L5988D View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
L5988D
STMicroelectronics
'L5988D' PDF : 52 Pages View PDF
L5988D
6
Application information
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:
Equation 10
IRMS = IO
D
–
-2-------D-----2-
+
-D----22-
Where Io is the maximum DC output current, D is the duty cycles, is the efficiency. This
function has a maximum at D = 0.5 and, considering = 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:
Equation 11
DMAX = V-----I-N----M----I--N-----+--V----O---V-U---L-T--O--+-W-----_--S-V--I--DL---OE----W-–---_---S--V-I-D--H-E---I--G----H----_--S---I--D---E-
and
Equation 12
DMIN = V-----I-N----M----A----X----+--V----O---V-U----LT--O--+--W----_---SV---I-LD---O-E--W--–---_--S----IV-D---HE----I-G----H----_---S---I-D---E--
Where VHIGH_SIDE and VLOW_SIDE are the voltage drops across the embedded switches.
The peak to peak voltage across the input filter can be calculated as:
Equation 13
VPP
=
-C----I--N---I--O---f-S----W---
1
–
D---
D
+
D---
1
–
D
+ ESR IO
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:
Equation 14
CIN = -V----P---P--I--O----f--S---W--- 1 – D--- D + D--- 1 – D
DocID15324 Rev 4
25/52
52
Share Link: 