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參數(shù)資料
| 型號： | LX1686CPWT |
| 英文描述： | Ballast/Backlight Controller/Driver |
| 中文描述： | 鎮(zhèn)流器/背光控制器/驅(qū)動器 |
| 文件頁數(shù)： | 7/12頁 |
| 文件大小： | 292K |
| 代理商： | LX1686CPWT |
Microsemi
Linfinity Microelectronics Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 7
Copyright
2000
Rev. 1.0, 2001-09-19
W
M
.
C
LX1684
Voltage-Mode PWM Controller
P
RODUCTION
L I N F I N I T Y D I V I S I O N
A P P L I C A T I O N I N F O R M A TI O N
SOFT-START CAPACITOR
(continued)
The capacitor current to follow the SS-pin voltage is:
dV
C
I
=
(
)
SS
SS
OUT
C
R
t
SS
OUT
C
OUT
C
e
C
dt
/
×
=
where C
OUT
is the output capacitance. The typical value of C
SS
should be in the range of 0.1 to 0.2μF.
During the soft-start interval the load current from a
microprocessor is negligible; therefore, the capacitor current is
approximately the required inductor current.
OVER-CURRENT PROTECTION
Current limiting occurs at current level I
CL
, when the voltage
detected by the current sense comparator is greater than the current
sense comparator threshold, V
TRIP
(400mV).
I
R
I
+
×
)
(
TRIP
V
SET
SET
ON
DS
CL
R
=
×
So,
μA
R
I
mV
R
I
R
I
V
R
ON
DS
CL
SET
SET
ON
DS
CL
TRIP
SET
45
400
)
(
)
(
×
=
×
=
Example:
For 10A current limit, using
SUB45N05-20L
MOSFET (20m
R
DS(ON)
):
=
R
SET
45
Current Sensing Using Sense Resistor
The method of current sensing using the R
DS(ON)
of the upper
MOSFET is economical, but can have a large tolerance, since the
R
DS(ON)
can vary with temperature, etc. A more accurate alternative
is to use an external sense resistor (R
SENSE
). The sense resistor
could be a PCB trace (for construction details, see Application
Note AN-10 or LX1668 data sheet). The over-current trip point is
calculated as in the equations above, replacing R
DS(ON)
with R
SENSE
.
Example:
For 10A current limit, using a 5m
sense resistor:
=
I
=
×
×
10
k
42
.
4
020
6
0
10
4
0
.
.
=
×
×
10
45
=
×
k
R
R
I
V
R
SET
SET
SENSE
CL
TRIP
SET
8
005
6
.
10
4
)
(
OUTPUT ENABLE
The LX1684 FET driver outputs are driven to ground by pulling
the soft-start pin below 0.3V.
PROGRAMMING THE OUTPUT VOLTAGE
The output voltage is sensed by the feedback pin (V
FB
) which
has a 1.25V reference. The output voltage can be set to any voltage
above 1.25V (and lower than the input voltage) by means of a
resistor divider (see Product Highlight).
)
1
2
1
R
R
V
V
REF
OUT
+
=
Note:
Keep R
1
and R
2
close to 100
(order of magnitude).
FET SELECTION
To insure reliable operation, the operating junction temperature
of the FET switches must be kept below certain limits. The Intel
specification states that 115°C maximum junction temperature
should be maintained with an ambient of 50°C. This is achieved by
properly derating the part, and by adequate heat sinking. One of
the most critical parameters for FET selection is the R
DS(ON)
resistance. This parameter directly contributes to the power
dissipation of the FET devices, and thus impacts heat sink design,
mechanical layout, and reliability. In general, the larger the current
handling capability of the FET, the lower the R
DS(ON)
will be, since
more die area is available.
TABLE 1
- FET Selection Guide
This table gives selection of suitable FETs from VISHAY
Device
V
DS
(V )
R
DS(ON)
@4.5V
(m
)
R
DS(ON)
@10V
(m
)
Gate
Charge
typ(nC)
D
2
PAK and TO-220
30
30
40
40
SO-8
30
30
SUB70N03-09BP
SUB45N03-13L
SUB45N05-20L
SUB70N04-10
13
20
20
14
9
13
18
10
15.5
45
43
50
Si4810DY
Si4812DY
20
28
13.5
18
20
16
Heat Dissipated In Upper MOSFET
The heat dissipated in the top MOSFET will be:
)
5
+
)
(
)
(
2
S
SW
IN
ON
DS
D
f
t
V
I
Cycle
Duty
R
I
P
×
×
×
×
×
×
=
Where t
SW
is switching transition line for body diode (~100ns) and
f
S
is the switching frequency.
For the
SUB70N03-09BP
(13m
R
DS(ON)
), converting 12V to
3.3V at 15A will result in typical heat dissipation of 2.6W.
A
P
P
L
I
C
A
T
I
O
N
S
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