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参数资料
| 型号： | LX1682CDM-TR |
| 厂商： | MICROSEMI CORP-ANALOG MIXED SIGNAL GROUP |
| 元件分类： | 稳压器 |
| 英文描述： | 1 A SWITCHING CONTROLLER, 230 kHz SWITCHING FREQ-MAX, PDSO8 |
| 封装： | ROHS COMPLIANT, PLASTIC, SOIC-8 |
| 文件页数： | 6/9页 |
| 文件大小： | 199K |
| 代理商： | LX1682CDM-TR |
Microsemi
Linfinity Microelectronics Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 6
Copyright
2000
Rev. 1.1b,2005-03-09
WWW
.Microse
m
i
.CO
M
LX1681/1682
Voltage-Mode PWM Controllers
PRODUCTION DATA SHEET
TM
APPLI CATI ON I N FORMATI O N
SOFT-START CAPACITOR (continued)
The capacitor current to follow the SS-pin voltage is:
()
SS
OUT
C
R
t
SS
OUT
C
e
C
dt
dV
C
I
/
×
=
where COUT is the output capacitance. The typical value of CSS
should be in the range of 0.1 to 0.2F.
During the soft-start interval the load current from a micro-
processor is negligible; therefore, the capacitor current is
approximately the required inductor current.
OVER-CURRENT PROTECTION
Current limiting occurs at current level ICL , when the voltage
detected by the current sense comparator is greater than the current
sense comparator threshold, VTRIP (400mV).
TRIP
SET
ON
DS
CL
V
R
I
R
I
=
×
+
×
)
(
So,
A
R
I
mV
R
I
R
I
V
R
ON
DS
CL
SET
ON
DS
CL
TRIP
SET
45
400
)
(
)
(
×
=
×
=
Example:
For 10A current limit, using IRL3303 MOSFET (26m RDS(ON) ):
=
×
=
k
RSET
1
.
3
10
45
026
.
0
10
4
.
0
6
Current Sensing Using Sense Resistor
The method of current sensing using the RDS(ON) of the upper
MOSFET is economical, but can have a large tolerance, since the
RDS(ON) can vary with temperature, etc. A more accurate alternative
is to use an external sense resistor (RSENSE ). Since one input to the
current sense comparator is the supply voltage to the IC (VCC - pin
8), 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
RDS(ON) with RSENSE .
Example:
For 10A current limit, using a 5 sense resistor:
=
×
=
×
=
k
R
I
R
I
V
R
SET
SENSE
CL
TRIP
SET
8
.
7
10
45
005
.
0
10
4
.
0
)
(
6
OUTPUT ENABLE
The LX1681/82 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 (VFB ) 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
V
REF
OUT
+
=
Note: Keep R1 and R2 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 RDS(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 RDS(ON) will be, since more die area is
available.
This table gives selection of suitable FETs from International Rectifier.
Device
RDS(ON)
@10V(m)
ID @
TC=100C
Max. Break-
down Voltage
IRL3803
6
83
30
IRL22203N
7
71
30
IRL3103
14
40
30
IRL3102
13
56
20
IRL3303
26
24
30
IRL2703
40
17
30
All devices in TO-220 package. For surface mount devices (TO-263 /
D 2 -Pak), add 'S' to part number, e.g. IRL3103S.
TABLE 1 - FET Selection Guide
Heat Dissipated In Upper MOSFET
The heat dissipated in the top MOSFET will be:
)
5
.
0
(
)
(
)
(
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
fS is the switching frequency.
For the IRL3102 (13 RDS(ON) ), converting 5V to 2.0V at 15A
will result in typical heat dissipation of 1.92W.
AA
PP
LL
IICC
AA
TT
IIOO
NN
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