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GRM31CR61E106K Datasheet(PDF) 10 Page - Richtek Technology Corporation |
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GRM31CR61E106K Datasheet(HTML) 10 Page - Richtek Technology Corporation |
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10 / 14 page  RT8252A 10 DS8252A-02 March 2011 www.richtek.com 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= 0.24(IMAX) will be a reasonable starting point. The largest ripple current occurs at the highest VIN. To guarantee that the 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. 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 IN RMS OUT(MAX) IN OUT V V I = I 1 VV − 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, one 10 μF low ESR ceramic capacitors are recommended. For the recommended capacitor, please refer to Table 3 for more detail. 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 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 RMS current is given by : OUT OUT L IN VV I = 1 fL V ⎡⎤ ⎡ ⎤ Δ× − ⎢⎥ ⎢ ⎥ × ⎣⎦ ⎣ ⎦ Hiccup Mode For the RT8252AH, it provides Hiccup Mode Under Voltage Protection (UVP). When the FB voltage drops below half of the feedback reference voltage, VFB, the UVP function will be triggered and the RT8252AH will shut down for a period of time and then recover automatically. The Hiccup Mode UVP can reduce input current in short circuit conditions. Latch-Off Mode For the RT8252AL, it provides Latch-Off Mode Under Voltage Protection (UVP). When the FB voltage drops below half of the feedback reference voltage, VFB, UVP will be triggered and the RT8252AL will shutdown in Latch- Off Mode. In shutdown condition, the RT8252AL can be reset by the EN pin or power input VIN. 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. Figure 4. The Resistors can be Selected to Set IC Lockout Threshold. ripple current stays below the specified maximum, the inductor value should be chosen according to the following equation : VIN EN GND BOOT FB SW 7 5 2 3 1 L R1 R2 VOUT VIN RT8252A SS 8 CSS COMP CC RC CP 6 CBOOT COUT CIN 100k 8V 12V REN2 REN1 10µF 4, 9 (Exposed Pad) |
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