Datasheet

LM26484
SNVS573F –SEPTEMBER 2008–REVISED MARCH 2013 www.ti.com
NFET SELECTION
There are a few major concerns when selecting an NFET for the LM26484 controller. The most important factor
to consider is the maximum power rating. It is important for the NFET to have a maximum power rating larger
than the application will need. The LM26484 has the ability to drive the gate voltage very close to VIN and down
to approximately 1.5V. Selecting an NFET where the ensured operation of the V
GS
is ≥1.5V is important.
Table 3. Recommended NFET
Part Vendor V
GS
P
DISSIPATION
Number
Si1450DH Vishay 1.5V 2.78W
EXTERNAL CAPACITORS
The LDO on the LM26484 requires external capacitors for regulator stability. These are specifically designed for
portable applications requiring minimum board space and smallest components. These capacitors must be
correctly selected for good performance. The tolerance and temperature coefficient must be considered when
selecting the capacitor to ensure that the capacitance will remain close to ideal over the entire operating
temperature range.
FEEDBACK CAPACITOR
A Feedback capacitor is required for stability; recommended values can be seen in Table 2. This capacitor must
be located a distance of not more than 1 cm from the LDO_FB pin and LDO_OUT. Any good quality ceramic or
film capacitor should be used.
OUTPUT CAPACITOR
The LDO on the LM26484 is designed specifically to work with very small ceramic output capacitors. A 10.0 µF
ceramic capacitor, marked as C10 in the Typical Application Circuit on page 1, temperature types Z5U, Y5V or
X7R with ESR between 5 mΩ to 500 mΩ, is suitable for proper operation.
It is also possible to use tantalum or film capacitors, but these are not as attractive for reasons of size and cost.
The output capacitor must meet the requirement for the minimum value of capacitance and also have an ESR
value that is within the range 50 mΩ to 500 mΩ for stability.
CAPACITOR CHARACTERISTICS
The LDO is designed to work with ceramic capacitors on the output to take advantage of the benefits they offer.
For capacitance values in the range of 0.47 μF to 44 μF, ceramic capacitors are the smallest, least expensive
and have the lowest ESR values, thus making them best for eliminating high frequency noise. The ESR of a
typical 10 μF ceramic capacitor is in the range of 20 mΩ to 40 mΩ, which easily meets the ESR requirement for
stability for the LDO.
For both input and output capacitors, careful interpretation of the capacitor specification is required to ensure
correct device operation. The capacitor value can change greatly, depending on the operating conditions and
capacitor type.
In particular, the output capacitor selection should take account of all the capacitor parameters, to ensure that the
specification is met within the application. The capacitance can vary with DC bias conditions as well as
temperature and frequency of operation. Capacitor values will also show some decrease over time due to aging.
The capacitor parameters are also dependent on the particular case size, with smaller sizes giving poorer
performance figures in general.
Buck Regulator Functional Description
The LM26484 incorporates two high efficiency synchronous switching buck regulators which are 180° out of
phase, SW1 and SW2 that deliver voltages from a single DC input voltage. Using a voltage mode architecture
with synchronous rectification, both bucks have the ability to deliver up to 2A depending on the input voltage and
output voltage (voltage head room), and the inductor chosen (maximum current capability).
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