Datasheet

LM3208
SNVS404B –APRIL 2006–REVISED MARCH 2013 www.ti.com
If a smaller inductance inductor is used in the application, the LM3208 may become unstable during line and load
transients, 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 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 LM3208.
Capacitor Selection
The LM3208 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 capacitor types such as X5R, X7R and B are recommended for
both filters. Table 2 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. If it is necessary to choose
a 0603-size capacitor for C
IN
and C
OUT
, the operation of the LM3208 should be carefully evaluated on the system
board. Use of multiple 2.2µF or 1µF capacitors in parallel may also be considered.
Table 2. Suggested Capacitors And Their Suppliers
Model Vendor
C2012X5R0J106M,10µF, 6.3V TDK
C1608X5R0J475M, 4.7µF, 6.3V TDK
0805ZD475KA 4.7µF, 10V AVX
The input filter capacitor supplies AC current drawn by the PFET switch of the LM3208 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 LM3208 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.
Set EN low to turn off the LM3208 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 LM3208 is designed for mobile phones where the system controller controls operation mode for maximizing
battery life and requirements for small package size outweigh the additional size required for inclusion of UVLO
(Under Voltage Lock-Out) circuitry.
DSBGA Package Assembly and Use
Use of the DSBGA package requires specialized board layout, precision mounting and careful re-flow
techniques, as detailed in TI Application Note 1112 (SNVA009). Refer to the section Surface Mount Technology
(SMD) Assembly Considerations. For best results in assembly, alignment ordinals on the PC board should be
used to facilitate placement of the device. The pad style used with DSBGA 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 (SNVA009) for specific instructions how to do this.
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