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| 型号： | LTC2847IUHF#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 7/20页 |
| 文件大小： | 0K |
| 描述： | IC TXRX MULTIPROTOCOL 38-QFN |
| 标准包装： | 52 |
| 类型： | 收发器 |
| 驱动器/接收器数： | 3/3 |
| 规程： | 多协议 |
| 电源电压： | 5V |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 38-WFQFN 裸露焊盘 |
| 供应商设备封装： | 38-QFN（5x7） |
| 包装： | 管件 |
| 产品目录页面： | 1355 (CN2011-ZH PDF) |
15
LTC2847
sn2847 2847fs
TYPICAL APPLICATIO S
U
DTE vs DCE Operation
The DCE/DTE pin acts as an enable for Driver 3/Receiver 1
in the LTC2847, and Driver 3/Receiver 1 in the LTC2845.
The LTC2847/LTC2845 can be configured for either DTE
or DCE operation in one of two ways: a dedicated DTE or
DCE port with a connector of appropriate gender or a port
with one connector that can be configured for DTE or DCE
operation by rerouting the signals to the LTC2847/LTC2845
using a dedicated DTE cable or dedicated DCE cable.
A dedicated DTE port using a DB-25 male connector is
shown in Figure 29. The interface mode is selected by logic
outputs from the controller or from jumpers to either VIN
or GND on the mode select pins. A dedicated DCE port
using a DB-25 female connector is shown in Figure 30.
A port with one DB-25 connector, that can be configured
for either DTE or DCE operation is shown in Figure 31. The
configuration requires separate cables for proper signal
routing in DTE or DCE operation. For example, in DTE
mode, the TXD signal is routed to Pins 2 and 14 via the
LTC2847’s Driver 1. In DCE mode, Driver 1 now routes the
RXD signal to Pins 2 and 14.
Power Dissipation Calculations
The LTC2847 takes in 5V VCC. VDD and VEE are in turn
produced from VCC with an internal charge pump at
approximately 80% and 70% efficiency respectively. Cur-
rent drawn internally from VDD or VEE translates directly
into a higher ICC. The LTC2847 dissipates power accord-
ing to the equation:
PDISS(2847) = VCC ICC – ND PRT + NR PRT
(1)
PRT refers to the power dissipated by each driver in a
receiver termination on the far end of the cable while ND
is the number of drivers. Conversely, current from the
far end drivers dissipate power NR PRT in the internal
receiver termination where NR is the number of receiv-
ers.
LTC2847 Power Dissipation
Consider an LTC2847 in X.21, DCE mode (three V.11
drivers and two V.11 receivers). From the Electrical Char-
acteristics Table, ICC at no load = 14mA, ICC at full load =
100mA. Each receiver termination is 100
(RRT) and
current going into each receiver termination = (100mA –
14mA)/3 = 28.7mA (IRT).
PRT = (IRT)2 RRT
(2)
From Equation (2), PRT = 82.4mW and from Equation (1),
DC power dissipation PDISS(2847) = 5V 100mA – 3
82.4mW + 2 82.4mW = 418mW.
Consider the above example running at a baud rate of
10MBd. From the Typical Characteristic for “V.11 Mode
ICC vs Data Rate,” the ICC at 10MBd is 160mA. ICC
increases with baud rate due to driver transient dissipa-
tion. From Equation (1), AC power dissipation PDISS(2847)
= 5V 160mA –3 82.4mW + 2 82.4mW = 718mW.
LTC2845 Power Dissipation
If a LTC2845 is used to form a complete DCE port with the
LTC2847, it will be running in the X.21 mode (three V.11
drivers and two V.10 drivers, two V.11 receivers and two
V.10 receivers, all with internal 30k termination). In addi-
tion to VCC, it uses the VDD and VEE outputs from the
LTC2847. Negligible power is dissipated in the large
internal receiver termination of the LTC2845 so the NR
PRT term of Equation (1) can be omitted. Thus Equation (1)
is modified as follows:
PDISS(2845) = (VCC ICC) + (VDD IDD)
+ (VEE IEE) – ND PRT
(3)
Since power is drawn from the supplies of the LTC2847
(VDD and VEE) at less than 100% efficiency, the LTC2847
dissipates extra power to source PDISS(2845) and PRT :
PDISS1(2847) = 125% (VDD IDD) + 143%
(4)
(VEE IEE) – PDISS(2845) – ND PRT
= 25% (VDD IDD) + 43% (VEE IEE)
From the LTC2845 Electrical Characteristics Table, for
VCC = 5V, VDD = 8V and VEE = – 5.5V:
ICC at no load
2.7mA
ICC at full load with all drivers high
110mA
IEE at no load
2mA
IEE at full load with both V.10 drivers low
23mA
IDD at no load
0.3mA
IDD at full load
0.3mA
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