Datasheet

'I
L
=
500 kHz x 15 PH
1.2V
x
5V
(5V - 1.2V)
= 121.6 mA
'I
L
=
f x L
D x (V
IN
- V
OUT
)
LM2852
www.ti.com SNVS325D –JANUARY 2005–REVISED APRIL 2013
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 equation:
where
• ΔI
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 PVIN pin.
• V
OUT
is the output voltage of the switcher.
• f is the switching frequency.
• L is the inductance of the output filter inductor. (3)
Knowing the current ripple is important for inductor selection since the peak current through the inductor 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 LM2852.
As an example, consider a 5V to 1.2V conversion and a 500kHz switching frequency. According to Table 1, a
15µH inductor may be used. Calculating the expected peak-to-peak ripple,
(4)
The maximum inductor current for a 2A load would therefore be 2A plus 60.8 mA, 2.0608A. 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 manufacturer 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. Below is a table of inductors that
may be suitable in LM2852 applications.
Table 2. LM2852 Output Filter Inductors
Inductance (µH) Part Number Vendor
1 DO1608C-102 Coilcraft
1 DO1813P-102HC Coilcraft
6.8 DO3316P-682 Coilcraft
7 MSS1038-702NBC Coilcraft
10 DO3316P-103 Coilcraft
10 MSS1038-103NBC Coilcraft
12 MSS1038-123NBC Coilcraft
15 D03316P-153 Coilcraft
15 MSS1038-153NBC Coilcraft
18 MSS1038-183NBC Coilcraft
22 DO3316P-223 Coilcraft
22 MSS1038-223NBC Coilcraft
22 DO3340P-223 Coilcraft
27 MSS1038-273NBC Coilcraft
33 MSS1038-333NBC Coilcraft
33 DO3340P-333 Coilcraft
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