lx1665 Microsemi Corporation, lx1665 Datasheet - Page 14

• Manufacturer Part Number: lx1665
• Description: Dual Output Pwm Controllers With 5-bit Dac
• Manufacturer: Microsemi Corporation

LX1664/64A, LX1665/65A
14
CURRENT LIMIT
be lower than the desired short-circuit current limit, a resistor (R
can be put in parallel with C
of components is as follows:
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.
and IRL3303 for the low side FET, for the best combination of cost
and performance. Alternative FET’s from any manufacturer could
be used, provided they meet the same criteria for R
Heat Dissipated In Upper MOSFET
The heat dissipated in the top MOSFET will be:
and f
This table gives selection of suitable FETs from International Rectifier.
All devices in TO-220 package. For surface mount devices (TO-263 /
D
2
In cases where R
Again, select (R
The recommended solution is to use IRL3102 for the high side
Where t
-Pak), add 'S' to part number, e.g. IRL3103S.
S
C
P
is the switching frequency.
D
S
R
R
=
= (I
L (Required)
L (Actual)
SW
R
2
L (Actual)
is switching transition line for body diode (~100ns)
* R
(continued)
TABLE 4 - FET Selection Guide
DS(ON)
=
S2
L
* (R
//R
L
is so large that the trip point current would
R
* Duty Cycle) + (0.51 * V
S2
S
S2
) < 10kΩ.
R
Ω Ω Ω Ω Ω
+ R
// R
S2
S
, as shown in Figure 11. The selection
S
S
)
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
=
D
UAL
R
L (Actual)
U S I N G T H E L X 1 6 6 4 / 6 5 D E V I C E S
L
O
P
UTPUT
R O D U C T I O N
*
IN
* t
R
R
PWM C
SW
S
S2
DS(ON)
+ R
* R
* f
DS
S2
S
S
ON will
)
.
DS
ON
ONTROLLERS WITH
S2
)
D
FET SELECTION
will result in typical heat dissipation of 1.48W.
Synchronous Rectification – Lower MOSFET
The lower pass element can be either a MOSFET or a Schottky
diode. The use of a MOSFET (synchronous rectification) will result
in higher efficiency, but at higher cost than using a Schottky diode
(non-synchronous).
Catch Diode – Lower MOSFET
A low-power Schottky diode, such as a 1N5817, is recommended
to be connected between the gate and source of the lower
MOSFET when operating from a 12V-power supply (see Figure 9).
This will help protect the controller IC against latch-up due to the
inductor voltage going negative. Although latch-up is unlikely, the
use of such a catch diode will improve reliability and is highly
recommended.
Non-Synchronous Operation - Schottky Diode
A typical Schottky diode, with a forward drop of 0.6V will dissipate
0.6 * 14 * [1 – 2.8/5] = 3.7W (compared to the 1.1 to 2.2W dissipated
by a MOSFET under the same conditions). This power loss
becomes much more significant at lower duty cycles – synchro-
nous rectification is recommended especially when a 12V-power
input is used. The use of a dual Schottky diode in a single TO-220
package (e.g. the MBR2535) helps improve thermal dissipation.
MOSFET GATE BIAS
The power MOSFETs can be biased by one of two methods:
charge pump or 12V supply connected to V
A T A
For the IRL3102 (13mΩ R
Power dissipated in the bottom MOSFET will be:
1) Charge Pump (Bootstrap)
2) 12V Supply
P
[IRL3303 or 1.12W for the IRL3102]
When 12V is supplied to the drain of the MOSFET, as in
Figure 9, the gate drive needs to be higher than 12V in order
to turn the MOSFET on. Capacitor C
are used as a charge pump voltage doubling circuit to raise
the voltage of V
high enough voltage to turn on Q
always be connected to V
itself, as well as gate drive for the bottom MOSFET.
When 5V is supplied to the drain of Q
be connected to both V
D
= I
S
2
H E E T
* R
5-B
(continued)
DS(ON)
IT
* [1 - Duty Cycle] = 2.24W
C1
DAC
so that the TDRV pin always provides a
DS(ON)
CC
CC
and V
), converting 5V to 2.8V at 14A
to provide power for the IC
C1
1
.
. The 12V supply must
10
1
, a 12V supply should
C1
and diodes D
.
2
& D
3