LP3971 datasheet(43/52 Pages) TI1

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VPP-C =

IRIPPLE

IRMS = IOUTMAX

where r =

L * f * IOUTMAX* VIN

(VIN - VOUT) * VOUT

1 -

VOUT

VIN

VOUT

VIN

LP3971

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SNVS432V – JANUARY 2006 – REVISED MAY 2013

• IRIPPLE: Average to peak inductor current

• IOUTMAX: Maximum load current (1500 mA)

• VIN: Maximum input voltage in application

• L: Min inductor value including worst case tolerances (30% drop can be considered for method 1)

• f: Minimum switching frequency (1.6 MHz)

• VOUT: Output voltage

Method 2

A more conservative and recommended approach is to choose an inductor that has saturation current rating

greater than the max current limit of TBD mA.

A 2.2

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

inductor’s resistance should be less than 0.3

Ω for a good efficiency. Table 2 lists suggested inductors and

suppliers. For low-cost applications, an unshielded bobbin inductor could be considered. 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 shielded inductor,

in the event that noise from low-cost bobbin models is unacceptable.

Input Capacitor Selection

A ceramic input capacitor of 10

μF, 6.3V is sufficient for most applications. Place the input capacitor as close as

possible to the VIN pin of the device. A larger value may be used for improved input voltage filtering. Use X7R or

X5R types, do not use Y5V. DC bias characteristics of ceramic capacitors must be considered when selecting

case sizes like 0805 and 0603. The input filter capacitor supplies current to the PFET switch of the converter 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 a

capacitor with sufficient ripple current rating. The input current ripple can be calculated as:

(3)

The worst case is when VIN = 2 * VOUT

Table 2. Suggested Inductors and Their Suppliers

Model

Vendor

Dimensions LxWxH (mm)

D.C.R (Typ)

FDSE0312-2R2M

Toko

3.0 x 3.0 x 1.2

160 m

Ω

DO1608C-222

Coilcraft

6.6 x 4.5 x 1.8

80 m

Ω

Output Capacitor Selection

Use a 10

μF, 6.3V ceramic capacitor. Use X7R or X5R types, do not use Y5V. 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 voltage ripple is caused by the charging and discharging of the output capacitor and also due to its

ESR and can be calculated as:

(4)

Voltage peak-to-peak ripple due to ESR can be expressed as follows

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