R5975D Datasheet, PDF (26/47 Pages) STMicroelectronics – Up to 3 A step-down switching regulator for aerospace applications

Application information
R5975D
Table 7. Output capacitor selection
Manufacturer
Series
Cap value (µF) Rated voltage (V)
ESR (m)
Sanyo POSCAP⁽¹⁾
TAE
47 to 680
2.5 to 10
25 to 35
TV
68 to 330
4 to 6.3
25 to 40
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
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
capacitor. The ripple current is usually fixed at 20 - 40% of I_Omax, which is 0.6 - 1.2 A with
I_Omax = 3 A. The approximate inductor value is obtained using the following formula:
Equation 20
L = ^---^V----^I--^N-----–-_---^V-_I---^O----^U----^T---^-  T_ON
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 = 0.9 A, the inductor value is about 12 µH.
The peak current through the inductor is given by:
Equation 21
I_PK = I_O + ^--₂---^I
and it can be seen 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
Coilcraft
Wurth Elektronik
Table 8. Inductor selection
Series
Inductor value (µH)
DO3316T
MSS1260T
5.6 to 12
5.6 to 15
WE-PD L
4.7 to 27
Saturation current (A)
3.5 to 4.7
3.5 to 8
3.55 to 6
8.2
Layout considerations
The layout of the 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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