Datasheet LT1945 (Analog Devices) - 5

LT1945
OPERATION
current in inductors L1 and L2 begins ramping up. Once The second switching regulator is a step-up converter the switch current reaches 350mA, comparator A2 resets (which generates a positive output) but the basic operation the one-shot, which turns off Q3 for 400ns. L2 continues is the same.The LT1945 contains additional circuitry to to deliver current to the output while Q3 is off. Q3 turns on provide protection during start-up and under short-circuit again and the inductor currents ramp back up to 350mA, conditions. When the FB2 pin voltage is less than approxi- then A2 again resets the one-shot. This switching action mately 600mV, the switch off-time is increased to 1.5μs continues until the output voltage is charged up (until the and the current limit is reduced to around 250mA (70% NFB1 pin reaches –1.23V), then A1 turns off the internal of its normal value). This reduces the average inductor circuitry and the cycle repeats. current and helps minimize the power dissipation in the power switch and in the external inductor and diode.
APPLICATIONS INFORMATION Choosing an Inductor
A smaller value can be used (especially for systems with output voltages greater than 12V) to give a smaller physical Several recommended inductors that work well with the size. Inductance can be calculated as: LT1945 are listed in Table 1, although there are many other manufacturers and devices that can be used. Consult each V − V + V OUT IN MIN D ( ) manufacturer for more detailed information and for their L = t I OFF entire selection of related parts. Many different sizes and LIM shapes are available. Use the equations and recommenda- where VD = 0.4V (Schottky diode voltage), ILIM = 350mA tions in the next few sections to fi nd the correct inductance and tOFF = 400ns; for designs with varying VIN such as value for your design. battery powered applications, use the minimum VIN value in the above equation. For most regulators with output
Table 1. Recommended Inductors
voltages below 7V, a 4.7μH inductor is the best choice,
PART VALUE (μH) MAX DCR (Ω) VENDOR
even though the equation above might specify a smaller LQH3C4R7 4.7 0.26 Murata LQH3C100 10 0.30 (714) 852-2001 value. This is due to the inductor current overshoot that LQH3C220 22 0.92 www.murata.com occurs when very small inductor values are used (see CD43-4R7 4.7 0.11 Sumida Current Limit Overshoot section). CD43-100 10 0.18 (847) 956-0666 CDRH4D18-4R7 4.7 0.16 www.sumida.com For higher output voltages, the formula above will give large CDRH4D18-100 10 0.20 inductance values. For a 2V to 20V converter (typical LCD DO1608-472 4.7 0.09 Coilcraft Bias application), a 21μH inductor is called for with the DO1608-103 10 0.16 (847) 639-6400 DO1608-223 22 0.37 www.coilcraft.com above equation, but a 10μH inductor could be used without excessive reduction in maximum output current.
Inductor Selection—Boost Regulator
The formula below calculates the appropriate inductor value
Inductor Selection—SEPIC Regulator
to be used for a boost regulator using the LT1945 (or at The formula below calculates the approximate inductor least provides a good starting point). This value provides value to be used for a SEPIC regulator using the LT1945. a good tradeoff in inductor size and system performance. As for the boost inductor selection, a larger or smaller Pick a standard inductor close to this value. A larger value value can be used. can be used to slightly increase the available output current, but limit it to around twice the value calculated below, as ⎛ V + V ⎞ L OUT D = 2 t too large of an inductance will increase the output voltage OFF ⎝⎜ ILIM ⎠⎟ ripple without providing much additional output current. 1945fa 5