Datasheet LT1615, LT1615-1 (Analog Devices) - 5

LT1615/LT1615-1
U U W U APPLICATIO S I FOR ATIO Choosing an Inductor
VIN value in the above equation. For most systems with output voltages below 7V, a 4.7µH inductor is the best Several recommended inductors that work well with the choice, even though the equation above might specify a LT1615 and LT1615-1 are listed in Table 1, although there smaller value. This is due to the inductor current over- are many other manufacturers and devices that can be shoot that occurs when very small inductor values are used. Consult each manufacturer for more detailed infor- used (see Current Limit Overshoot section). mation and for their entire selection of related parts. Many different sizes and shapes are available. Use the equations For higher output voltages, the formula above will give and recommendations in the next few sections to find the large inductance values. For a 2V to 20V converter (typical correct inductance value for your design. LCD Bias application), a 21µH inductor is called for with
Table 1. Recommended Inductors
the above equation, but a 10µH inductor could be used without excessive reduction in maximum output current.
PART VALUE (
µ
H) MAX DCR (
Ω
) VENDOR
LQH3C4R7 4.7 0.26 Murata
Inductor Selection—SEPIC Regulator
LQH3C100 10 0.30 (814) 237-1431 LQH3C220 22 0.92 www.murata.com The formula below calculates the approximate inductor CD43-4R7 4.7 0.11 Sumida value to be used for a SEPIC regulator using the LT1615. CD43-100 10 0.18 (847) 956-0666 As for the boost inductor selection, a larger or smaller CDRH4D18-4R7 4.7 0.16 www.sumida.com CDRH4D18-100 10 0.20 value can be used. DO1608-472 4.7 0.09 Coilcraft DO1608-103 10 0.16 (847) 639-6400  V V  OUT D DO1608-223 22 0.37 www.coilcraft.com L = + 2 tOFF  ILIM 
Inductor Selection—Boost Regulator Current Limit Overshoot
The formula below calculates the appropriate inductor value to be used for a boost regulator using the LT1615 or For the constant off-time control scheme of the LT1615, LT1615-1 (or at least provides a good starting point). This the power switch is turned off only after the 350mA (or value provides a good tradeoff in inductor size and system 100mA) current limit is reached. There is a 100ns delay performance. Pick a standard inductor close to this value. between the time when the current limit is reached and A larger value can be used to slightly increase the available when the switch actually turns off. During this delay, the output current, but limit it to around twice the value inductor current exceeds the current limit by a small calculated below, as too large of an inductance will in- amount. The peak inductor current can be calculated by: crease the output voltage ripple without providing much additional output current. A smaller value can be used  V ( ) − V  IN MAX SAT = + (especially for systems with output voltages greater than I I ns PEAK LIM  100  L  12V) to give a smaller physical size. Inductance can be calculated as: Where VSAT = 0.25V (switch saturation voltage). The current overshoot will be most evident for systems with V − V + V OUT IN MIN D ( ) high input voltages and for systems where smaller induc- L = tOFF tor values are used. This overshoot can be beneficial as it ILIM helps increase the amount of available output current for where VD = 0.4V (Schottky diode voltage), ILIM = 350mA or smaller inductor values. This will be the peak current seen 100mA, and tOFF = 400ns; for designs with varying VIN by the inductor (and the diode) during normal operation. such as battery powered applications, use the minimum For designs using small inductance values (especially at sn16151 16151fas 5