the maximum junction temperature should be lower than 125 °C. 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 IN RMS OUT(MAX) IN OUT V V I = I 1 VV − CIN and COUT Selection The input capacitance, CIN, is needed to filter the trapezoidal current at the Source of the high side MOSFET. To prevent large ripple current, a low ESR input capacitor sized for the maximum RMS current should be used. The approximate RMS current equation is given : This formula has a maximum at VIN = 2VOUT, where IRMS = IOUT / 2. This simple worst case condition is commonly used for design because even significant deviations do not offer much relief. Choose a capacitor rated at a higher temperature than required. Several capacitors may also be paralleled to meet size or height requirements in the design. For the input capacitor, two 10 μF low ESR ceramic capacitors are suggested. For the suggested capacitor, please refer to Table 3 for more details. The selection of COUT is determined by the required ESR to minimize voltage ripple. Moreover, the amount of bulk capacitance is also a key for COUT selection to ensure that the control loop is stable. Loop stability can be checked by viewing the load transient response as described in a later section. The output ripple, ΔVOUT , is determined by : OUT L OUT 1 VI ESR 8fC ⎡⎤ Δ≤ Δ + ⎢⎥ ⎣⎦ |
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