Datasheet LT3473, LT3473A (Analog Devices) - 9

LT3473/LT3473A
U U W U APPLICATIO S I FOR ATIO Inrush Current Setting the Output Voltages
The LT3473 has an integrated Schottky power diode. The LT3473 has both an internal 1.25V reference and an When supply voltage is abruptly applied to the VIN pin external reference input. This allows the user to select while the output capacitor is discharged, the voltage between using the built-in reference and supplying an difference between VIN and CAP generates inrush current external reference voltage. The voltage at the CTRL pin can flowing from the input through the inductor and the be adjusted while the device is operating to alter the output internal Schottky diode to charge the output capacitor at voltage for purposes such as display dimming or contrast the CAP pin. The maximum current the LT3473’s Schottky adjustment. To use the internal 1.25V reference, the CTRL can sustain is 2A. The selection of inductor and capacitor pin must be held higher than 1.5V. When the CTRL pin is values should ensure that the peak inrush current is less held between 0V and 1.2V, the LT3473 will regulate the than 2A. Peak inrush current can be calculated as follows: output such that the FB pin voltage is equal to the CTRL pin voltage. V – . 0 6 ⎛ α ⎛ ω ⎞⎞ ⎛ ⎛ ω ⎞⎞ I IN The CAP pin should be used as the feedback node. To set P = • exp⎜– •arctan⎜ ⎟⎟ •sin⎜arctan⎜ ⎟⎟ L • ω ⎝ ω ⎝ α ⎠⎠ ⎝ ⎝ α ⎠⎠ the output voltage, select the values of R1 and R2 accord- r ing to the following equation. + . 1 5 α = 2•L ⎛ R2⎞ V = V INT REF • ⎜1+ ⎟ 1 r ⎝ R ⎠ 1 ω = – L • C 4 • L2 where VREF = 1.25V if the internal reference is used, or V where L is the inductance, r is the resistance of the REF = VCTRL if VCTRL is between 0V and 1.2V. inductor and C is the output capacitance. For a low DCR To maintain output voltage accuracy, 1% resistors are inductor, which is usually the case for this application, the recommended. peak inrush current can be simplified as follows:
Soft-Start
V – . 0 6 ⎛ α π⎞ I IN The SHDN pin also functions as soft-start. Use an RC filter P = • ⎜ exp – • ⎟ L • ω ⎝ ω 2 ⎠ 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 abrupt voltage step at VIN and/or a large capacitor a large RC time constant. at the CAP pin generate larger inrush current. Table 3 gives inrush peak currents for some component selections. An LT3473 20k inductor with low saturation current could generate very ON/OFF SHDN large inrush current. For this case, inrush current should 100nF be measured to ensure safe operation. Note that inrush 3473 F04 current is not a concern if the input voltage rises slowly.
Figure 4. Soft-Start Circuitry Table 3. Inrush Peak Current Output Disconnect Considerations VIN (V) R (
Ω
) L (
µ
H) C (
µ
F) IP (A)
5 0.05 6.8 0.47 0.86 The LT3473 has an output disconnect PNP that isolates the load from the input during shutdown. The drive circuit 10 0.05 6.8 0.47 1.83 maintains the PNP at the edge of saturation, adaptively 3.6 0.05 6.8 0.47 0.58 according to the load, thus yielding the best compromise 3.6 0.05 4.7 0.47 0.67 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 3473f 9