LM3205

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SNVS388D – JULY 2005 – REVISED MAY 2009

INDUCTOR SELECTION

A 3.3µH inductor with saturation current rating over 1200mA and low inductance drop at the full DC bias

condition is recommended for almost all applications. The inductor’s DC resistance should be less than 0.2

Ω for

good efficiency. For low dropout voltage, lower DCR inductors are advantageous. The lower limit of acceptable

inductance is 2.3µH at 1200mA over the operating temperature range. Full attention should be paid to this limit,

because some small inductors show large inductance drops at high DC bias. These can not be used with the

LM3205. Taiyo-Yuden NR3015T3R3M is an example of an inductor with the lowest acceptable limit (as of

Nov./05). Table 2 suggests some inductors and suppliers.

Table 2. Suggested inductors and their suppliers

Model

Size (WxLxH) [mm]

Vendor

NR3015T3R3M

3.0 x 3.0 x 1.5

Taiyo-Yuden

DO3314-332MXC

3.3 x 3.3 x 1.4

Coilcraft

If a smaller inductance inductor is used in the application, the LM3205 may become unstable during line and load

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 footprints accommodating

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. Saturation occurs when the magnetic flux density from current

through the windings of the inductor exceeds what the inductor’s core material can support with a corresponding

magnetic field. This can cause poor efficiency, regulation errors or stress to a DC-DC converter like the LM3205.

CAPACITOR SELECTION

The LM3205 is designed for use with ceramic capacitors for its input and output filters. Use a 10µF ceramic

capacitor for input and a 4.7µF ceramic capacitor for output. They should maintain at least 50% capacitance at

DC bias and temperature conditions. Ceramic capacitors types such as X5R, X7R are recommended for both

filters. These provide an optimal balance between small size, cost, reliability and performance for cell phones

and similar applications. Table 3 lists some suggested part numbers and suppliers. DC bias characteristics of the

capacitors must be considered when selecting the voltage rating and case size of the capacitor. A few

manufactures can supply 4.7µF capacitors in the 0805 case size which maintain at least 50% of their value, but

TDK is currently the only manufacturer which can provide such capacitors in the 0603 case size. As of

November, 2005, no manufacture can supply 10µF capacitors in the 0603 case size which maintain 50% of their

evaluated on the system board. Output capacitors with smaller case sizes mitigate piezo electric vibrations when

the output voltage is stepped up and down at fast rates. However, they have a larger percentage drop in value

with dc bias. Use of multiple 2.2µF or 1µF capacitors in parallel may also be considered.

Table 3. Suggested capacitors and their suppliers

Model

Vendor

0805ZD475KA 4.7µF, 10V

AVX

C1608X5R0J475M, 4.7µF, 6.3V

TDK

C2012X5R0J106M,10µF, 6.3V

TDK

The input filter capacitor supplies AC current drawn by the PFET switch of the LM3205 in the first part of each

cycle and reduces the voltage ripple imposed on the input power source. The output filter capacitor absorbs the

AC inductor current, 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

(Equivalent Series Resistance) to perform these functions. The ESR of the filter capacitors is generally a major

factor in voltage ripple.

EN PIN CONTROL

Drive the EN pin using the system controller to turn the LM3205 ON and OFF. Use a comparator, Schmidt trigger

or logic gate to drive the EN pin. Set EN high (>1.2V) for normal operation and low (<0.5V) for a 0.01µA (typ.)

shutdown mode.

Copyright © 2005–2009, Texas Instruments Incorporated

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