Datasheet LT3470 (Analog Devices) - 10

LT3470
APPLICATIONS INFORMATION Table 2. Inductor Vendors VENDOR URL PART SERIES INDUCTANCE RANGE (µH) SIZE (mm)
Coilcraft www.coilcraft.com DO1605 10 to 47 1.8 × 5.4 × 4.2 ME3220 10 to 47 2.0 × 3.2 × 2.5 DO3314 10 to 47 1.4 × 3.3 × 3.3 Sumida www.sumida.com CR32 10 to 47 3.0 × 3.8 × 4.1 CDRH3D16/HP 10 to 33 1.8 × 4.0 × 4.0 CDRH3D28 10 to 47 3.0 × 4.0 × 4.0 CDRH2D18/HP 10 to 15 2.0 × 3.2 × 3.2 Toko www.tokoam.com DB320C 10 to 27 2.0 × 3.8 × 3.8 D52LC 10 to 47 2.0 × 5.0 × 5.0 Wurth Elektronik www.we-online.com WE-PD2 Typ S 10 to 47 3.2 × 4.0 × 4.5 WE-TPC Typ S 10 to 22 1.6 × 3.8 × 3.8 Coiltronics www.cooperet.com SD10 10 to 47 1.0 × 5.0 × 5.0 Murata www.murata.com LQH43C 10 to 47 2.6 × 3.2 × 4.5 LQH32C 10 to 15 1.6 × 2.5 × 3.2 be used. It is important to note that inductor saturation LT3470’s switching frequency. The capacitor’s equivalent current is reduced at high temperatures—see inductor series resistance (ESR) determines this impedance. Choose vendors for more information. one with low ESR intended for use in switching regulators. The contribution to ripple voltage due to the ESR is ap-
Input Capacitor
proximately ILIM • ESR. ESR should be less than ~150mΩ. Step-down regulators draw current from the input sup- The value of the output capacitor must be large enough to ply in pulses with very fast rise and fall times. The input accept the energy stored in the inductor without a large capacitor is required to reduce the resulting voltage ripple change in output voltage. Setting this voltage step equal at the V to 1% of the output voltage, the output capacitor must be: IN pin of the LT3470 and to force this switching current into a tight local loop, minimizing EMI. The input 2 ⎛ I ⎞ capacitor must have low impedance at the switching C LIM OUT > 50 • L • frequency to do this effectively. A 1µF to 2.2µF ceramic ⎝⎜ V OUT ⎠⎟ capacitor satisfies these requirements. where ILIM is the top current limit with VFB = 0V (see Elec- If the input source impedance is high, a larger value ca- trical Characteristics). For example, an LT3470 producing pacitor may be required to keep input ripple low. In this 3.3V with L = 33µH requires 22µF. The calculated value case, an electrolytic of 10µF or more in parallel with a 1µF can be relaxed if small circuit size is more important than ceramic is a good combination. Be aware that the input low output ripple. capacitor is subject to large surge currents if the LT3470 Sanyo’s POSCAP series in B-case and provides very good circuit is connected to a low impedance supply, and that performance in a small package for the LT3470. Similar some electrolytic capacitors (in particular tantalum) must performance in traditional tantalum capacitors requires be specified for such use. a larger package (C-case). With a high quality capacitor
Output Capacitor and Output Ripple
filtering the ripple current from the inductor, the output voltage ripple is determined by the delay in the LT3470’s The output capacitor filters the inductor’s ripple current feedback comparator. This ripple can be reduced further and stores energy to satisfy the load current when the by adding a small (typically 22pF) phase lead capacitor LT3470 is quiescent. In order to keep output voltage ripple between the output and the feedback pin. low, the impedance of the capacitor must be low at the Rev. E 10 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 Package Description Revision History Related Parts