參數(shù)資料
型號(hào)： L6997STR
廠商： 意法半導(dǎo)體
英文描述： STEP DOWN CONTROLLER FOR LOW VOLTAGE OPERATIONS
中文描述： 降壓控制器低電壓運(yùn)作
文件頁(yè)數(shù)： 16/30頁(yè)
文件大?。?/td> 515K
代理商： L6997STR
L6997S
16/30
6
Application conditions: V
IN
= 3.3V, ±10% V
OUT
= 1.25V I
OUT
= 5A F
SW
= 270kHz
STEP BY STEP DESIGN
6.1 Input capacitor.
A pulsed current (with zero average value) flows through the input capacitor of a buck converter. The AC com-
ponent of this current is quite high and dissipates a considerable amount of power on the ESR of the capacitor:
(17)
The RMS current, which the capacitor must provide, is given by:
(18)
Where
δ
is the duty cycle of the application
Neglecting the last term, the equation reduces to:
(19)
which maximum value corresponds to to
δ
= 1/2 and is equal I
out
/2
Therefore, in worst case, the input capacitors should be selected with a RMS ripple current rating as high as
half the respective maximum output current.
Electrolytic capacitors are the most used because theyare the cheapest ones and are available with a wide
range of RMS current ratings. The only drawback is that, for a givenripple current rating, they are physically larg-
er than other capacitors. Very good tantalum capacitors are coming available, with very low ESR and small size.
The only problem is that they occasionally can burn if subjected to very high current during the charge. So, it is
better avoid this type of capacitors for the input filter of the device. In fact, they can be subjected to high surge
current when connected to the power supply. If available for the requested capacitance value and voltage rating,
the ceramic capacitors have usually a higher RMS current rating for a given physical dimension (due to the very
low ESR). The drawback is the quite high cost. Possible solutions:
With our parameter from the equation 3 it is found:
Icin
rms
= 2.42A
6.2 Inductor
To define the inductor, it is necessary to determine firstly the inductance value. Its minimum value is given by:
(20)
where RF =
I/I
OUT
(basically it is approximately 30%).
10
μ
F
22
μ
F
C34Y5U1E106ZTE12 TOKIN
JMK325BJ226MM
TAIYO-YUDEN
ECJ4XF0J476Z
PANASONIC
C3225X5R0J476M
TDK
47
μ
F
33
μ
F
P
CIN
ESR
CIN
Iout
2
Vin
Vin
Vin
2
Vout
–
(
)
=
Icin
rms
Iout
2
δ
1
δ
–
(
)
12
------
I
L
(
)
2
+
=
Icin
rms
Iout
δ
1
δ
–
(
)
=
Lmin
V
Vin
)
F
SW
I
out
RF Vin
max
------------------------------------------–
≥
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