Datasheet LT3463, LT3463A (Analog Devices) - 6

LT3463/LT3463A
U U W U APPLICATIO S I FOR ATIO
output voltages from a high input voltage source will often inrush current include a larger more abrupt voltage step at exceed the 50V maximum switch rating. For instance, a VIN, a larger output capacitor tied to the outputs, and an 12V to – 40V converter using the inverting topology would inductor with a low saturation current. generate 52V on the SW pin, exceeding its maximum While the internal diode is designed to handle such events, rating. For this application, an inverting charge pump is the inrush current should not be allowed to exceed 1 amp. the best topology. For circuits that use output capacitor values within the The formula below calculates the approximate inductor recommended range and have input voltages of less than value to be used for an inverting charge pump regulator 5V, inrush current remains low, posing no hazard to the using the LT3463. As for the boost inductor selection, a device. In cases where there are large steps at VIN and/or larger or smaller value can be used. For designs with a large capacitor is used at the outputs, inrush current varying V should be measured to ensure safe operation. IN such as battery powered applications, use the minimum VIN value in the equation below.
Setting the Output Voltages
V − V + V OUT IN MIN D The output voltages are programmed using two feedback ( ) L = tOFF resistors. As shown in Figure 1, resistors R1 and R2 ILIM program the positive output voltage (for Switcher 1), and resistors R3 and R4 program the negative output voltage
Capacitor Selection
(for Switcher 2) according to the following formulas: The small size and low ESR of ceramic capacitors makes them ideal for LT3463 applications. Use only X5R and X7R  R2 types because they retain their capacitance over wider V = 1 2 . V OUT1 5 1+  R1 voltage and temperature ranges than other ceramic types. A 1µF input capacitor and a 0.22µF or 0.47µF output R4 V = 1 – 2 . V OUT2 5 capacitor are sufficient for most applications. Table 2 R3 shows a list of several ceramic capacitor manufacturers. Consult the manufacturers for more detailed information R1 and R3 are typically 1% resistors with values in the on their entire selection of ceramic capacitors. For appli- range of 50k to 250k. cations needing very low output voltage ripple, larger output capacitor values can be used.
Board Layout Considerations Table 2. Recommended Ceramic Capacitor Manufacturers
As with all switching regulators, careful attention must be
MANUFACTURER PHONE URL
paid to the PCB board layout and component placement. AVX 843-448-9411 www.avxcorp.com To maximize efficiency, switch rise and fall times are made Kemet 408-986-0424 www.kemet.com as short as possible. To prevent electromagnetic interfer- Murata 814-237-1431 www.murata.com ence (EMI) problems, proper layout of the high frequency switching path is essential. The voltage signal of the SW Taiyo Yuden 408-573-4150 www.t-yuden.com pin has sharp rising and falling edges. Minimize the length and area of all traces connected to the SW pin and always
Inrush Current
use a ground plane under the switching regulator to When V minimize interplane coupling. In addition, the ground IN is increased from ground to operating voltage while the output capacitor is discharged, an inrush current connection for the feedback resistor R1 should be tied will flow through the inductor and integrated Schottky directly to the GND pin and not shared with any other diode into the output capacitor. Conditions that increase component, ensuring a clean, noise-free connection. 3463f 6