Datasheet LT3470A (Analog Devices) - 10

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 Würth 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 switching regulators. The contribution to ripple voltage due to the Step-down regulators draw current from the input sup- ESR is approximately I ply in pulses with very fast rise and fall times. The input LIM • ESR. ESR should be less than ~150mΩ. The value of the output capacitor must be large capacitor is required to reduce the resulting voltage ripple enough to accept the energy stored in the inductor without at the VIN pin of the LT3470A and to force this switching a large change in output voltage. Setting this voltage step current into a tight local loop, minimizing EMI. The input equal to 1% of the output voltage, the output capacitor capacitor must have low impedance at the switching must be: frequency to do this effectively. A 1μF to 2.2μF ceramic capacitor satisfies these requirements. ⎛ 2 I ⎞ C LIM ⎜ ⎟ If the input source impedance is high, a larger value ca- OUT > 50 • L • V ⎝ OUT ⎠ pacitor 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 Electri- ceramic is a good combination. Be aware that the input cal Characteristics). For example, an LT3470A producing capacitor is subject to large surge currents if the LT3470A 3.3V with L = 33μH requires 22μF. The calculated value circuit is connected to a low impedance supply, and that can be relaxed if small circuit size is more important than some electrolytic capacitors (in particular tantalum) must 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 voltage ripple is determined by the delay in the LT3470A’s ripple low, the impedance of the capacitor must be low at feedback comparator. This ripple can be reduced further the 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. 3470afb 10