Datasheet LT3470A (Analog Devices) - 10

link to page 3 LT3470A
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
Input Capacitor
Choose one with low ESR intended for use in switch- Step-down regulators draw current from the input sup- ing regulators. The contribution to ripple voltage due to ply in pulses with very fast rise and fall times. The input the ESR is approximately ILIM • ESR. ESR should be less capacitor is required to reduce the resulting voltage ripple than ~150mΩ. The value of the output capacitor must be at the V large enough to accept the energy stored in the induc- IN pin of the LT3470A and to force this switch- ing current into a tight local loop, minimizing EMI. The tor without a large change in output voltage. Setting this input capacitor must have low impedance at the switching voltage step equal to 1% of the output voltage, the output frequency to do this effectively. A 1µF to 2.2µF ceramic capacitor must be: capacitor satisfies these requirements. ⎛ 2 I ⎞ C LIM If the input source impedance is high, a larger value OUT > 50 •L • ⎝⎜ VOUT ⎠⎟ capacitor may be required to keep input ripple low. In this case, an electrolytic of 10µF or more in parallel with a 1µF Where ILIM is the top current limit with VFB = 0V (see ceramic is a good combination. Be aware that the input Electrical Characteristics). For example, an LT3470A pro- capacitor is subject to large surge currents if the LT3470A ducing 3.3V with L = 33µH requires 22µF. The calculated circuit is connected to a low impedance supply, and that value can be relaxed if small circuit size is more important some electrolytic capacitors (in particular tantalum) must than low output ripple. be specified for such use. Sanyo’s POSCAP series in B-case and provides very good performance in a small package for the LT3470A. Similar
Output Capacitor and Output Ripple
performance in traditional tantalum capacitors requires The output capacitor filters the inductor’s ripple current a larger package (C-case). With a high quality capacitor and stores energy to satisfy the load current when the filtering the ripple current from the inductor, the output LT3470A is quiescent. In order to keep output voltage rip- voltage ripple is determined by the delay in the LT3470A’s ple low, the impedance of the capacitor must be low at the feedback comparator. This ripple can be reduced further LT3470A’s switching frequency. The capacitor’s equiva- by adding a small (typically 22pF) phase lead capacitor lent series resistance (ESR) determines this impedance. between the output and the feedback pin. Rev. C 10 For more information www.analog.com Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Order Information Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History related parts