Datasheet LT3472 (Analog Devices) - 7

LT3472
U U W U APPLICATIO S I FOR ATIO Table 1. Recommended Inductors
1A. The selection of inductor and capacitor value should
Part No. Inductance DCR Current Manufacturer
ensure the peak of the inrush current to be below 1A. The
(
µ
H) (
Ω
) Rating (mA)
peak inrush current can be calculated as follows: LQH32CN220 22 0.71 250 Murata LQH32CN470 47 1.3 170 (814) 237-1431 V – . 0 6 ⎡ α ω ⎤ LQH2MCN220 22 2.1 185 www.murata.com ⎛ ⎞ I IN = • EXP P ⎢– • arctan ⎜ ⎟ • LQH2MCN470 47 5.1 120 L • ω ω ⎝ α ⎠ ⎥ ⎣ ⎦ D1067FB-220M 22 2.0 270 TOKO ⎡ ω ⎤ (408) 432-8281 ⎛ ⎞ SIN⎢arctan•⎜ ⎟ www.tokoam.com ⎝ α ⎠ ⎥ ⎣ ⎦ ELJPC220KF 22 4.0 160 Panasonic r + . 1 5 (714) 373-7334 α = www.panasonic.com 2 • L CDRH3D16-220 22 0.53 350 Sumida 1 r (847) 956-0666 ω = – 2 www.sumida.com L • C 4 • L LB2012B220M 22 1.7 75 Taiyo Yuden LEM2520-220 22 5.5 125 (408) 573-4150 where L is the inductance, r is the resistance of the www.t-yuden.com inductor and C is the output capacitance. For low DCR inductors, which is usually the case for this application,
Capacitor Selection
the peak inrush current can be simplified as follows: The small size of ceramic capacitors makes them suitable VIN – . 0 6 ⎛ α π⎞ for LT3472 applications. X5R and X57 types of ceramic I = • EXP P – • L ⎜ ⎟ • ω ω 2 capacitors are recommended because they retain their ⎝ ⎠ capacitance over wider voltage and temperature ranges Table 3 gives inrush peak currents for some component than other types such as Y5V or Z5U. A 2.2µF input selections. Note that inrush current is not a concern if the capacitor and a 2.2µF output capacitor are sufficient for input voltage rises slowly. most LT3472 applications.
Table 3. Inrush Peak Current Table 2. Recommended Ceramic Capacitor Manufacturers VIN (V) r (
Ω
) L (
µ
H) C (
µ
F) IP (A) Manufacturer Phone URL
5 0.5 22 2.2 0.89 Taiyo Yuden (408) 573-4150 www.t-yuden.com 3.6 0.7 22 2.2 0.59 Murata (814) 237-1431 www.murata.com 3.6 2.1 22 2.2 0.46 Kemet (408) 986-0424 www.kemet.com 3.6 1.3 47 1 0.32 3.6 0.7 22 1 0.46
Inrush Current
The LT3472 uses internal Schottky diodes. When supply
External Diode Selection
voltage is abruptly applied to VIN pin, for the positive As stated previously the LT3472 has internal Schottky channel, the voltage difference between VIN and VPOS diodes. The Schottky diode DP is sufficient for most step- generates inrush current flowing from input through the up applications. However, for high current inverter appli- inductor LP and the internal Schottky diode DP to charge cations, a properly selected external Schottky diode in the output capacitor COP. For the inverter channel, there is parallel with DN can improve efficiency. For external diode a similar inrush current flowing from input through the selection, both forward voltage drop and diode capaci- inductor LN1 path, charging the capacitor CNF, and return- tance need to be considered. Schottky diodes rated for ing through the internal Schottky diode DN. The maximum higher current usually have lower forward voltage drop current the Schottky diodes in the LT3472 can sustain is 3472f 7