Datasheet LT3483, LT3483A (Analog Devices) - 7

LT3483/LT3483A
APPLICATIONS INFORMATION
value recommended, as too large of an inductance will to 4.7µF. The following formula is useful to estimate the increase the output voltage ripple without providing much output capacitor value needed: additional output current. L •I 2 SW
Table 1. Recommended Inductors
COUT = –VOUT •∆VOUT
MAX L IDC DCR HEIGHT PART (µH) (mA) (
Ω
) (mm) MANUFACTURER
where ISW = 0.25A (LT3483) or ISW = 0.5A (LT3483A) and LQH2MCN4R7M02L 4.7 300 0.84 0.95 Murata ∆VOUT = 30mV. The flying capacitor in the inverting charge LQH2MCN6R8M02L 6.8 255 1.0 www.murata.com pump configuration ranges from 0.1µF to 0.47µF. Multiply LQH2MCN100M02L 10 225 1.2 the value predicted by the above equation for COUT by 1/10 SDQ12 4.7 1.45 0.40 1.2 Cooper Electronics to determine the value needed for the flying capacitor. Coupled 10 980 0.72 Tech Inductor 15 780 1.15 www.cooperet.com
Table 2. Recommended Ceramic Capacitor Manufacturers
LPD3015 4.7 860 0.52 1.4 Coilcraft Coupled 10 580 1.0 www.coilcraft.com
MANUFACTURER URL
Inductor AVX www.avxcorp.com Kemet www.kemet.com
Capacitor Selection
Murata www.murata.com The small size and low ESR of ceramic capacitors make Taiyo Yuden www.tyuden.com them ideal for LT3483/LT3483A applications. Use of X5R and X7R types is recommended because they retain their
Setting the Output Voltage
capacitance over wider voltage and temperature ranges The output voltage is programmed using one feedback than other dielectric types. Always verify the proper volt- resistor according to the following formula: age rating. Table 2 shows a list of several ceramic capaci- tor manufacturers. Consult the manufacturers for more V R1= – OUT detailed information on their entire selection of ceramic 10µA capacitors. A 4.7µF ceramic bypass capacitor on the V
Inrush Current
IN pin is rec- ommended where the distance to the power supply or When VIN is increased from ground to operating voltage, battery could be more than a couple inches. Otherwise, an inrush current will flow through the input inductor and a 1µF is adequate. integrated Schottky diode to charge the flying capacitor. A capacitor in parallel with feedback resistor R1 is recom- Conditions that increase inrush current include a larger, mended to reduce the output voltage ripple. Use a 5pF more abrupt voltage step at VIN, a larger flying capacitor, capacitor for the inverting charge pump, and a 22pF value and an inductor with a low saturation current. for the inverting flyback or other dual inductor configura- While the internal diode is designed to handle such events, tions. Output voltage ripple can be reduced to 20mV in the inrush current should not be allowed to exceed 1.5A. some cases using this capacitor in combination with an For circuits that use flying capacitors within the recom- appropriately selected output capacitor. mended range and have input voltages less than 5V, inrush The output capacitor is selected based on desired output current remains low, posing no hazard to the device. In voltage ripple. For low output voltage ripple in the inverting cases where there are large steps at VIN, inrush current flyback configuration, use a 4.7µF to 10µF capacitor. The should be measured to ensure operation within the limits inverting charge pump utilizes values ranging from 0.22µF of the device. 3483fc 7 Document Outline Features Description Applications Typical Application Absolute Maximum Ratings Pin Configuration order information Electrical Characteristics Typical Performance Characteristics pin functions block diagram operation applications information typical applications Package Description Revision History Typical Application Related Parts