LTC6820
applicaTions inForMaTion
Table 6. Typical RB1 and RB2 Values
MAX CABLE
LENGTH
TURNS
RATIO
TERMINATION
RESISTANCE
READY
I
V
V
V
R
R
B1
IDRV
20mA
10mA
20mA
10mA
CURRENT
B
A
TCMP
ICMP
B2
100m
50m
1 :1
1 :1
1 :1
1 :1
120Ω
120Ω
75Ω
1mA
0.5mA
1mA
1.2V
0.6V
0.3V
0.3V
0.6V
0.6V
604Ω
1.21k
374Ω
750Ω
1.4k
2.8k
4.7mA
3.2mA
4.7mA
3.2mA
100m
50m
0.75V
0.375V
0.19V
0.19V
0.38V
0.38V
1.62k
3.24k
75Ω
0.5mA
Transformer Selection Guide
Foroptimalcommonmodenoiserejection,chooseacenter-
tapped transformer or a transformer with an integrated
common mode choke. The center tap can be tied to a 27pF
or smaller capacitor (larger will restrict the driver’s ability
to set the common mode voltage). If the transformer has
bothacentertapandcommonmodechokeontheprimary
side, a larger capacitor may be used.
As shown in Figure 1, a transformer or a pair of transform-
ers are used to isolate the IP and IM signals between the
two LTC6820’s. The isoSPI signals have programmable
pulse amplitudes up to 1.6V, and pulse widths of 50ns
and 150ns. To meet these requirements, choose a trans-
former having a magnetizing inductance ranging from
50µH to 350µH, and a 1:1 or 2:1 turns ratio. Minimizing
transformer insertion loss will reduce required transmit
power; generally an insertion loss of less than –1.5dB is
recommended.
Table 7 shows a recommendedlist oftransformers for use
with the LTC6820. 10/100BaseTX Ethernet transformers
are inexpensive and work very well in this application.
Ethernet transformers often include a common mode
choke, which will improve common mode rejection as
compared to other transformers.
Table 7. Recommended Transformers
MANUFACTURER
PCA
PART NUMBER
EPF8119SE
ISOLATION VOLTAGE
1500VRMS
TURNS RATIO
CENTER TAP
CM CHOKE
1:1
1:1
1:1
1:1
2:1
1:1
1:1
Yes
Yes
No
Yes
Yes
No
Halo
TG110-AE050N5LF
PE-68386NL
78613/3C
1500VRMS
Pulse
1500V DC
Murata
Murata
Pulse
1000VRMS
Yes
No
No
78604/3C
1000VRMS
No
HX1188NL
1500VRMS
Yes
No
Yes
No
EPCOS
B82804A0354A110
1500V DC
2:1 Transformers
µC
LTC6820
MSTR
LTC2452
LTC6820
MSTR
2:1
1:2
IP
IP
MOSI
MISO
SCK
CS
SDO
MOSI
MISO
IM SCK
CS
480Ω
480Ω
SDI
SCK
CS
SDO
SCK
CS
IM
Single-Transformer Isolation
µC
LTC6820
MSTR
LTC6802
LTC6820
MSTR
IP
IP
MOSI
MISO
SCK
CS
SDO
SDI
SCK
CS
SDI
SDO
SCK
CS
MOSI
120Ω
120Ω
MISO
IM SCK
CS
IM
6820 F17
Figure 17. Alternative Isolation Barriers
6820f
20
Linear [ Linear ]
