R5973AD Application information
Manufacturer
Sanyo POSCAP
(1)
AVX
KEMET
Sprague
Table 7. Output capacitor selection
Series
TAE
THB/C/E
TPS
T494/5
595D
Cap value ( µ F) Rated voltage (V)
100 to 470
100 to 470
100 to 470
100 to 470
220 to 390
4 to 16
4 to 16
4 to 35
4 to 20
4 to 20
ESR (m )
25 to 35
25 to 55
50 to 200
30 to 200
160 to 650
1. POSCAP capacitors have some characteristics which are very similar to tantalum.
Inductor
The inductor value is very important as it fixes the ripple current flowing through the output
I capacitor. The ripple current is usually fixed at 20 - 40% of I omax
O
, which is 0.3 - 0.6 A with max = 1.5 A. The approximate inductor value is obtained using the following formula:
Equation 20
L =
V
IN
–
V
I
OUT
T
ON where T
ON
V
OUT
is the ON time of the internal switch, given by D · T. For example, with
= 3.3 V, V
IN
= 12 V and I
O
= 0.45 A, the inductor value is about 12 µH. The peak current through the inductor is given by:
Equation 21
I
PK
= I
O
+
I
2 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.
Manufacturer
Coilcraft
Coiltronics
BI
Epcos
Wurth Elektronik
Table 8. Inductor selection
Series Inductor value ( µ H) Saturation current (A)
DO3316T
UP1B
HM76-3
B82476
74456115
15 to 33
22 to 33
15 to 33
15 to 33
15 to 33
2.0 to 3.0
2.0 to 2.4
2.5 to 3.3
2 to 3
2.5 to 3
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 high switching current loop areas should be kept as small as possible and lead lengths as short as possible.
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