LTC2845 Datasheet, PDF (14/20 Pages) Linear Technology – 3.3V Software-Selectable Multiprotocol Transceiver

LTC2845
APPLICATIONS INFORMATION
LTC2846
A
51.5Ω
V.35 DRIVER
S1
124Ω S2
51.5Ω
B
C
2845 F20
Figure 20. V.35 Driver
No-Cable Mode
The no-cable mode (M0=M1=M2=D4ENB=1, R4EN = 0)
is intended for the case when the cable is disconnected
from the connector. The bias circuitry, drivers and receiv-
ers are turned off, the driver outputs are forced into a high
impedance state, and the supply current drops to less than
700µA.
LTC2846 and LTC2847 Supplies
The LTC2846 and LTC2847 use an internal capacitive
charge pump to generate V_DD and V_EE as shown in Figure
21. A voltage doubler generates about 8V on V_DD and a
voltage inverter generates about – 7.5V for V_EE. Three 1µF
surface mounted tantalum or ceramic capacitors are re-
quired for C1, C2 and C3. The V_EE capacitor C4 should be
a minimum of 3.3µF. All capacitors are 16V and should be
placed as close as possible to the LTC2846 to reduce EMI.
The LTC2846 has an internal boost switching regulator
which generates a 5V output from the 3.3V supply as
shown in Figure 22. The 5V V_CC supplies its internal charge
pump and transceivers as well as its companion chip. The
LTC2847 requires an external 5V supply.
Receiver Fail-Safe
All LTC2846/LTC2845 receivers feature fail-safe opera-
tion in all modes. If the receiver inputs are left floating or
shorted together by a termination resistor, the receiver
output will always be forced to a logic high.
C3
1µF
5V
V_DD
C2⁺
C1⁺ _LTC2846 C2^–
C1
OR
1µF
C1^{– LTC2847} V_EE
V_CC
GND
C5
10µF
C2
1µF
C4
+ 3.3µF
2845 F21
Figure 21. Charge Pump
14
DTE vs DCE Operation
The DCE/DTE pin acts as an enable for Driver 3/Receiver␣ 1
in the LTC2846, and Driver 3/Receiver 1 in the LTC2845.
The LTC2846/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 LTC2846/LTC2845
using a dedicated DTE cable or dedicated DCE cable.
A dedicated DTE port using a DB-25 male connector is
shown in Figure 23. 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 24.
A port with one DB-25 connector, can be configured for
either DTE or DCE operation is shown in Figure 25. 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 Driver␣ 1
in the LTC2846. In DCE mode, Driver 1 now routes the RXD
signal to Pins 2 and 14.
Compliance Testing
The LTC2846/LTC2845 chipset has been tested by TUV
Rheinland of North America Inc. and passed the NET1,
NET2 and TBR2 requirements. Copies of the test report are
available from LTC or TUV Rheinland of North America Inc.
The title of the report is Test Report No.TBR2/050101/02
The address of TUV Rheinland of North America Inc. is:
TUV Rheinland of North America Inc.
1775, Old Highway 8 NW, Suite 107
St. Paul, MN 55112
Tel. [phone]
Fax [phone]
L1
5.6µH
D1
V_IN
3.3V
C6
V_IN
SW
R1
10µF
BOOST
13k
SWITCHING
REGULATOR
SHDN SHDN
FB
GND
R2
4.3k
C1,C2: TAIYO YUDEN X5R JMK316BJ106ML
D1: ON SEMICONDUCTOR MBR0520
L1: SUMIDA CR43-5R6
V_CC
5V
480mA
C5
10µF
2845 F22
Figure 22. LTC2846 Boost Switching Regulator
sn2845 2845fs