LX1681 Microsemi Corporation, LX1681 Datasheet - Page 7

• Manufacturer Part Number: LX1681
• Description: VOLTAGE - MODE PWM CONTROLLERS
• Manufacturer: Microsemi Corporation

Copyright © 1999
Rev. 1.0 5/99
SOFT-START CAPACITOR
capacitor current plus the load current. The soft-start capacitor
should be selected so that the overall inductor current does not
exceed it maximum.
should be in the range of 0.1 to 0.2µF.
processor is negligible; therefore, the capacitor current is ap-
proximately the required inductor current.
OVER-CURRENT PROTECTION
Current limiting occurs at current level I
detected by the current sense comparator is greater than the
current sense comparator threshold, V
Example:
For 10A current limit, using IRL3303 MOSFET (26m
Current Sensing Using Sense Resistor
The method of current sensing using the R
MOSFET is economical, but can have a large tolerance, since the
R
tive is to use an external sense resistor (R
to the current sense comparator is the supply voltage to the IC
(V
construction details, see Application Note AN-10 or LX1668 data
sheet).
above, replacing R
Example:
For 10A current limit, using a 5m
DS(ON)
CC
The capacitor current to follow the SS-pin voltage is:
where C
During the soft-start interval the load current from a micro-
So,
The over-current trip point is calculated as in the equations
R
SET
- pin 8), the sense resistor could be a PCB trace (for
can vary with temperature, etc. A more accurate alterna-
I
=
Cout
I
R
R
V
CL
SET
SET
= C
TRIP
OUT
* R
=
=
OUT
DS(ON)
- (I
is the output capacitance. The typical value of C
V
0.4 - 10 * 0.026
TRIP
I
CL
SET
DS(ON)
dV
* R
+ I
dt
45 * 10
- I
I
SET
CL
SENSE
SET
with R
* R
=
* R
(continued)
)
-6
DS(ON)
SET
C
=
C
OUT
SENSE
SS
= V
0.4 - 10 * 0.005
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
=
= 3.1k
TRIP
* e
.
45 x 10
400mV - I
sense resistor:
V
-(t/R ss C ss )
TRIP
O L T A G E
P
SENSE
A P P L I C A T I O N I N F O R M A T I O N
CL
(400mV).
45µA
-6
R O D U C T I O N
, when the voltage
DS(ON)
). Since one input
CL
* R
= 7.8k
- M
of the upper
DS(ON)
R
O D E
DS(ON)
):
P W M C
SS
D
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 (V
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
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
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
be, since more die area is available.
Heat Dissipated In Upper MOSFET
The heat dissipated in the top MOSFET will be:
and f
will result in typical heat dissipation of 1.92W.
This table gives selection of suitable FETs from International Rectifier.
All devices in TO-220 package. For surface mount devices (TO-263 /
D
A T A
IRL22203N
2
Where t
For the IRL3102 (13m R
IRL3803
IRL3103
IRL3102
IRL3303
IRL2703
-Pak), add 'S' to part number, e.g. IRL3103S.
Device
S
P
O N T R O L L E R S
is the switching frequency.
D
= (I
V
Note: Keep R
S
OUT
SW
2
H E E T
is switching transition line for body diode (~100ns)
= V
* R
TABLE 1 - FET Selection Guide
DS(ON)
10V (m )
REF
R
DS(ON)
(1 + R
14
13
26
40
6
7
1
* Duty Cycle) + (0.5 * I * V
and R
@
1
/R
DS(ON)
2
2
close to 100
)
), converting 5V to 2.0V at 15A
T
C
LX1681/1682
= 100°C
I
D
83
71
40
56
24
17
@
(order of magnitude)
IN
down Voltage
Max. Break-
* t
FB
) which has
SW
DS(ON)
30
30
30
20
30
30
* f
S
)
DS(ON)
will
.
7