Datasheet LT3462, LT3462A (Analog Devices) - 6

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APPLICATIONS INFORMATION Inrush Current Capacitor Selection
The LT3462 has a built‑in Schottky diode. When supply Ceramic capacitors are recommended. An X7R or X5R voltage is applied to the VIN pin, the voltage difference dielectric should be used to avoid capacitance decreasing between VIN and VD generates inrush current flowing severely with applied voltage and at temperature limits. from input through the inductor and the Schottky diode The “flying” capacitor between the SW and D pins should to charge the flying capacitor to VIN. The maximum non‑ be a ceramic type of value 1µF or more. When used in repetitive surge current the Schottky diode in the LT3462 the dual inductor or coupled inductor topologies the fly‑ can sustain is 1.5A. The selection of inductor and capaci‑ ing capacitor should have a voltage rating that is more tor value should ensure the peak of the inrush current to than the difference between the input and output voltages. be below 1.5A. The peak inrush current can be calculated For the charge pump inverter topology, the voltage rat‑ as follows: ing should be more than the output voltage. The output capacitor should be a ceramic type. Acceptable output ⎛ ⎞ capacitance varies from 1µF for high VOUT (–36V), to 10µF V ⎜ π ⎟ I IN – 0.6 for low VOUT (–5V). The input capacitor should be a 1µF P = exp⎜ – L L ⎟ ceramic type and be placed as close as possible to the –1 ⎜ 2 –1⎟ LT3462/LT3462A. C ⎝ C ⎠ where L is the inductance between supply and SW, and C
Layout Hints
is the capacitance between SW and D. The high speed operation of the LT3462 demands care‑ Table 3 gives inrush peak currents for some component ful attention to board layout. You will not get advertised selections. performance with careless layout. Figure 2 shows the rec‑ ommended component placement. A ceramic capacitor
Table 3. Inrush Peak Current
of 1µF or more must be placed close to the IC for input
VIN (V) L (µH) C (µF) IP (A)
supply bypassing. 5 22 1 0.70 5 33 1 0.60 C1 + 12 47 1 1.40 GND L1
Inductor Selection
VIN Each of the two inductors used with LT3462 should have C2 1 6 a saturation current rating (where inductance is approxi‑ 2 5 mately 70% of zero current inductance ) of approximately 3 4 L2 0.25A or greater. If the device is used in the charge pump R2 mode, where there is only one inductor, then its rating C3 should be 0.35A or greater. DCR of the inductors should R1 C4 be less than 1Ω. For LT3462, a value of 22µH is suitable V if using a coupled inductor such as Sumida CLS62‑220. OUT 3462 F02 If using two separate inductors, increasing the value to 47µH will result in the same ripple current. For LT3462A,
Figure 2. Suggested Layout
a value of 10µH for the coupled inductor and 22µH for two inductors will be acceptable for most applications. Rev A 6 For more information www.analog.com Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts