Datasheet LT1617, LT1617-1 (Analog Devices) - 5

LT1617/LT1617-1
U U W U APPLICATIO S I FOR ATIO Choosing an Inductor
For higher output voltages, the formula above will give large inductance values. For a 2V to 20V converter (typical Several recommended inductors that work well with the LCD bias application), a 47µH inductor is called for with LT1617 and LT1617-1 are listed in Table 1, although there the above equation, but a 10µH or 22µH inductor could be are many other manufacturers and devices that can be used without excessive reduction in maximum output used. Consult each manufacturer for more detailed infor- current. mation and for their entire selection of related parts. Many different sizes and shapes are available. Use the equations
Inductor Selection—Inverting Charge Pump Regulator
and recommendations in the next few sections to find the correct inductance value for your design. For the inverting regulator, the voltage seen by the internal power switch is equal to the sum of the absolute value of
Table 1. Recommended Inductors
the input and output voltages, so that generating high
PART VALUE (
µ
H) MAX DCR (
Ω
) VENDOR
output voltages from a high input voltage source will often LQH3C4R7 4.7 0.26 Murata exceed the 36V maximum switch rating. For instance, a LQH3C100 10 0.30 (714) 852-2001 LQH3C220 22 0.92 www.murata.com 12V to – 30V converter using the inverting topology would CD43-4R7 4.7 0.11 Sumida generate 42V on the SW pin, exceeding its maximum CD43-100 10 0.18 (847) 956-0666 rating. For such a system, an inverting charge pump is the CDRH4D18-4R7 4.7 0.16 www.sumida.com best topology. CDRH4D18-100 10 0.20 DO1608-472 4.7 0.09 Coilcraft The formula below calculates the approximate inductor DO1608-103 10 0.16 (847) 639-6400 value to be used for an inverting charge pump regulator D01608-223 22 www.coilcraft.com using the LT1617. As for the boost inductor selection, a larger or smaller value can be used. For designs with
Inductor Selection—Inverting Regulator
varying VIN such as battery powered applications, use the The formula below calculates the appropriate inductor minimum VIN value in the equation below. value to be used for an inverting regulator using the LT1617 or LT1617-1 (or at least provides a good starting V − V + V OUT IN MIN D ( ) point). This value provides a good tradeoff in inductor size L = tOFF and system performance. Pick a standard inductor close ILIM to this value (both inductors should be the same value). A larger value can be used to slightly increase the available
Current Limit Overshoot
output current, but limit it to around twice the value For the constant off-time control scheme of the LT1617, calculated below, as too large of an inductance will in- the power switch is turned off only after the 350mA (or crease the output voltage ripple without providing much 100mA) current limit is reached. There is a 100ns delay additional output current. A smaller value can be used between the time when the current limit is reached and (especially for systems with output voltages greater than when the switch actually turns off. During this delay, the 12V) to give a smaller physical size. Inductance can be inductor current exceeds the current limit by a small calculated as: amount. The peak inductor current can be calculated by:  V + V    OUT D V ( ) − V IN MAX SAT L = 2  t I = I + ns 100 OFF  PEAK LIM   I   L LIM    Where V where V SAT = 0.25V (switch saturation voltage). The D = 0.4V (Schottky diode voltage), ILIM = 350mA or current overshoot will be most evident for systems with 100mA, and tOFF = 400ns. high input voltages and for systems where smaller induc- 5