Datasheet

DocID14117 Rev 4 25/44
B5973D Application information
44
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 0.4 - 0.8 A with
I
O
max = 2 A. The approximate inductor value is obtained using the following equation:
Equation 20
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
= 2 V and I
O
= 0.6 A, the inductor value is about 17 µH. The peak current
through the inductor is given by:
Equation 21
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 8, some
inductor manufacturers are listed.
Table 7. Output capacitor selection
Manufacturer Series Cap. value (µF) Rated voltage (V) ESR (m)
Sanyo POSCAP
(1)
1. POSCAP capacitors have some characteristics which are very similar to tantalum.
TAE 100 to 470 4 to 16 25 to 35
THB/C/E 100 to 470 4 to 16 25 to 55
AVX TPS 100 to 470 4 to 35 50 to 200
KEMET T494/5 100 to 470 4 to 20 30 to 200
Sprague 595D 220 to 390 4 to 20 160 to 650
Table 8. Inductor selection
Manufacturer Series Inductor value (µH) Saturation current (A)
Coilcraft DO3316T 15 to 33 2.0 to 3.0
Coiltronics UP1B 22 to 33 2.0 to 2.4
BI HM76-3 15 to 33 2.5 to 3.3
Epcos B82476 15 to 33 2 to 3
Wurth Elektronik 74456115 15 to 33 2.5 to 3
L
V
IN
V
OUT
–
I
---------------------------------- T
ON
=
I
PK
I
O
I
2
-----+=