參數(shù)資料
型號(hào)： L5989D
廠商： STMICROELECTRONICS
元件分類： 穩(wěn)壓器
英文描述： 5.06 A SWITCHING REGULATOR, 1000 kHz SWITCHING FREQ-MAX, PDSO16
封裝： ROHS COMPLIANT, HTSSOP-16
文件頁(yè)數(shù)： 17/51頁(yè)
文件大?。?/td> 1947K
代理商： L5989D
Application information
L5989D
24/51
Doc ID 15778 Rev 3
6
Application information
6.1
Input capacitor selection
The capacitor connected to the input has to be capable to support the maximum input
operating voltage and the maximum RMS input current required by the device. The input
capacitor is subject to a pulsed current, the RMS value of which is dissipated over its ESR,
affecting the overall system efficiency.
So the input capacitor must have a RMS current rating higher than the maximum RMS input
current and an ESR value compliant with the expected efficiency.
The maximum RMS input current flowing through the capacitor can be calculated as:
Where Io is the maximum DC output current, D is the duty cycles,
η is the efficiency. This
function has a maximum at D = 0.5 and, considering
η = 1, it is equal to Io/2.
In a specific application the range of possible duty cycles has to be considered in order to
find out the maximum RMS input current. The maximum and minimum duty cycles can be
calculated as:
and
Where
ΔV
HIGH_SIDE and ΔVLOW_SIDE are the voltage drops across the embedded switches.
The peak to peak voltage across the input filter can be calculated as:
Given a physical dimension, ceramic capacitors can met well the requirements of the input
filter substaining an higher input current than electrolytic / tantalum types. In this case the
equation of CIN as a function of the target VPP can be written as follows:
I
RMS
I
O
D
2D
2
η
---------------
–
D
2
η
2
-------
+
=
D
MAX
V
OUT
ΔV
LOW_SIDE
+
V
INMIN
ΔV
LOW_SIDE
ΔV
HIGH_SIDE
–
+
--------------------------------------------------------------------------------------------------
=
D
MIN
V
OUT
ΔV
LOW_SIDE
+
V
INMAX
ΔV
LOW_SIDE
ΔV
HIGH_SIDE
–
+
----------------------------------------------------------------------------------------------------
=
V
PP
I
O
C
IN
f
SW
-----------------------
1
D
η
----
–
D
D
η
----
1D
–
()
+
ESR I
O
+
=
C
IN
I
O
V
PP
f
SW
-------------------------
1
D
η
----
–
D
D
η
----
1D
–
()
+
=
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