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LT3495B Datasheet(PDF) 11 Page - Linear Technology |
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LT3495B Datasheet(HTML) 11 Page - Linear Technology |
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11 / 16 page  LT3495/LT3495B/ LT3495-1/LT3495B-1 11 3495b1b1fa APPLICATIONS INFORMATION Maximum Output Load Current The maximum output current of a particular LT3495 series circuit is a function of several circuit variables. The fol- lowing method can be helpful in predicting the maximum load current for a given circuit: Step 1: Calculate the peak inductor current: IPK =ILIMIT + VIN •200•10–9 L amps where ILIMIT is 0.65A and 0.35A for LT3495/LT3495B and LT3495-1/LT3495B-1 respectively. L is the inductance value in Henrys and VIN is the input voltage to the boost circuit. Step 2: Calculate the inductor ripple current: IRIPPLE = VOUT +1– VIN ( )•200•10–9 L amps where VOUT is the desired output voltage. If the inductor ripple current is greater than the peak current, then the circuitwillonlyoperateindiscontinuousconductionmode. TheinductorvalueshouldbeincreasedsothatIRIPPLE<IPK. An application circuit can be designed to operate only in discontinuous mode, but the output current capability will be reduced. Step 3: Calculate the average input current: IIN(AVG) =IPK – IRIPPLE 2 amps Step 4: Calculate the nominal output current: IOUT(NOM) = IIN(AVG) • VIN •0.8 VOUT amps Step 5: Derate output current: IOUT = IOUT(NOM)•0.8amps For low output voltages the output current capability will be increased. When using output disconnect (load cur- rent taken from VOUT), these higher currents will cause the drop in the PMOS switch to be higher resulting in reduced output current capability than those predicted by the preceding equations. Inrush Current When VCC is stepped from ground to the operating voltage while the output capacitor is discharged, a higher level of inrushcurrentmayflowthroughtheinductorandSchottky diode into the output capacitor. Conditions that increase inrush current include a larger more abrupt voltage step at VIN, a larger output capacitor tied to the CAP pin and an inductor with a low saturation current. While the chip is designed to handle such events, the inrush current should not be allowed to exceed 1.5A. For circuits that use output capacitor values within the recommended range and have input voltages of less than 5V, inrush current remains low, posing no hazard to the device. In cases where there are large steps at VCC (more than 5V) and/or a large capacitor is used at the CAP pin, inrush current should be measured to ensure safe operation. Soft-Start By connecting the SHDN and CTRL pins as shown in Figure 4, using an RC filter at the CTRL pin to limit the start-up current, the LT3495 is able to achieve soft-start. The small bias current of the CTRL pin allows using a small capacitor for a large RC time constant. The soft- start waveform is shown in Figure 5. The soft-start time Figure 5. Soft-Start Waveform Figure 4. Soft-Start Circuitry SW CAP VCC SHDN CTRL VOUT FB GND 3495 F04 LT3495 CCTRL RCTRL CHIP ENABLE 500µs/DIV 3495 F05 SHDN VOLTAGE 5V/DIV INDUCTOR CURRENT 500mA/DIV VOUT VOLTAGE 5V/DIV CTRL VOLTAGE 2V/DIV VCC = 3.6V VOUT = 16V |
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