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LTC6820

isoSPI Isolated Communications Interface

Linear Technology 

LTC6820

Operation

25

20

15

10

5

VDD = 3V

IS = 1mA

0

0 0.5 1 1.5 2 2.5 3

PULSE AMPLITUDE (V)

6820 F10

Figure 10. AIB Current Gain vs Amplitude

This type of driver does not require a center-tapped

transformer, but such a transformer may improve noise

immunity, especially if it has a common mode choke. See

the Applications Information section for additional details.

Receiver Common Mode Bias

When not transmitting, the output driver maintains IP

and IM near VDD with a pair of 35k (RIN) resistors to a

voltage of VDD – VICMP/3 – 167mV. This weak bias net-

work holds the outputs near their desired operating point

without significantly loading the cable, which allows a large

number of LTC6820’s to be paralleled without affecting

signal amplitude.

Figure 11 shows the differential and single-ended IP and

IM signals while transmitting and receiving data. The

driver forces the common mode voltage it needs while

transmitting, then it returns to the bias level with a time

constant of RIN • CLOAD/2, where CLOAD is the sum of the

capacitance at the IP and IM pins.

When the LTC6820 is in low power IDLE mode, the bias

voltage is disconnected from the 35k resistors, resulting

in a 70k differential load.

State Diagram

During periods of no communication, a low current IDLE

(or shutdown) state is available to reduce power. In the

IDLE state the LTC6820 shuts down most of the circuitry.

A slave device uses a low current comparator to monitor

for activity, so it has larger IDLE current.

16

3.0

TRANSMIT SHORT +1

2.5

2.0

IP IM

1.5

RECEIVE SHORT –1

1.0

0.5

IP-IM

0

–0.5

–1.0

–1.5

0

VDD = 3V

IB = 1mA

200 400 600 800 1000

TIME (ns)

6820 F11

Figure 11. Transmitting and Receiving Data

IDLE

TIMEOUT

(tIDLE)

IDLE

WAKE-UP SIGNAL

(tREADY)

READY

NO ACTIVITY

ON isoSPI

PORT

TRANSMIT/RECEIVE

ACTIVE

6820 F12

Figure 12. State Diagram

In the READY state all circuitry is enabled and ready to

transmit or receive, but is not actively transmitting on IP

and IM.

Supply current increases when actively communicating,

so this condition is referred to as the ACTIVE state.

Supply Current

Table 5 provides equations for estimating IDD in each state.

The results are for average supply current (as opposed

to peak currents), and make the assumption that a slave

is returning an equal number of 0s and 1s (significant

because the slave doesn’t generate +1 data pulses, so the

average driver current is smaller).

6820f

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