参数资料
型号: L5988D
厂商: STMICROELECTRONICS
元件分类: 稳压器
英文描述: 5.6 A SWITCHING REGULATOR, 1000 kHz SWITCHING FREQ-MAX, PDSO16
封装: LEAD FREE, HTSSOP-16
文件页数: 19/52页
文件大小: 1858K
代理商: L5988D
Application information
L5988D
26/52
Doc ID 15324 Rev 3
Considering
η=1 this function has its maximum in D = 0.5:
Typically CIN is dimensioned to keep the maximum peak-peak voltage across the input filter
in the order of 1% VIN_MAX
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:
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
ΔI
L = 20% to 40% of the maximum
output current, the minimum inductance value can be calculated:
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
ΔI
L = 30% of IO is about 4.7 H.
The peak current through the inductor is given by:
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.
Table 10.
Input capacitors
Manufacture
Series
Cap value (
μF)
Rated voltage (V)
MURATA
GRM31
10
25
GRM55
10
25
TDK
C3225
10
25
C
IN_MIN
I
O
2V
PP_MAX
f
SW
----------------------------------------------
=
ΔI
L
V
IN
V
OUT
L
------------------------------ T
ON
V
OUT
L
-------------- T
OFF
==
L
MIN
V
OUT
V
F
+
ΔI
MAX
----------------------------
1D
MIN
F
SW
-----------------------
=
I
LPK
,
I
O
ΔI
L
2
--------
+
=
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