STMicroelectronics Evaluation board for A5970AD EVALA5970AD EVALA5970AD Fiche De Données
Codes de produits
EVALA5970AD
DocID14661 Rev 5
23/42
A5970AD
Application information
42
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
capacitor. The ripple current is usually fixed at 20 - 40% of I
omax
, which is 0.2 - 0.4 A with
I
O
max = 1 A. The approximate inductor value is obtained using the following formula:
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
= 12 V and
I
O
= 0.3 A, the inductor value is about 15 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
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
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
-----
+
=