Datasheet LT3464 (Analog Devices) - 10

LT3464
U U W U APPLICATIO S I FOR ATIO
300 7 6 7 6 250 VIN SW V SW 3 IN 3 OUT V 200 OUT OUT VOUT 1 1 CTRL CTRL (SAT) 5 5 150 LT3464 CAP R2 LT3464 CAP 8 8 SHDN SHDN R2 V CAP-OUT 100 2 2 FB FB GND 50 R1 GND R1 4 4 0 0 5 10 15 20 25 30 3464 F01 COLLECTOR CURRENT (mA) 3464 F02
Figure 4. Feedback Connection Using the CAP Pin and the OUT Pin Figure 5: Output Disconnect Voltage Drop (VDROP) vs Current Output Disconnect Considerations
indefinite short, but care must be taken to avoid exceeding The LT3464 is equipped with an output disconnect that the maximum junction temperature. isolates the load from the input during shutdown. See the
Inrush Current
Operation section for a functional diagram. The output disconnect uses a pass PNP coupled with circuitry that When VIN is stepped from ground to operating voltage varies the base current such that the transistor is consis- while the output capacitor is discharged, an inrush current tently at the edge of saturation, thus yielding the best will flow through the inductor and integrated Schottky compromise between V diode into the output capacitor. Conditions that increase CE(SAT) and low quiescent current. To remain stable, this circuit requires a bypass capacitor inrush current include a larger more abrupt voltage step at connected between the OUT pin and the CAP pin or VIN, a larger output capacitor tied to the CAP pin, and an between the OUT pin and ground. A ceramic capacitor with inductor with a low saturation current. a value of at least 0.1µF is a good choice. While the internal diode is designed to handle such events, The PNP V the inrush current should not be allowed to exceed 1 amp. CE(SAT) varies with load current as shown in Figure 5. This voltage drop (V For circuits that use output capacitor values within the DROP) can be accounted for when using the CAP pin as the feedback node by setting recommended range and have input voltages of less than the output voltage according to the following formula: 5V, inrush current remains low, posing no hazard to the device. In cases where there are large steps at V IN  V (more than 5V) and/or a large capacitor is used at the CAP OUT + V  R2 = R DROP 1 – 1 pin, inrush current should be measured to ensure safe  VREF  operation. In addition, the disconnect circuit has a built in current limit of 25mA (minimum) to protect the chip during short- circuit. This feature allows the LT3464 to tolerate an 3464f 10