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| 型号： | LX1681CDM |
| 元件分类： | 稳压器 |
| 英文描述： | 1 A SWITCHING CONTROLLER, 230 kHz SWITCHING FREQ-MAX, PDSO8 |
| 封装： | PLASTIC, SOIC-8 |
| 文件页数： | 7/9页 |
| 文件大小： | 145K |
| 代理商： | LX1681CDM |
V O L T AGE -M ODE PWM C ONTROLLERS
LX1681/1682
PRODUCT DA T ABOOK 1996/1997
7
Copyright 1999
Rev. 1.0 5/99
P RODUCTION D AT A S HEET
APPLICA TION INFORMA TION
SOFT-START CAPACITOR (continued)
capacitor current plus the load current. The soft-start capacitor
should be selected so that the overall inductor current does not
exceed it maximum.
The capacitor current to follow the SS-pin voltage is:
where C
OUT is the output capacitance. The typical value of CSS
should be in the range of 0.1 to 0.2F.
During the soft-start interval the load current from a micro-
processor is negligible; therefore, the capacitor current is ap-
proximately the required inductor current.
OVER-CURRENT PROTECTION
Current limiting occurs at current level I
CL, when the voltage
detected by the current sense comparator is greater than the
current sense comparator threshold, V
TRIP (400mV).
I
CL * RDS(ON) + ISET * RSET = VTRIP
So,
R
SET =
=
Example:
For 10A current limit, using IRL3303 MOSFET (26m
R
DS(ON)):
R
SET =
= 3.1k
Current Sensing Using Sense Resistor
The method of current sensing using the R
DS(ON) of the upper
MOSFET is economical, but can have a large tolerance, since the
R
DS(ON) can vary with temperature, etc.
A more accurate alterna-
tive is to use an external sense resistor (R
SENSE).
Since one input
to the current sense comparator is the supply voltage to the IC
(V
CC -
pin 8), the sense resistor 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) with RSENSE.
Example:
For 10A current limit, using a 5m
sense resistor:
R
SET =
=
= 7.8k
V
TRIP - ICL * RDS(ON)
I
SET
I
Cout = COUT
=
* e
-(t/RssCss )
dV
dt
C
OUT
C
SS
400mV - I
CL * RDS(ON)
45A
0.4 - 10 * 0.026
45 * 10
-6
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
FB) which 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).
V
OUT = VREF (1 + R1 /R2 )
Note: Keep R
1 and R2 close to 100 (order of magnitude).
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
DS(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 R
DS(ON) will
be, since more die area is available.
Device
R
DS(ON) @
I
D @
Max. Break-
10V (m
)T
C = 100°C
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
TABLE 1 - FET Selection Guide
This table gives selection of suitable FETs from International Rectifier.
All devices in TO-220 package. For surface mount devices (TO-263 /
D2-Pak), add 'S' to part number, e.g. IRL3103S.
Heat Dissipated In Upper MOSFET
The heat dissipated in the top MOSFET will be:
P
D = (I
2 * R
DS(ON) * Duty Cycle) + (0.5 * I * VIN * tSW * fS )
Where t
SW is switching transition line for body diode (~100ns)
and f
S is the switching frequency.
For the IRL3102 (13m
R
DS(ON)), converting 5V to 2.0V at 15A
will result in typical heat dissipation of 1.92W.
V
TRIP - (ICL * RSENSE )
I
SET
0.4 - 10 * 0.005
45 x 10-6
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