L7986 STMicroelectronics, L7986 Datasheet - Page 15

• Manufacturer Part Number: L7986
• Description: 3A step-down switching regulator
• Manufacturer: STMicroelectronics

Specifications of L7986

• Low Dropout Operation: 100% duty cycle

L7986
6
6.1
Application information
Input capacitor selection
The capacitor connected to the input must be capable of supporting 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 an 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 must 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
across 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 022450 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
SW
Application information
⋅
is the voltage drop
O
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