f*8*C

The worst case IRMS is:

1 -

¸¸

¹

·

¨¨

©

§

2

(duty cycle = 50%)

*

*

L >= (

1

f

)

) * (

) * (

LM3370

www.ti.com

SNVS406M – NOVEMBER 2005 – REVISED AUGUST 2011

(7)

A 2.2 µH inductor with a saturation current rating of at least 1400 mA is recommended for most applications. The

inductor’s resistance should be less than around 0.2

Ω for good efficiency. Table 5 lists suggested inductors and

suppliers.

For low-cost applications, an unshielded bobbin inductor is suggested. For noise critical applications, a toroidal or

shielded-bobbin inductor should be used. A good practice is to lay out the board with overlapping footprints of

both types for design flexibility. This allows substitution of a low-noise toroidal inductor, in the event that noise

from low-cost bobbin models is unacceptable.

Below are some suggested inductor manufacturers include but are not limited to:

Table 5. Suggested Inductors and Suppliers

Model

Vendor

Dimensions (mm)

DO3314-222

Coilcraft

3.3 x 3.3 x 1.4

1.6A

LPO3310-222

3.3 x 3.3 x 1.0

1.1A

SD3114-2R2

Cooper

3.1 x 3.1 x 1.4

1.48A

NR3010T2R2M

Taiyo Yuden

3.0 x 3.0 x 1.0

1.1A

NR3015T2R2M

3.0 x 3.0 x 1.5

1.48A

VLF3010AT- 2R2M1R0

TDK

2.6 x 2.8 x 1.0

1.0A

INPUT CAPACITOR SELECTION

A ceramic input capacitor of 4.7

μF, 6.3V is sufficient for most applications. A larger value may be used for

improved input voltage filtering. The input filter capacitor supplies current to the PFET switch of the LM3370 in

the first half of each cycle and reduces voltage ripple imposed on the input power source. A ceramic capacitor's

low ESR provides the best noise filtering of the input voltage spikes due to this rapidly changing current. Select

an input filter capacitor with a surge current rating sufficient for the power-up surge from the input power source.

The power-up surge current is approximately the capacitor’s value (µF) times the voltage rise rate (V/µs).

The input current ripple can be calculated as:

(8)

OUTPUT CAPACITOR SELECTION

DC bias characteristics of ceramic capacitors must be considered when selecting case sizes like 0805 and 0603.

DC bias characteristics vary from manufacturer to manufacturer and dc bias curves should be requested from

them as part of the capacitor selection process.

The output filter capacitor smooths out current flow from the inductor to the load, helps maintain a steady output

voltage during transient load changes and reduces output voltage ripple. These capacitors must be selected with

sufficient capacitance and sufficiently low ESR to perform these functions. The output ripple voltage can be

calculated as:

Voltage peak-to-peak ripple due to capacitance =

(9)

Voltage peak-to-peak ripple, root mean squared =

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