Datasheet

Applications Information (Continued)
CHOOSING AN INDUCTANCE VALUE
The current ripple present in the output filter inductor is
determined by the input voltage, output voltage, switching
frequency and inductance according to the following equa-
tion:
where ∆I
L
is the peak to peak current ripple, D is the duty
cycle V
OUT
/V
IN
,V
IN
is the input voltage applied to the output
stage, V
OUT
is the output voltage of the switcher, f is the
switching frequency and L
O
is the inductance of the output
filter inductor. Knowing the current ripple is important for
inductor selection since the peak current through the induc-
tor is the load current plus one half the ripple current. Care
must be taken to ensure the peak inductor current does not
reach a level high enough to trip the current limit circuitry of
the LM2853. As an example, consider a 5V to 1.2V conver-
sion and a 550 kHz switching frequency. According to Table
1, a 4.7 µH inductor may be used. Calculating the expected
peak-to-peak ripple,
The maximum inductor current for a 3A load would therefore
be 3A plus 177 mA, 3.177A. As shown in the ripple equation,
the current ripple is inversely proportional to inductance.
OUTPUT FILTER INDUCTORS
Once the inductance value is chosen, the key parameter for
selecting the output filter inductor is its saturation current
(I
SAT
) specification. Typically I
SAT
is given by the manufac-
turer as the current at which the inductance of the coil falls to
a certain percentage of the nominal inductance. The I
SAT
of
an inductor used in an application should be greater than the
maximum expected inductor current to avoid saturation. Be-
low is a table of inductors that are suitable in LM2853
applications.
TABLE 2. Recommended Inductors
Inductance Part Number Vendor
4.7 µF DO3308P-472ML Coilcraft
4.7 µF DO3316P-472ML Coilcraft
4.7 µF MSS1260-472ML Coilcraft
5.2 µF MSS1038-522NL Coilcraft
5.6 µF MSS1260-562ML Coilcraft
6.8 µF DO3316P-682ML Coilcraft
6.8 µF MSS1260-682ML Coilcraft
OUTPUT FILTER CAPACITORS
The recommended capacitors that may be used in the output
filter with the LM2853 are limited in value and ESR range
according to Table 1.
Below are some examples of capacitors that can typically be
used in an LM2853 application.
TABLE 3. Recommended Capacitors
Capacitance (µF) Part Number Chemistry Vendor
100 594D107X_010C2T Tantalum Vishay-Sprague
100 593D107X_010D2_E3 Tantalum Vishay-Sprague
100 TPSC107M006#0075 Tantalum AVX
100 NOSD107M006#0080 Niobium Oxide AVX
100 NOSC107M004#0070 Niobium Oxide AVX
120 594D127X_6R3C2T Tantalum Vishay-Sprague
150 594D157X_010C2T Tantalum Vishay-Sprague
150 595D157X_010D2T Tantalum Vishay-Sprague
150 591D157X_6R3C2_20H Tantalum Vishay-Sprague
150 TPSD157M006#0050 Tantalum AVX
150 TPSC157M004#0070 Tantalum AVX
150 NOSD157M006#0070 Niobium Oxide AVX
220 594D227X_6R3D2T Tantalum Vishay-Sprague
220 591D227X_6R3D2_20H Tantalum Vishay-Sprague
220 591D227X_010D2_20H Tantalum Vishay-Sprague
220 593D227X_6R3D2_E3 Tantalum Vishay-Sprague
LM2853
www.national.com9