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參數(shù)資料
| 型號： | MC75174BDWG |
| 廠商： | ON Semiconductor |
| 文件頁數(shù)： | 13/13頁 |
| 文件大?。?/td> | 0K |
| 描述： | IC LINEDRVR QUAD EIA485 20-SOICW |
| 產(chǎn)品變化通告： | Product Obsolescence 08/Apr/2011 |
| 標(biāo)準(zhǔn)包裝： | 38 |
| 類型： | 線路驅(qū)動(dòng)器，發(fā)射器 |
| 驅(qū)動(dòng)器/接收器數(shù)： | 4/0 |
| 規(guī)程： | RS422，RS485 |
| 電源電壓： | 4.75 V ~ 5.25 V |
| 安裝類型： | 表面貼裝 |
| 封裝/外殼： | 20-SOIC（0.295"，7.50mm 寬） |
| 供應(yīng)商設(shè)備封裝： | 20-SOIC |
| 包裝： | 管件 |
MC75172B, MC75174B
http://onsemi.com
9
The
maximum
ambient
operating
temperature
(applicable to both EIA485 and EIA422A) is listed as
85
°C. However, a lower ambient may be required
depending on system use (i.e. specifically how many
drivers within a package are used) and at what current
levels they are operating. The maximum power which may
be dissipated within the package is determined by:
PDmax +
T
Jmax
–T
A
RqJA
RqJA = package thermal resistance (typical
70
°C/W for the DIP package, 85°C/W for
SOIC package);
TJmax = max. operating junction
temperature, and
TA = ambient temperature.
where:
Since the thermal shutdown feature has a trip point of
150
°C, ±20°C, TJmax is selected to be 130°C. The power
dissipated within the package is calculated from:
= {[(VCC VOH)
IOH] + VOL IOL)} each
driver = + (VCC
ICC)
VCC = the supply voltage;
VOH, VOL are measured or estimated from
Figures 7 to 10;
ICC = the quiescent power supply current
(typical 60 mA).
PD
where:
As indicated in the equation, the first term (in brackets)
must be calculated and summed for each of the four drivers,
while the last term is common to the entire package.
Example 1: TA = 25
°C, IOL = IOH = 55 mA for each
driver, VCC = 5.0 V, DIP package. How many drivers per
package can be used?
Maximum allowable power dissipation is:
PDmax +
130
°C * 25°C
70
°C W
+ 1.5 W
Since the power supply current of 60 mA dissipates
300 mW, that leaves 1.2 W (1.5 W 0.3 W) for the drivers.
From Figures 7 and 9, VOL
[1.75 V, and VOH [3.85 V.
The power dissipated in each driver is:
{(5.0 3.85)
0.055} + (1.75 0.055) = 160 mW.
Since each driver dissipates 160 mW, the four drivers per
package could be used in this application.
Example 2: TA = 85
°C, IOL = 27.8 mA, IOH = 20 mA for
each driver, VCC =5.0 V, SOIC package. How many drivers
per package can be used?
Maximum allowable power dissipation is:
PDmax +
130
°C * 85°C
85
°C W
+ 0.53 W
Since the power supply current of 60 mA dissipates
300 mW, that leaves 230 mW (530 mW 300 mW) for the
drivers. From Figures 8 and 10 (adjusted for VCC = 5.0 V),
VOL
[1.38 V, and VOH [4.27 V. The power dissipated
in each driver is:
{(5.0 4.27)
0.020} + (1.38 0.0278) = 53 mW
Since each driver dissipates 53 mW, the use of all four
drivers in a package would be marginal. Options include
reducing
the
load
current,
reducing
the
ambient
temperature, and/or providing a heat sink.
System Requirements
EIA485 requires each driver to be capable of
transmitting data differentially to at least 32 unit loads, plus
an equivalent DC termination resistance of 60
W, over a
common mode voltage of 7.0 to 12 V. A unit load (U.L.),
as defined by EIA485, is shown in Figure 17.
Figure 17. Unit Load Definition
1.0 mA
V
5.0 V
3.0 V
7.0 V
12 V
0.8 mA
Reprinted from EIA485, Electronic Industries Association,
Washington,DC.
I
A load current within the shaded regions represents an
impedance of less than one U.L., while a load current of a
magnitude outside the shaded area is greater than one U.L.
A system’s total load is the sum of the unit load equivalents
of each receiver’s input current, and each disabled driver’s
output leakage current. The 60
W termination resistance
mentioned above allows for two 120
W terminating
resistors.
Using the EIA485 requirements (worst case limits), and
the graphs of Figures 7 and 9, it can be determined that the
maximum current an MC75172B or MC75174B driver will
source or sink is
[65 mA.
System Example
An example of a typical EIA485 system is shown in
Figure 18. In this example, it is assumed each receiver’s
input characteristics correspond to 1.0 U.L. as defined in
Figure 17. Each “off” driver, with a maximum leakage of
±50 mA over the common mode range, presents a load of
[0.06 U.L. The total load for the active driver is therefore
8.3 unit loads, plus the parallel combination of the two
terminating resistors (60
W). It is up to the system software
to control the driver Enable pins to ensure that only one
driver is active at any time.
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