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LT3473 Datasheet(PDF) 9 Page - Linear Technology |
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LT3473 Datasheet(HTML) 9 Page - Linear Technology |
9 / 12 page 9 LT3473/LT3473A 3473f Inrush Current The LT3473 has an integrated Schottky power diode. When supply voltage is abruptly applied to the VIN pin while the output capacitor is discharged, the voltage difference between VIN and CAP generates inrush current flowing from the input through the inductor and the internal Schottky diode to charge the output capacitor at the CAP pin. The maximum current the LT3473’s Schottky can sustain is 2A. The selection of inductor and capacitor values should ensure that the peak inrush current is less than 2A. Peak inrush current can be calculated as follows: I V L r L LC r L P IN = ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ = + = –. • • exp – • arctan • sin arctan . • • – • 06 15 2 1 4 2 ω α ω ω α ω α α ω where L is the inductance, r is the resistance of the inductor and C is the output capacitance. For a low DCR inductor, which is usually the case for this application, the peak inrush current can be simplified as follows: I V L P IN = ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ –. • • exp – • 06 2 ω α ω π A large abrupt voltage step at VIN and/or a large capacitor at the CAP pin generate larger inrush current. Table 3 gives inrush peak currents for some component selections. An inductor with low saturation current could generate very large inrush current. For this case, inrush current should be measured to ensure safe operation. Note that inrush current is not a concern if the input voltage rises slowly. Table 3. Inrush Peak Current VIN (V) R (Ω)L (µH) C (µF) IP (A) 5 0.05 6.8 0.47 0.86 10 0.05 6.8 0.47 1.83 3.6 0.05 6.8 0.47 0.58 3.6 0.05 4.7 0.47 0.67 Setting the Output Voltages The LT3473 has both an internal 1.25V reference and an external reference input. This allows the user to select between using the built-in reference and supplying an external reference voltage. The voltage at the CTRL pin can be adjusted while the device is operating to alter the output voltage for purposes such as display dimming or contrast adjustment. To use the internal 1.25V reference, the CTRL pin must be held higher than 1.5V. When the CTRL pin is held between 0V and 1.2V, the LT3473 will regulate the output such that the FB pin voltage is equal to the CTRL pin voltage. The CAP pin should be used as the feedback node. To set the output voltage, select the values of R1 and R2 accord- ing to the following equation. VV R R INT REF =+ ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ •1 2 1 where VREF = 1.25V if the internal reference is used, or VREF = VCTRL if VCTRL is between 0V and 1.2V. To maintain output voltage accuracy, 1% resistors are recommended. Soft-Start The SHDN pin also functions as soft-start. Use an RC filter at the SHDN pin to limit the start-up current. The small bias current of the SHDN pin allows using a small capacitor for a large RC time constant. APPLICATIO S I FOR ATIO Figure 4. Soft-Start Circuitry SHDN 100nF ON/OFF 3473 F04 20k LT3473 Output Disconnect Considerations The LT3473 has an output disconnect PNP that isolates the load from the input during shutdown. The drive circuit maintains the PNP at the edge of saturation, adaptively according to the load, thus yielding the best compromise between VCESAT and quiescent current to minimize power loss. To remain stable, it requires a bypass capacitor connected between the OUT pin and the CAP pin or |
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