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LTC3400BES6 Datasheet(PDF) 6 Page - Linear Technology |
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LTC3400BES6 Datasheet(HTML) 6 Page - Linear Technology |
6 / 12 page LTC3400/LTC3400B 6 3400f PCB LAYOUT GUIDELINES The high speed operation of the LTC3400/LTC3400B demands careful attention to board layout. You will not get advertised performance with careless layout. Figure 2 shows the recommended component placement. A large ground pin copper area will help to lower the chip tempera- ture. A multilayer board with a separate ground plane is ideal, but not absolutely necessary. APPLICATIO S I FOR ATIO II VD fL D OUT MAX P IN () •– • •• •– = () η 2 1 where: η = estimated efficiency IP = peak current limit value (0.6A) VIN = input (battery) voltage D = steady-state duty ratio = (VOUT – VIN)/VOUT f = switching frequency (1.2MHz typical) L = inductance value The inductor current ripple is typically set for 20% to 40% of the maximum inductor current (IP). High frequency ferrite core inductor materials reduce frequency depen- dent power losses compared to cheaper powdered iron types, improving efficiency. The inductor should have low ESR (series resistance of the windings) to reduce the I2R power losses, and must be able to handle the peak inductor current without saturating. Molded chokes and some chip inductors usually do not have enough core to support the peak inductor currents of 850mA seen on the LTC3400/LTC3400B. To minimize radiated noise, use a toroid, pot core or shielded bobbin inductor. See Table 1 for some suggested components and suppliers. SW GND FB 1 2 3 6 5 4 VIN VOUT SHDN SHDN (OPTIONAL) 3400 F02 VOUT VIN RECOMMENDED COMPONENT PLACEMENT. TRACES CARRYING HIGH CURRENT ARE DIRECT. TRACE AREA AT FB PIN IS SMALL. LEAD LENGTH TO BATTERY IS SHORT Figure 2. Recommended Component Placement for Single Layer Board COMPONENT SELECTION Inductor Selection The LTC3400/LTC3400B can utilize small surface mount and chip inductors due to their fast 1.2MHz switching frequency. A minimum inductance value of 3.3 µH is necessary for 3.6V and lower voltage applications and 4.7 µH for output voltages greater than 3.6V. Larger values of inductance will allow greater output current capability by reducing the inductor ripple current. Increasing the inductance above 10 µH will increase size while providing little improvement in output current capability. The approximate output current capability of the LTC3400/ LTC3400B versus inductance value is given in the equa- tion below and illustrated graphically in Figure 3. INDUCTANCE ( µH) 3 60 80 110 120 160 7 11 13 21 180 140 59 15 17 19 23 3400 F03 VIN =1.2V VOUT = 3V VOUT = 3.3V VOUT = 3.6V VOUT = 5V Figure 3. Maximum Output Current vs Inductance Based On 90% Efficiency |
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