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參數(shù)資料
| 型號： | L6997STR |
| 廠商： | 意法半導體 |
| 英文描述： | STEP DOWN CONTROLLER FOR LOW VOLTAGE OPERATIONS |
| 中文描述： | 降壓控制器低電壓運作 |
| 文件頁數(shù)： | 17/30頁 |
| 文件大?。?/td> | 515K |
| 代理商： | L6997STR |
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L6997S
With our parameters:
Lmin
≥
2
μ
H
The saturation current must be higher then 5A
6.3 Output capacitor and ripple voltage
The output capacitor is selected based on both static and dynamic output voltage accuracy. The static output
voltage accuracy depends mostly on the ERS of the output capacitor, while the dynamic accuracy usually de-
pends both on the ESR and capacitance value.
If the static precision is ±1% for the 1.25V output voltage, the output ripple is ±12.5mV.
To determine the ESR value from the output precision is necessary to calculate the ripple current:
(21)
Where F
SW
= 270kHz.
From the Eq. above the ripple current is around 1.25A.
So the ESR is given by:
(22)
The dynamic specifications are sometimes more relaxed than the static requirements, Anyway a minimum out-
put capacitance must be ensured to avoid output voltage variation due to the charge and discharge of Cout dur-
ing load transients.
To allow the device control loop to work properly, the zero introduced by the output capacitor ESR (
τ
= ESR ·
Cout) must be at least ten times smaller than switching frequency. Low ESR tantalum capacitors, which ESR
zero is close to ten kHz, are suitable for output filtering. Output capacitor value C
OUT
and its ESR, ESRC
OUT
,
should be large enough and small enough, respectively, to keep output voltage within the accuracy range during
a load transient, and to give the device a minimum signal to noise ratio.
The current ripple flows through the output capacitors, so the should be calculated also to sustain this ripple:
the RMS current value is given by Eq. 18.
(23)
But this is usually a negligible constrain.
Possible solutions:
Multilayer capacitors can not be used because their very low ESR.
6.4 MOSFET’s and Schottky Diodes
A 3.3V bus powers the gate drivers of the device, the use ultra low level MOSFET is highly recommended, es-
pecially for high current applications. The MOSFET breakdown voltage V
BRDSS
must be greater than VINMAX
with a certain margin.
The RDS
ON
can be selected once the allowable power dissipation has been established. By selecting identical
330
μ
F
EEFUE0D331R
PANASONIC
2R5TPE220M
POSCAP
220
μ
F
I
-----------–
L
Vin
---------
T
sw
=
ESR
V
I
2
---------------------
----
1.25
---------------
=
20m
=
=
Icout
rms
2 3
----------
I
L
=
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