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PM6675AS Datasheet(PDF) 38 Page - STMicroelectronics

No. de pieza PM6675AS
Descripción Electrónicos  High efficiency step-down controller with embedded 2 A LDO regulator
PDF  48 Pages
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Fabricante Electrónico  STMICROELECTRONICS [STMicroelectronics]
Página de inicio  http://www.st.com
Logo STMICROELECTRONICS - STMicroelectronics

PM6675AS Datasheet(HTML) 38 Page - STMicroelectronics

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Application information
PM6675AS
38/48
8.1.3
Output capacitor selection
Using tantalum or electrolytic capacitors, the selection is made referring to ESR and voltage
rating rather than by a specific capacitance value.
The output capacitor has to satisfy the output voltage ripple requirements. At a given
switching frequency, small inductor values are useful to reduce the size of the choke but
increase the inductor current ripple. Thus, to reduce the output voltage ripple a low ESR
capacitor is required.
To reduce jitter noise between different switching regulators in the system, it is preferable to
work with an output voltage ripple greater than 25 mV.
As far as it concerns the load transient requirements, the Equivalent Series Resistance
(ESR) of the output capacitor must satisfy this relationship:
Equation 37
where VRIPPLE is the maximum tolerable ripple voltage.
In addition, the ESR must be enough high to meet stability requirements. The output
capacitor zero must be lower than the switching frequency:
Equation 38
If ceramic capacitors are used, the output voltage ripple due to inductor current ripple is
negligible; then the inductance could be smaller, reducing the size of the choke. In this case
it is important that the output capacitor can adsorb the inductor energy without generating
an over-voltage condition when the system changes from a full load to a no load condition.
The minimum output capacitance can be chosen by the following equation:
Equation 39
where Vf is the output capacitor voltage after the load transient and Vi is the output capacitor
voltage before the load transient.
MAX
,
L
MAX
,
RIPPLE
I
V
ESR
∆
≤
out
Z
SW
C
ESR
2
1
f
f
⋅
⋅
π
=
>
2
2
2
MAX
,
LOAD
min
,
OUT
Vi
Vf
I
L
C
−
⋅
=



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