L7986TA STMicroelectronics, L7986TA Datasheet - Page 15

• Manufacturer Part Number: L7986TA
• Description: 3A step-down switching regulator
• Manufacturer: STMicroelectronics

Specifications of L7986TA

• Low Dropout Operation: 100% duty cycle

L7986TA
6
6.1
Application informations
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:
Equation 6
Where Io is the maximum DC output current, D is the duty cycle, η is the efficiency.
Considering η=1, this function has a maximum at D=0.5 and 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:
Equation 7
and
Equation 8
Where V
the internal PDMOS.
The peak to peak voltage across the input capacitor can be calculated as:
Equation 9
where ESR is the equivalent series resistance of the capacitor.
F
is the forward voltage on the freewheeling diode and V
V
PP
=
-------------------------
C
IN
Doc ID 022098 Rev 1
I
⋅
O
F
I
RMS
SW
D
D
MAX
MIN
⋅
=
⎛
⎝
1 D
I
=
O
–
=
⋅
--- -
------------------------------------- -
V
η
------------------------------------ -
V
⎞ D
⎠
INMAX
V
V
INMIN
D
⋅
OUT
OUT
–
2 D
-------------- -
+
+
⋅
+
–
–
D
--- -
η
η
V
V
V
V
⋅
2
F
SW
SW
(
F
+
1 D
D
------ -
–
η
2
2
)
+
ESR I
Application informations
SW
⋅
is voltage drop across
O
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