Datasheet

Application information ST1S14
24/46 DocID17977 Rev 2
A list of some tantalum capacitor manufacturers is provided in Table 9.
7.1.3 Inductor
The inductor value is very important as it fixes the ripple current flowing through the output
capacitor. The ripple current is usually fixed at 20 - 40 % of I
omax
, which is 0.6 - 1.2 A with
IOmax
= 3 A. The approximate inductor value is obtained using the following formula:
Equation 22
where T
ON
is the ON time of the internal switch, given by D · T. For example, with
V
OUT
= 3.3 V, V
IN
= 24 V, and ΔI
O
= 0.8 A, the inductor value is about 4.7 µH. The peak
current through the inductor is given by:
Equation 23
and it can be observed that if the inductor value decreases, the peak current (which must be
lower than the current limit of the device) increases. So, when the peak current is fixed, a
higher inductor value allows a higher value for the output current. In Table 10, some inductor
manufacturers are listed.
7.2 Layout considerations
The layout of switching DC-DC converters is very important to minimize noise and
interference. Power-generating portions of the layout are the main cause of noise and so
Table 9. Output capacitor selection
Manufacturer Series Rated voltage (V) Cap value (µF)
(1)
1. see Section 6: Closing the loop for the selection of the output capacitor
ESR (mΩ)
(1)
Nippon Chemicon KZE 6.3 to 50
Sanyo POSCAP
(2)
2. POSCAP capacitors have some characteristics which are very similar to tantalum.
TAE 4 to 16
THB/C/E 4 to 16
AVX TPS 4 to 35
f
Z
1
2 π ESR COUT⋅⋅ ⋅
----------------------------------------------------------
BW<=
Table 10. Inductor selection
Manufacturer Series Inductor value (µH) Saturation current (A)
Wurth Elektronik
WE-HCI 7040 1 to 4.7 20 to 7
WE-HCI 7050 4.9 to 10 20 to 4.0
Coilcraft XPL 7030 2.2 to 10 29 to 7.2
L
V
IN
V
OUT
–()
ΔI
----------------------------------
T
ON
⋅=
I
PK
I
O
ΔI
2
-----+=