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 V
IN
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 V
CC
. V
DD
and V
EE
are in turn
produced from V
CC
with an internal charge pump at
approximately 80% and 70% efficiency respectively. Cur-
rent drawn internally from V
DD
or V
EE
translates directly
into a higher I
CC
. The LTC2847 dissipates power accord-
ing to the equation:
P
DISS(2847)
= V
CC
• I
CC
– N
D
• P
RT
+ N
R
• P
RT
(1)
P
RT
refers to the power dissipated by each driver in a
receiver termination on the far end of the cable while N
D
is the number of drivers. Conversely, current from the
far end drivers dissipate power N
R
• P
RT
in the internal
receiver termination where N
R
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, I
CC
at no load = 14mA, I
CC
at full load =
100mA. Each receiver termination is 100Ω (R
RT
) and
current going into each receiver termination = (100mA –
14mA)/3 = 28.7mA (I
RT
).
P
RT
= (I
RT
)
2
• R
RT
(2)
From Equation (2), P
RT
= 82.4mW and from Equation (1),
DC power dissipation P
DISS(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
I
CC
vs Data Rate,” the I
CC
at 10MBd is 160mA. I
CC
increases with baud rate due to driver transient dissipa-
tion. From Equation (1), AC power dissipation P
DISS(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 V
CC
, it uses the V
DD
and V
EE
outputs from the
LTC2847. Negligible power is dissipated in the large
internal receiver termination of the LTC2845 so the N
R
•
P
RT
term of Equation (1) can be omitted. Thus Equation (1)
is modified as follows:
P
DISS(2845)
= (V
CC
• I
CC
) + (V
DD
• I
DD
)
+ (V
EE
• I
EE
) – N
D
• P
RT
(3)
Since power is drawn from the supplies of the LTC2847
(V
DD
and V
EE
) at less than 100% efficiency, the LTC2847
dissipates extra power to source P
DISS(2845)
and P
RT
:
P
DISS1(2847)
= 125% • (V
DD
• I
DD
) + 143% • (4)
(V
EE
• I
EE
) – P
DISS(2845)
– N
D
• P
RT
= 25% • (V
DD
• I
DD
) + 43% • (V
EE
• I
EE
)
From the LTC2845 Electrical Characteristics Table, for
V
CC
= 5V, V
DD
= 8V and V
EE
= –5.5V:
I
CC
at no load 2.7mA
I
CC
at full load with all drivers high 110mA
I
EE
at no load 2mA
I
EE
at full load with both V.10 drivers low 23mA
I
DD
at no load 0.3mA
I
DD
at full load 0.3mA
