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LTC1474CMS8 Datasheet(PDF) 7 Page - Linear Technology |
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LTC1474CMS8 Datasheet(HTML) 7 Page - Linear Technology |
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7 / 20 page  7 LTC1474/LTC1475 APPLICATIONS INFORMATION ments. Lower peak currents have the advantage of lower output ripple ( ∆VOUT = IPEAK • ESR), lower noise, and less stress on alkaline batteries and other circuit components. Also, lower peak currents allow the use of inductors with smaller physical size. Peak currents as low as 10mA can be programmed with the appropriate sense resistor. Increasing RSENSE above 10 Ω, however, gives no further reduction of IPEAK. For RSENSE values less than 1Ω, it is recommended that the user parallel standard resistors (available in values ≥ 1 Ω) instead of using a special low valued shunt resistor. Although a single resisor could be used with the desired value, these low valued shunt resistor types are much more expensive and are currently not available in case sizes smaller than 1206. Three or four 0603 size standard resistors require about the same area as one 1206 size current shunt resistor at a fraction of the cost. At higher supply voltages and lower inductances, the peak currents may be slightly higher due to current comparator overshoot and can be predicted from the second term in the following equation: I R VV L PEAK SENSE IN OUT = + + () − () − 01 025 25 10 7 . . . (2) Note that RSENSE only sets the maximum inductor current peak. At lower dI/dt (lower input voltages and higher inductances), the observed peak current at loads less than IMAX may be less than this calculated peak value due to the voltage comparator tripping before the current ramps up high enough to trip the current comparator. This effect improves efficiency at lower loads by keeping the I2R losses down (see Efficiency Considerations section). Inductor Value Selection Once RSENSE and IPEAK are known, the inductor value can be determined. The minimum inductance recommended as a function of IPEAK and IMAX can be calculated from: L VV t II MIN OUT D OFF PEAK MAX ≥ + () − 075 . (3) where tOFF = 4.75µs. MAXIMUM OUTPUT CURRENT (mA) 0 5 4 3 2 1 0 50 100 150 200 1474/75 F02 250 300 FOR LOWEST NOISE FOR BEST EFFICIENCY Figure 2. RSENSE Selection The basic LTC1474/LTC1475 application circuit is shown in Figure 1, a high efficiency step-down converter. External component selection is driven by the load requirement and begins with the selection of RSENSE. Once RSENSE is known, L can be chosen. Finally D1, CIN and COUT are selected. RSENSE Selection The current sense resistor (RSENSE) allows the user to program the maximum inductor/switch current to opti- mize the inductor size for the maximum load. The LTC1474/ LTC1475 current comparator has a maximum threshold of 100mV/(RSENSE + 0.25). The maximum average output current IMAX is equal to this peak value less half the peak- to-peak ripple current ∆IL. Allowing a margin for variations in the LTC1474/LTC1475 and external components, the required RSENSE can be calculated from Figure 2 and the following equation: RSENSE = (0.067/IMAX) – 0.25 (1) for 10mA < IMAX < 200mA. For IMAX above 200mA, RSENSE is set to zero by shorting Pins 6 and 7 to provide the maximum peak current of 400mA (limited by the fixed internal sense resistor). This 400mA default peak current can be used for lower IMAX if desired to eliminate the need for the sense resistor and associated decoupling capacitor. However, for optimal performance, the peak inductor current should be set to no more than what is needed to meet the load current require- |
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