Linear Technology LTC6804-1, Руководство пользователя

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LTC6804-1/LTC6804-2
68
680412fc
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applicaTions inForMaTion
The following guidelines should be used when setting the
bias current (100µA to 1mA) I
B
and the receiver compara-
tor threshold voltage V
ICMP
/2:
R
M
= Transmission Line Characteristic Impedance Z
0
Signal Amplitude V
A
= (20 • I
B
) • (R
M
/2)
V
TCMP
(Receiver Comparator Threshold) = K • V
A
V
ICMP
(voltage on ICMP pin) = 2 • V
TCMP
R
B2
= V
ICMP
/I
B
R
B1
= (2/I
B
) – R
B2
Select I
B
and K (Signal Amplitude V
A
to Receiver Compara-
tor Threshold ratio) according to the application:
For lower power links: I
B
= 0.5mA and K = 0.5
For full power links: I
B
= 1mA and K = 0.5
For long links (>50m): I
B
= 1mA and K = 0.25
For addressable multi-drop: I
B
= 1mA and K = 0.4
For applications with little system noise, setting I
B
to 0.5mA
is a good compromise between power consumption and
noise immunity. Using this I
B
setting with a 1:1 transformer
and R
M
= 100Ω, R
B1
should be set to 3.01k and R
B2
set
to 1k. With typical CAT5 twisted pair, these settings will
allow for communication up to 50m. For applications in
very noisy environments or that require cables longer than
50m it is recommended to increase I
B
to 1mA. Higher drive
current compensates for the increased insertion loss in
the cable and provides high noise immunity. When using
cables over 50m and a transformer with a 1:1 turns ratio
and R
M
= 100Ω, R
B1
would be 1.5k and R
B2
would be 499Ω.
R
M
IPA ISOMD V
REG
IMA
+
–
+
–
IBIAS
ICMP
680412 F41
LTC6804
R
M
R
B1
R
B2
R
B1
R
B2
IPB ISOMD
IMB
IBIAS
ICMP
MOSI
MISO
SCK
CS
SDO
SDI
SCK
CS
LTC6804
TWISTED-PAIR CABLE
WITH CHARACTERISTIC IMPEDANCE R
M
ISOLATION BARRIER
(MAY USE ONE OR TWO TRANFORMERS)
MASTER
• • • •
2V
V
A
2V
V
A
Figure 41. isoSPI Circuit
The maximum clock rate of an isoSPI link is determined
by the length of the isoSPI cable. For cables 10 meters
or less, the maximum 1MHz SPI clock frequency is pos-
sible. As the length of the cable increases, the maximum
possible SPI clock rate decreases. This dependence is a
result of the increased propagation delays that can cre-
ate possible timing violations. Figure 42 shows how the
maximum data rate reduces as the cable length increases
when using a CAT5 twisted pair.
Cable delay affects three timing specifications: t
CLK
, t
6
and t
7
. In the Electrical Characteristics table, each of these
specifications is de-rated by 100ns to allow for 50ns of
cable delay. For longer cables, the minimum timing pa-
rameters may be calculated as shown below:
t
CLK
, t
6
and t
7
> 0.9μs + 2 • t
CABLE
(0.2m per ns)
CABLE LENGTH (METERS)
1
DA
TA RA
TE (Mbps)
1.2
0.8
0.4
0.2
1.0
0.6
0
10
680412 F42
100
CAT5 ASSUMED
Figure 42. Data Rate vs Cable Length