lx1684 Microsemi Corporation, lx1684 Datasheet - Page 7

• Manufacturer Part Number: lx1684
• Description: Voltage-mode Pwm Controller
• Manufacturer: Microsemi Corporation

Copyright © 2000
Rev. 1.0c, 2005-03-08
SOFT-START CAPACITOR
The capacitor current to follow the SS-pin voltage is:
where C
should be in the range of 0.1 to 0.2µF.
microprocessor 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
R
MOSFET is economical, but can have a large tolerance, since the
R
is to use an external sense resistor (R
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)
DS(ON)
During the soft-start interval the load current from a
Current limiting occurs at current level I
The method of current sensing using the R
For 10A current limit, using a 5mΩ sense resistor:
Current Sensing Using Sense Resistor
Example:
):
can vary with temperature, etc. A more accurate alternative
TM
OUT
I
R
C
I
is the output capacitance. The typical value of C
OUT
CL
R
R
SET
R
R
SET
SET
SET
SET
×
=
=
R
C
=
=
=
=
DS
0
OUT
V
0
.
V
(
400
4
ON
TRIP
4 .
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: [phone]
TRIP
45
−
dV
45
−
dt
)
mV
10
SUB45N05-20L
+
10
×
−
TRIP
×
−
I
10
(continued)
=
(
×
10
SET
×
I
I
(400mV).
I
−
CL
I
CL
0
SET
C
45
SET
−
. 0
−
C
.
6
I
×
6
OUT
020
×
×
CL
005
SS
µA
R
R
R
SENSE
×
SET
DS
SENSE
×
=
R
=
MOSFET (20mΩ
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
). The sense resistor
ON
4
7
−
=
(
, when the voltage
.
8 .
t
(
DS(ON)
V
42
DS(ON)
)
)
/
ON
R
k
TRIP
SS
Ω
k
)
Integrated Products Division
C
Ω
with R
of the upper
SS
)
®
Microsemi
SENSE
.
SS
Where t
OUTPUT ENABLE
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.
f
3.3V at 15A will result in typical heat dissipation of 2.6W.
S
SUB70N03-09BP
P
is the switching frequency.
The LX1684 FET driver outputs are driven to ground by pulling
The output voltage is sensed by the feedback pin (V
Note: Keep R
To insure reliable operation, the operating junction temperature
TABLE 1 - FET Selection Guide
This table gives selection of suitable FETs from VISHAY
SUB45N03-13L
SUB45N05-20L
Heat Dissipated In Upper MOSFET
The heat dissipated in the top MOSFET will be:
For the
SUB70N04-10
D
Si4810DY
Si4812DY
=
Device
Voltage-Mode PWM Controller
(
SW
I
2
is switching transition line for body diode (~100ns) and
SUB70N03-09BP
×
R
P
DS
RODUCTION
1
and R
(
ON
V
)
OUT
×
V
(V)
30
30
40
40
30
30
2
D
Duty
DS
close to 100Ω (order of magnitude).
2
PAK and TO-220
=
(13mΩ R
DS(ON)
V
Cycle
SO-8
D
REF
R
@4.5V
(mΩ)
ATA
DS(ON)
13
20
20
14
20
28
will be, since more die area is
1 (
)
+
DS(ON)
+
S
0 (
HEET
R
R
5 .
1
2
R
×
), converting 12V to
@10V
)
(mΩ)
13.5
DS(ON)
I
13
18
10
18
9
×
V
DS(ON)
IN
×
t
LX1684
resistance.
SW
Charge
typ(nC)
FB
Gate
15.5
) which
×
45
43
50
20
16
f
S
)
Page 7