LX1668CDW Microsemi Analog Mixed Signal Group, LX1668CDW Datasheet - Page 13

• Manufacturer Part Number: LX1668CDW
• Description: IC CTRLR DC/DC PROG SW 20SOIC
• Manufacturer: Microsemi Analog Mixed Signal Group

Specifications of LX1668CDW

• Applications: Controller, Intel Pentium® II, III
• Voltage - Input: 5V, 12V
• Number Of Outputs: 3
• Voltage - Output: 1.3 ~ 3.5 V
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 20-SOIC
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Copyright © 1999
Rev. 1.0a
CURRENT LIMIT
General Guidelines for Selecting R
dependency of the inductance.
2.5µH at 0A current.
be lower than the desired short-circuit current limit, a resistor (R
can be put in parallel with C
of components is as follows:
more information.
OUTPUT ENABLE
The LX1668 FET driver outputs are driven to ground by pulling
the soft-start pin below 0.5V.
PROGRAMMING THE OUTPUT VOLTAGE
The output voltage is set by the DAC with a 5-bit digital voltage-
identification (VID) code input (see Table 1). The DAC input is
designed to be compatible with digital circuits. The VID code
may be hard-wired into the package of the processor [as in the
case of a Pentium II or Pentium Pro processor]. If the processor
does not have a VID code, the output voltage can be set by
means of a DIP-switch, jumpers or TTL-compatible digital
circuits. When using a DIP-switch or jumpers, connect the VID
pin to ground (DIP-switch ON) for a low or “0” signal and leave
the VID pin open (DIP-switch OFF) for a high or “1” signal.
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 C
The above equation has taken into account the current-
Typical values are: R
In cases where R
Again, select (R
R
C
L
R
S
R
=
L (Required)
=
S
L (Actual)
according to:
R
I
V
trip,S
L (Actual)
trip
(continued)
S2
=
L
//R
* (R
L
is so large that the trip point current would
R
S
S2
) < 10k . See Application Note AN-7 for
L
S2
R
= 3m , R
+ R
S2
// R
S
, as shown in Figure 10. The selection
Select: R
C
S
S n
S
)
P
=
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
=
ROGRAMMABLE
S
R
S
= 9k , C
R
L
L
S
, C
n
L (Actual)
R
S
L
S
10 k
P
, and R
A P P L I C A T I O N I N F O R M A T I O N
R O D U C T I O N
S
*
= 0.1µF, and L is
L
R
R
S
S2
M
+ R
* R
DS(ON)
U LTIPLE
S2
S
DS(ON)
will
S2
)
O
UTPUT
D
FET SELECTION
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
will result in typical heat dissipation of 1.92W.
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).
Non-Synchronous Operation - Schottky Diode
A typical Schottky diode, with a forward drop of 0.6V will dissipate
0.6 * 15 * [1 – 2/5] = 5.4W (compared to the 1.8 to 3.5W dissipated
by a MOSFET under the same conditions). This power loss
becomes much more significant at lower duty cycles. The use of
a dual Schottky diode in a single TO-220 package (e.g. the
MBR2535) helps improve thermal dissipation.
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
The recommended solution is to use IRL3102 for the high side
Where t
For the IRL3102 (13m R
Power dissipated in the bottom MOSFET will be:
IRL3803
IRL3103
IRL3102
IRL3303
IRL2703
-Pak), add 'S' to part number, e.g. IRL3103S.
Device
S
DC:DC C
P
P
[IRL3303 or 1.76W for the IRL3102]
is the switching frequency.
D
D
= (I
= I
S
SW
2
2
H E E T
* R
is switching transition line for body diode (~100ns)
* R
DS(ON)
(continued)
TABLE 4 - FET Selection Guide
DS(ON)
ONTROLLER
10V (m )
R
DS(ON)
14
13
26
40
6
7
* [1 - Duty Cycle] = 3.51W
* Duty Cycle) + (0.5 * I * V
@
DS(ON)
T
), converting 5V to 2.0V at 15A
C
= 100°C
I
D
83
71
40
56
24
17
@
IN
down Voltage
LX1668
Max. Break-
* t
DS(ON)
SW
30
30
30
20
30
30
* f
.
S
)
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