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LT3470A

3470afa

APPLICATIONS INFORMATION

be used. It is important to note that inductor saturation

current is reduced at high temperatures—see inductor

vendors for more information.

Input Capacitor

Step-down regulators draw current from the input sup-

ply in pulses with very fast rise and fall times. The input

capacitor is required to reduce the resulting voltage ripple

at the VIN pin of the LT3470A and to force this switching

current into a tight local loop, minimizing EMI. The input

capacitor must have low impedance at the switching

frequency to do this effectively. A 1μF to 2.2μF ceramic

capacitor satisﬁes these requirements.

If the input source impedance is high, a larger value ca-

pacitor may be required to keep input ripple low. In this

case, an electrolytic of 10μF or more in parallel with a 1μF

ceramic is a good combination. Be aware that the input

capacitor is subject to large surge currents if the LT3470A

circuit is connected to a low impedance supply, and that

some electrolytic capacitors (in particular tantalum) must

be speciﬁed for such use.

Output Capacitor and Output Ripple

The output capacitor ﬁlters the inductor’s ripple current

and stores energy to satisfy the load current when the

LT3470A is quiescent. In order to keep output voltage

ripple low, the impedance of the capacitor must be low

at the LT3470A’s switching frequency. The capacitor’s

equivalent series resistance (ESR) determines this im-

pedance. Choose one with low ESR intended for use in

switching regulators. The contribution to ripple voltage

due to the ESR is approximately ILIM • ESR. ESR should

be less than ~150mΩ. The value of the output capacitor

must be large enough to accept the energy stored in the

inductor without a large change in output voltage. Setting

this voltage step equal to 1% of the output voltage, the

output capacitor must be:

COUT > 50 •L •

ILIM

VOUT

Where ILIM is the top current limit with VFB= 0V (see Electri-

cal Characteristics). For example, an LT3470A producing

3.3V with L = 33μH requires 22μF. The calculated value

can be relaxed if small circuit size is more important than

low output ripple.

Sanyo’s POSCAP series in B-case and provides very good

performance in a small package for the LT3470A. Similar

performance in traditional tantalum capacitors requires

a larger package (C-case). With a high quality capacitor

ﬁltering the ripple current from the inductor, the output

voltage ripple is determined by the delay in the LT3470A’s

feedback comparator. This ripple can be reduced further

by adding a small (typically 22pF) phase lead capacitor

between the output and the feedback pin.

Table 2. Inductor Vendors

VENDOR

URL

PART SERIES

INDUCTANCE RANGE (μH)

SIZE (mm)

Coilcraft

www.coilcraft.com

DO1605

ME3220

DO3314

10 to 47

1.8

× 5.4 × 4.2

2.0

× 3.2 × 2.5

1.4

× 3.3 × 3.3

Sumida

www.sumida.com

CR32

CDRH3D16/HP

CDRH3D28

CDRH2D18/HP

10 to 47

10 to 33

10 to 47

10 to 15

3.0

× 3.8 × 4.1

1.8

× 4.0 × 4.0

3.0

× 4.0 × 4.0

2.0

× 3.2 × 3.2

Toko

www.tokoam.com

DB320C

D52LC

10 to 27

10 to 47

2.0

× 3.8 × 3.8

2.0

× 5.0 × 5.0

Würth Elektronik

www.we-online.com

WE-PD2 Typ S

WE-TPC Typ S

10 to 47

10 to 22

3.2

× 4.0 × 4.5

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

LQH32C

10 to 47

10 to 15

2.6

× 3.2 × 4.5

1.6

× 2.5 × 3.2

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