Datasheet LT8410, LT8410-1 (Analog Devices) - 10

LT8410/LT8410-1
APPLICATIONS INFORMATION
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 to 1.235V. Since the V OUT(NOM) = mA OUT voltage is set by the FBP pin VOUT 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 LT8410 series has an output disconnect PMOS that drop in the PMOS switch to be higher resulting in lower blocks the load from the input during shutdown. The output current capability than predicted by the preceding maximum current through the PMOS is limited by circuitry equations. inside the chip, helping the chip survive output shorts.
Inrush Current SHDN Pin Comparator and Hysteresis Current
When VCC is stepped from ground to the operating voltage An internal comparator compares the SHDN pin voltage while the output capacitor is discharged, a high level of with an internal voltage reference (1.3V) which gives a inrush current may flow through the inductor and Schottky precise turn-on voltage level. The internal hysteresis of this diode into the output capacitor. Conditions that increase turn-on voltage is about 60mV. When the chip is turned on, inrush current include a larger more abrupt voltage step and the SHDN pin voltage is close to this turn-on voltage, at VCC, a larger output capacitor tied to the CAP pin and 0.1μA current flows out of the SHDN pin. This current is an inductor with a low saturation current. While the chip is called SHDN pin hysteresis current, and will go away when designed to handle such events, the inrush current should the chip is off. By connecting the external resistors as in not be allowed to exceed 0.3A. For circuits that use output Figure 2, a user-programmable enable voltage function capacitor values within the recommended range and have can be realized. input voltages of less than 6V, inrush current remains low, posing no hazard to the device. In cases where there are The turn-on voltage for the configuration is: large steps at VCC (more than 6V) and/or a large capacitor   is used at the CAP pin, inrush current should be measured 1.30 •  1+ R1   to ensure safe operation.  R2 and the turn-off voltage is:
Soft-Start
  The LT8410 series contains a soft-start circuit to limit 1.24−R3•10−7 ( )• 1+R1   −(R1•10−7)  R2 peak switch currents during start-up. High start-up cur- rent is inherent in switching regulators, in general, since where R1, R2 and R3 are resistance value in Ω. the feedback loop is saturated due to VOUT being far from its final value. The regulator tries to charge the output ENABLE VOLTAGE capacitor as quickly as possible, which results in large peak current. R1 R3 CONNECT TO When the FBP pin voltage is generated by a resistor divider SHDN PIN from the VREF pin, the start-up current can be limited by R2 connecting an external capacitor (typically 47nF to 220nF) to the VREF pin. When the part is brought out of shutdown, 84101 F02 this capacitor is first discharged for about 70μs (providing
Figure 2. Programming Enable Voltage by Using External Resistors
84101fc 10 For more information www.linear.com/LT8410 Document Outline Description Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Features Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts