Datasheet LT1944-1 (Analog Devices) - 5

LT1944-1
U U W U APPLICATIO S I FOR ATIO Choosing an Inductor
systems with output voltages below 7V, a 10µH inductor is the best choice, even though the equation above might Several recommended inductors that work well with the specify a smaller value. This is due to the inductor current LT1944-1 are listed in Table 1, although there are many overshoot that occurs when very small inductor values are other manufacturers and devices that can be used. Con- used (see Current Limit Overshoot section). sult each manufacturer for more detailed information and for their entire selection of related parts. Many different For higher output voltages, the formula above will give sizes and shapes are available. Use the equations and large inductance values. For a 2V to 20V converter (typical recommendations in the next few sections to find the LCD Bias application), a 74µH inductor is called for with correct inductance value for your design. the above equation, but a 22µH inductor could be used
Table 1. Recommended Inductors
without excessive reduction in maximum output current.
PART VALUE (
µ
H) MAX DCR (
Ω
) VENDOR Inductor Selection—SEPIC Regulator
LQH3C4R7 4.7 0.26 Murata LQH3C100 10 0.30 (714) 852-2001 The formula below calculates the approximate inductor LQH3C220 22 0.92 www.murata.com value to be used for a SEPIC regulator using the LT1944-1. CD43-4R7 4.7 0.11 Sumida As for the boost inductor selection, a larger or smaller CD43-100 10 0.18 (847) 956-0666 value can be used. CDRH4D18-4R7 4.7 0.16 www.sumida.com CDRH4D18-100 10 0.20   DO1608-472 4.7 0.09 Coilcraft V V L OUT D = + 2 t DO1608-103 10 0.16 (847) 639-6400 OFF  I  DO1608-223 22 0.37 www.coilcraft.com LIM
Inductor Selection—Boost Regulator Current Limit Overshoot
The formula below calculates the appropriate inductor For the constant off-time control scheme of the LT1944-1, value to be used for a boost regulator using the LT1944-1 the power switch is turned off only after the current limit (or at least provides a good starting point). This value is reached. There is a 100ns delay between the time when provides a good tradeoff in inductor size and system the current limit is reached and when the switch actually performance. Pick a standard inductor close to this value. turns off. During this delay, the inductor current exceeds A larger value can be used to slightly increase the available the current limit by a small amount. The peak inductor output current, but limit it to around twice the value current can be calculated by: calculated below, as too large of an inductance will in- crease the output voltage ripple without providing much  V ( ) − V  IN MAX SAT = + additional output current. A smaller value can be used I I ns PEAK LIM  100  L  (especially for systems with output voltages greater than 12V) to give a smaller physical size. Inductance can be Where VSAT = 0.25V (switch saturation voltage). The calculated as: current overshoot will be most evident for systems with high input voltages and for systems where smaller induc- V − V + V OUT IN MIN D ( ) tor values are used. This overshoot can be beneficial as it L = tOFF helps increase the amount of available output current for ILIM smaller inductor values. This will be the peak current seen where VD = 0.4V (Schottky diode voltage), ILIM = 100mA by the inductor (and the diode) during normal operation. (or 175mA) and tOFF = 400ns (or 1.5µs); for designs with For designs using small inductance values (especially at varying VIN such as battery powered applications, use the input voltages greater than 5V), the current limit over- minimum VIN value in the above equation. For most shoot can be quite high. Although it is internally current 5