L5985 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'L5985' PDF : 36 Pages View PDF
L5985
Table 6. Inductors
Manufacturer
Series
Coiltronics
CD1
UP2.8B
BI
SUMIDA
HM76-3
CDRH8D28
CDRH8D28/HP
Application informations
Inductor Value (µH) Saturation Current (A)
15 to 22
4.7 to 10
15 to 33
4.7 to 10
15 to 22
2.9 to 3.6
2.7 to 3.9
2.5 to 3.7
2.5 to 3.4
2.5 to 2.8
5.3
Output capacitor selection
The current in the capacitor has a triangular waveform (with zero average value) which
generates a voltage ripple across it. This ripple is due to the capacitive component and the
resistive component (ESR). So the output capacitor has to be selected in order to have a
voltage ripple compliant with the application requirements.
The amount of the voltage ripple can be calculated starting from the current ripple obtained
by the inductor selection.
Equation 9
∆VOUT
=
ESR
⋅
∆IMAX
+
-----------∆----I--M----A---X------------
8 ⋅ COUT ⋅ fSW
Usually the resistive component of the ripple is much higher than the capacitive one, if the
output capacitor adopted is not a multi layer ceramic capacitor (MLCC) with very low ESR
value.
The output capacitor is important also for loop stability: it fixes the double LC filter pole and
the zero due to its ESR. In Chapter 5.4, it will be illustrated how to consider its effect in the
system stability.
For example with VOUT = 3.3V, VIN = 12V, ∆IL = 0.6A (resulting by the inductor value), in
order to have a ∆VOUT = 0.01·VOUT, if the multi layer capacitor are adopted, 10uF are
needed and the ESR effect on the output voltage ripple can be neglected. In case of not
negligible ESR (electrolytic or tantalum capacitors), the capacitor is chosen taking into
account its ESR value. So 100uF with ESR = 40mΩ is compliant with the requested output
voltage ripple.
The output capacitor is also important to sustain the output voltage when a load transient
with high slew rate is required by the load. When the load transient slew rate exceeds the
system bandwidth the output capacitor provides the current to the load. So if the high slew
rate load transient is required by the application the output capacitor and system bandwidth
have to be chosen in order to sustain the load transient.
In the table below some capacitor series are listed.
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