lx1682 Microsemi Corporation, lx1682 Datasheet - Page 6

• Manufacturer Part Number: lx1682
• Description: Voltage - Mode Pwm Controllers
• Manufacturer: Microsemi Corporation

Copyright © 2000
Rev. 1.1b,2005-03-09
SOFT-START CAPACITOR (continued)
The capacitor current to follow the SS-pin voltage is:
where C
should be in the range of 0.1 to 0.2µF.
processor is negligible; therefore, the capacitor current is
approximately the required inductor current.
OVER-CURRENT PROTECTION
detected by the current sense comparator is greater than the current
sense comparator threshold, V
So,
Example:
For 10A current limit, using IRL3303 MOSFET (26mΩ R
Current Sensing Using Sense Resistor
MOSFET is economical, but can have a large tolerance, since the
R
is to use an external sense resistor (R
current sense comparator is the supply voltage to the IC (V
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
R
Example:
DS(ON)
DS(ON)
During the soft-start interval the load current from a micro-
Current limiting occurs at current level I
The method of current sensing using the R
For 10A current limit, using a 5µ sense resistor:
can vary with temperature, etc. A more accurate alternative
with R
TM
OUT
I
C
I
is the output capacitance. The typical value of C
OUT
CL
R
R
R
SENSE
R
R
SET
SET
SET
SET
SET
×
=
.
R
C
=
=
=
=
=
DS
OUT
V
0
0
V
(
400
ON
4 .
TRIP
4 .
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: [phone]
TRIP
dV
45
45
−
−
dt
)
mV
+
10
10
−
TRIP
×
×
−
I
=
(
10
10
SET
×
×
I
I
(400mV).
I
−
CL
I
CL
SET
C
45
SET
. 0
. 0
−
−
C
I
×
6
6
OUT
×
×
CL
026
005
SENSE
SS
µA
R
R
R
×
SET
DS
SENSE
×
). Since one input to the
R
=
=
A P P L I C A T I O N I N F O R M A T I O N
e
(
CL
DS
ON
3
7
−
=
(
, when the voltage
1 .
8 .
t
(
V
DS(ON)
)
)
/
ON
k
R
k
TRIP
Linfinity Microelectronics Division
Ω
SS
Ω
)
C
of the upper
SS
)
®
DS(ON)
Microsemi
CC
- pin
):
SS
Where t
TABLE 1 - FET Selection Guide
OUTPUT ENABLE
pulling the soft-start pin below 0.3V.
PROGRAMMING THE OUTPUT VOLTAGE
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).
FET SELECTION
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
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
available.
Heat Dissipated In Upper MOSFET
f
will result in typical heat dissipation of 1.92W.
S
IRL22203N
P
is the switching frequency.
The LX1681/82 FET driver outputs are driven to ground by
The output voltage is sensed by the feedback pin (V
Note: Keep R
To insure reliable operation, the operating junction temperature
This table gives selection of suitable FETs from International Rectifier.
All devices in TO-220 package. For surface mount devices (TO-263 /
D
The heat dissipated in the top MOSFET will be:
For the IRL3102 (13µ R
Device
IRL3803
IRL3103
IRL3102
IRL3303
IRL2703
D
2
-Pak), add 'S' to part number, e.g. IRL3103S.
=
Voltage-Mode PWM Controllers
(
SW
I
2
is switching transition line for body diode (~100ns) and
×
R
P
DS
RODUCTION
1
@10V(mΩ)
and R
(
R
ON
V
DS(ON)
14
13
26
40
6
7
)
OUT
×
2
Duty
close to 100
=
DS(ON)
DS(ON)
V
Cycle
D
REF
T
C
ATA
), converting 5V to 2.0V at 15A
=100ºC
I
will be, since more die area is
D
83
71
40
56
24
17
1 (
(order of magnitude
)
@
+
+
S
0 (
HEET
R
R
5 .
1
2
×
)
LX1681/1682
I
down Voltage
×
Max. Break-
V
DS(ON)
IN
×
).
30
30
30
20
30
30
t
resistance.
SW
FB
) which
×
f
S
)
Page 6