LX1669 查看數據表（PDF） - Microsemi Corporation
零件编号
产品描述 (功能)
生产厂家
LX1669 Datasheet PDF : 14 Pages
| |||
PRODUCT DATABOOK 1996/1997
PROGRAMMABLE DC:DC CONTROLLER
PR O D U C T I O N D ATA S H E E T
LX1669
APPLICATION INFORMATION
CURRENT LIMIT (continued)
General Guidelines for Selecting RS , CS , and RL
RL =
Vtrip
Itrip,S
and CS according to:
Select: RS ≤ 10 kΩ
CS n =
Ln
RL RS
The above equation has taken into account the current-
dependency of the inductance.
Typical values are: RL = 3mΩ, RS = 9kΩ, CS = 0.1µF, and L is
2.5µH at 0A current.
In cases where RL is so large that the trip point current would
be lower than the desired short-circuit current limit, a resistor (RS2)
can be put in parallel with CS, as shown in Figure 9. The selection
of components is as follows:
RL (Required) =
RS2
RL (Actual)
RS2 + RS
L
L
CS = RL (Actual) * (RS2 // RS ) = RL (Actual) *
RS + RS2
RS2 * RS
Again, select (RS2//RS) < 10kΩ. See Application Note AN-7 for
more information.
OUTPUT ENABLE
The LX1669 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 RDS(ON)
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 RDS(ON) will
be, since more die area is available.
FET SELECTION (continued)
TABLE 4 - FET Selection Guide
This table gives selection of suitable FETs from International Rectifier.
Device
RDS(ON) @
10V (mΩ)
ID @
TC = 100°C
Max. Break-
down Voltage
IRL3803
6
83
30
IRL22203N
7
71
30
IRL3103
14
40
30
IRL3102
13
56
20
IRL3303
26
24
30
IRL2703
40
17
30
All devices in TO-220 package. For surface mount devices (TO-263 /
D2-Pak), add 'S' to part number, e.g. IRL3103S.
The recommended solution is to use IRL3102 for the high side
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 RDS(ON).
Heat Dissipated In Upper MOSFET
The heat dissipated in the top MOSFET will be:
PD = (I2 * RDS(ON) * Duty Cycle) + (0.5 * I * VIN * tSW * fS )
Where tSW is switching transition line for body diode (~100ns)
and fS is the switching frequency.
For the IRL3102 (13mΩ RDS(ON)), converting 5V to 2.0V at 15A
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).
Power dissipated in the bottom MOSFET will be:
PD = I2 * RDS(ON) * [1 - Duty Cycle] = 3.51W
[IRL3303 or 1.76W for the IRL3102]
Non-Synchronous Operation - Schottky Diode
A typical Schottky diode with a forward drop of 0.6V will dissipate
0.6 x 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 – synchro-
nous rectification is recommended. The use of a dual Schottky
diode in a single TO-220 package (e.g. the MBR2535) helps
improve thermal dissipation.
Copyright © 1999
Rev. 1.0 4/99
13
Share Link: 