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RT8278 Datasheet(PDF) 10 Page - Richtek Technology Corporation |
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RT8278 Datasheet(HTML) 10 Page - Richtek Technology Corporation |
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10 / 16 page  RT8278 10 DS8278-02 March 2011 www.richtek.com Chip Enable Operation The EN pin is the enable input. Pull the EN pin low (<0.4V) to shutdown the device. During shutdown mode, the RT8278 quiescent current drops to lower than 25 μA.Drive the EN pin high (>1.4V, < 5.5V), to turn on the device. If the EN pin is open, it will be pulled high by the internal circuit. For external timing control (e.g.RC), the EN pin can also be externally pulled high by adding a 100k Ω or greater resistor from the VIN pin (see Figure 6). In some cases, the output voltage of the RT8278 may still be under UVP threshold when soft-start finishes. Then the RT8278 will restart again and the output voltage of the RT8278 will rise to the regulation voltage. This phenomenon often happens in high frequency operation and with slow rising input voltage. It can easily be solved by adding a voltage divider on the EN pin. The RT8278 will be enabled when the input voltage rises close to the nominal input voltage. 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 SW L(MAX) IN(MAX) VV L = x 1 f x I V ⎡⎤ ⎡ ⎤ − ⎢⎥ ⎢ ⎥ Δ ⎣⎦ ⎣ ⎦ The inductor's current rating (defined by that which causes a temperature rise from 25 °C ambient to 40°C) should be greater than the maximum load current and its saturation current should be greater than the short circuit peak current limit. Refer to Table 2 for the suggested inductor selection. Figure 5. Limit of Maximum Duty Figure 4. Minimum On-Time Constraint to Input Voltage Minimum On-Time Constraint 0.0 2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0 22.5 25.0 27.5 30.0 1000 1400 1800 2200 2600 3000 Switching Frequency (kHz) VOUT = 3.3V VOUT = 1.8V Figure 3. Switching Frequency vs. RRT Switching Frequency vs. RRT 0 250 500 750 1000 1250 1500 1750 2000 2250 2500 2750 3000 0 200 400 600 800 1000 RRT (kΩ) RRT (k Ω) OUT OUT L SW IN VV I = x 1 f x L V ⎡⎤ ⎡ ⎤ Δ− ⎢⎥ ⎢ ⎥ ⎣⎦ ⎣ ⎦ Having a lower ripple current reduces not only the ESR losses in the output capacitors but also the output voltage ripple. Higher frequency combined with smaller ripple current is necessary to achieve high efficiency operation. However, it requires a large inductor to achieve this goal. For the ripple current selection, setting the maximum value of the ripple current ΔIL= 0.24(IMAX) is 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 : Output Voltage vs. Load Current 2.80 2.85 2.90 2.95 3.00 3.05 3.10 3.15 3.20 3.25 3.30 3.35 3.40 3.45 3.50 0.0 0.4 0.8 1.2 1.6 2.0 Load Current (A) VIN = 5V VIN = 12V fSW = 2.2MHz |
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Similar Description - RT8278 |
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