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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 |
| 文件頁數(shù)： | 10/30頁 |
| 文件大?。?/td> | 515K |
| 代理商： | L6997STR |
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L6997S
10/30
so considering zero the output load the output voltage is given by:
(9))
indicating with V
out
the final value, the output charging time can be estimated as:
(10)
the minimum C
SS
value is given imposing this condition:
T
out
=T
SS
(11)
4.5 Current limit
The current limit comparator senses the inductor current through the low side MOSFET RDS
ON
drop and com-
pares this value with the ILIM pin voltage value. While the current is above the current limit value, the control
inhibits the high side MOSFET Turn On.
To properly set the current limit threshold, it should be noted that this is a valley current limit. The Average cur-
rent depends on the inductor value, V
IN
V
OUT
and switching frequency.
The average output current in current limit is given by:
(12)
Thus, to set the current threshold, choose RILIM according to the following equation:
(13)
In overcurrent conditions the system keeps the current constant until the output voltage meets the undervoltage
threshold. The negative valley current limit, for the sink mode, is set automatically at the same value of the pos-
itive valley current limit. The average negative current limit differs from the positive average current limit by the
ripple current; this difference is due to the valley control technique.
The current limit system accuracy is function of the precision of the resistance connected to the ILIM pin and
the low side MOSFET RDS
ON
accuracy. Moreover the voltage on ILIM pin must range between 10mV and 1V
to ensure the system linearity.
Figure 9. Current limit schematic
V
out
t,C
SS
(
)
Q t,C
C
out
)
------------------------
R
/R
K
I
t
2
C
out
V
SS
C
SS
2
-----------------------------------------------------------------------------
)
(
)
=
=
V
out
C
SS
(
)
V
C
R
ilim
/R
dson
K
ILIM
I
SS
---------------------------------------------------------------------------
)
(
)
0.5
=
I
OUT
CL
I
max valley
I
2
----
+
=
I
max valley
R
Rds
on
-----------------
K
ILIM
=
Positive and negative current limit
R
ILIM
5μA
Current
Comparator
To
logic
PGN
D
PHASE
To inductor
LS
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