Datasheet LT8415 (Analog Devices) - 9

LT8415
APPLICATIONS INFORMATION Maximum Output Load Current Inrush Current
The maximum output current of a particular LT8415 circuit When VCC is stepped from ground to the operating voltage is a function of several circuit variables. The following while the output capacitor is discharged, a high level of method can be helpful in predicting the maximum load inrush current may flow through the inductor and Schottky current for a given circuit: diode into the output capacitor. Conditions that increase Step 1: Calculate the peak inductor current: inrush current include a larger more abrupt voltage step at VCC, a larger output capacitor tied to the CAP pin and an inductor with a low saturation current. While the chip is I VIN •150 •10−6 PK = ILIMIT + mA designed to handle such events, the inrush current should L not be allowed to exceed 0.3A. For circuits that use output where I capacitor values within the recommended range and have LIMIT is 25mA. L is the inductance value in Henrys and V input voltages of less than 6V, inrush current remains low, IN is the input voltage to the boost circuit. posing no hazard to the device. In cases where there are Step 2: Calculate the inductor ripple current: large steps at VCC (more than 6V) and/or a large capacitor is used at the CAP pin, inrush current should be measured I (VOUT + 1− VIN)•200•10−6 to ensure safe operation. RIPPLE = mA L
Soft-Start
where VOUT is the desired output voltage. If the inductor ripple current is less than the peak current, then the circuit The LT8415 contains a soft-start circuit to limit peak switch will only operate in discontinuous conduction mode. The currents during start-up. High start-up current is inherent inductor value should be increased so that I in switching regulators in general since the feedback loop RIPPLE < IPK. An application circuit can be designed to operate only in is saturated due to VOUT being far from its final value. The discontinuous mode, but the output current capability regulator tries to charge the output capacitor as quickly will be reduced. as possible, which results in large peak current. Step 3: Calculate the average input current: When the FBP pin voltage is generated by a resistor divider from the VREF pin, the start-up current can be limited by I IRIPPLE connecting an external capacitor (typically 47nF to 220nF) IN(AVG) = IPK − mA 2 to the VREF pin. When the part is brought out of shutdown, this capacitor is first discharged for about 70μs (providing Step 4: Calculate the nominal output current: protection against pin glitches and slow ramping), then I an internal 10μA current source pulls the VREF pin slowly I IN(AVG) • VIN • 0.7 OUT(NOM) = mA V to 1.235V. Since the VOUT voltage is set by the FBP pin OUT voltage, the VOUT voltage will also slowly increase to the Step 5: Derate output current: regulated voltage, which results in lower peak inductor current. The voltage ramp rate on the pin can be set by IOUT = IOUT(NOM) • 0.8 the value of the VREF pin capacitor. For low output voltages the output current capability will be increased. When using output disconnect (load current
Output Disconnect
taken from VOUT), these higher currents will cause the The LT8415 has an output disconnect PMOS that blocks drop in the PMOS switch to be higher resulting in lower the load from the input during shutdown. The maximum output current capability than predicted by the preceding current through the PMOS is limited to 19mA by circuitry equations. inside the chip, helping the chip survive output shorts. 8415fb For more information www.linear.com/LT8415 9