LM3207TL datasheet(16/19 Pages) NSC

16 / 19 page

Application Information (Continued)

LDO CAPACITOR SELECTION

The output capacitor should be connected between the LDO

output and a good ground connection. This capacitor must

be selected within specified capacitance range and have

sufficiently low ESR. The ESR of the capacitor is generally a

major factor in LDO stability. Refer to manufacturer ESR

curves for more detail. Table 1 suggests acceptable capaci-

tors and their suppliers.

TABLE 1. Suggested capacitors and their suppliers

Model

Vendor

C1005X5R1A104KT, 100nF, 10V

TDK

C1005X5R0J224KT, 220nF, 6.3V

TDK

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 advanta-

geous. The lower limit of acceptable inductance is 1.7µH 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 LM3207. 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

MIPW3226D3R0M

3.2 x 2.6 x 1.0

FDK

If a smaller inductance inductor is used in the application, the

LM3207 may become unstable during line and load tran-

sients and V

CON transient response times may be affected.

For low-cost applications, an unshielded bobbin inductor is

suggested. For noise critical applications, a toroidal or

shielded-bobbin inductor is recommended. A good practice

is to layout 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 result

in poor efficiency, regulation errors or stress to DC-DC con-

verter like the LM3207.

DC-DC CONVERTER CAPACITOR SELECTION

The LM3207 is designed with a ceramic capacitor 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 tempera-

ture 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 sug-

gests acceptable part numbers and their suppliers. DC bias

characteristics of the capacitors must be considered when

selecting the voltage rating and case size of the capacitor. If

it is necessary to choose a 0603-size capacitor for V

IN, the

operation of the LM3207should be carefully 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

Taiyo-Yuden

C1608X5R0J475M, 4.7uF, 6.3V

TDK

C1608X5R0J106M, 10µF, 6.3V

TDK

C2012X5R0J106M, 10uF, 6.3V

TDK

C2012X5R1A475M, 4.7uF, 6.3V

TDK

The input filter capacitor supplies AC current drawn by the

PFET switch of the LM3207 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 tran-

sient load changes and reduces output voltage ripple. These

capacitors must be selected with sufficient capacitance and

sufficiently low ESR (Equivalent Series Resistance) to per-

form these functions. The ESR of the filter capacitors is

generally a factor in voltage ripple.

EN PIN CONTROL

Drive the EN and EN

LDO pins using the system controller to

turn the LM3207 ON and OFF. Use a comparator, Schmidt

trigger or logic gate to drive the EN and EN

LDO pins. Set EN

high (>1.2V) for normal operation and low (<0.5V) for a

0.01µA (typ.) shutdown mode.

Set EN low to turn off the LM3207 during power-up and

under voltage conditions when the power supply is less than

the 2.7V minimum operating voltage. The part is out of

regulation when the input voltage is less than 2.7V. The

LM3207 is designed for mobile phones where the system

controller controls operation mode for maximizing battery life

and requirements for small package size outweigh the addi-

tional size required for inclusion of UVLO (Under Voltage

Lock-Out) circuitry.

Micro SMD PACKAGE ASSEMBLY AND USE

Use of the Micro SMD package requires specialized board

layout, precision mounting and careful re-flow techniques, as

detailed in National Semiconductor Application Note 1112.

Refer to the section Surface Mount Technology (SMD) As-

sembly Considerations. For best results in assembly, align-

ment ordinals on the PC board should be used to facilitate

placement of the device. The pad style used with Micro SMD

package must be the NSMD (non-solder mask defined) type.

This means that the solder-mask opening is larger than the

pad size. This prevents a lip that otherwise forms if the

solder-mask and pad overlap, from holding the device off the

surface of the board and interfering with mounting. See

Application Note 1112 for specific instructions.

The 9-Bump package used for LM3207 has 300 micron

solder balls and requires 10.82mil pads for mounting the

circuit board. The trace to each pad should enter with a 90˚

entry angle to prevent debris from being caught in deep

corners. Initially, the trace to each pad should be 6-7mil

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