L5988D View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
L5988D
STMicroelectronics
'L5988D' PDF : 52 Pages View PDF
Application information
L5988D
Considering this function has its maximum in D = 0.5:
Equation 15
CIN_MIN = -2-------V-----P---P----_-I-M-O---A---X--------f--S---W---
Typically CIN is dimensioned to keep the maximum peak-peak voltage across the input filter
in the order of 1% VIN_MAX.
Manufacture
Table 11. Input capacitors
Series
Cap value (F)
Rated voltage (V)
GRM31
10
25
MURATA
GRM55
10
25
TDK
C3225
10
25
6.2
Inductor selection
The inductance value fixes the current ripple flowing through the output capacitor. So the
minimum inductance value in order to have the expected current ripple has to be selected.
The rule to fix the current ripple value is to have a ripple at 20% - 40% of the output current.
The inductance value can be calculated by the following equation:
Equation 16
IL = -V----I-N-----–---L--V----O----U----T- TON = V-----O-L---U---T-- TOFF
Where TON and TOFF are the on and off time of the internal power switch. The maximum
current ripple, at fixed Vout, is obtained at maximum TOFF that is at minimum duty cycle (see
previous section to calculate minimum duty). So fixing IL = 20% to 40% of the maximum
output current, the minimum inductance value can be calculated:
Equation 17
LMIN = V-----O----U-I--MT----A-+---X--V----F- 1-----–-F----DS----WM-----I-N--
where FSW is the switching frequency 1/(TON + TOFF).
For example for VOUT = 3.3 V, VIN = 12 V, IO = 4 A and FSW = 400 kHz the minimum
inductance value to have IL = 30% of IO is about 4.7 µH.
The peak current through the inductor is given by:
Equation 18
IL PK = IO + ---2--I-L-
So if the inductor value decreases, the peak current (that has to be lower than the current
limit of the device) increases. The higher is the inductor value, the higher is the average
output current that can be delivered, without reaching the current limit.
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