LTC6820 datasheet(84/92 Pages) AD

84 / 92 page

LTC6811-1/LTC6811-2

Rev. C

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APPLICATIONS INFORMATION

Transformer Selection Guide

As shown in Figure 45, a transformer or pair of transform-

ers isolates the isoSPI signals between two isoSPI ports.

The isoSPI signals have programmable pulse amplitudes

up to 1.6VP-P and pulse widths of 50ns and 150ns. To be

able to transmit these pulses with the necessary fidelity

the system requires that the transformers have primary

inductances above 60µH and a 1:1 turns ratio. It is also

necessary to use a transformer with less than 2.5µH of

leakage inductance. In terms of pulse shape the primary

inductance will mostly affect the pulse droop of the 50ns

and 150ns pulses. If the primary inductance is too low,

the pulse amplitude will begin to droop and decay over

the pulse period. When the pulse droop is severe enough,

the effective pulse width seen by the receiver will drop

substantially, reducing noise margin. Some droop is ac-

ceptable as long as it is a relatively small percentage of

thetotalpulseamplitude.Theleakageinductanceprimarily

affects the rise and fall times of the pulses. Slower rise

and fall times will effectively reduce the pulse width. Pulse

width is determined by the receiver as the time the signal

is above the threshold set at the ICMP pin. Slow rise and

fall times cut into the timing margins. Generally it is best

to keep pulse edges as fast as possible. When evaluating

transformers, it is also worth noting the parallel winding

capacitance. While transformers have very good CMRR at

lowfrequency,thisrejectionwilldegradeathigherfrequen-

cies, largely due to the winding to winding capacitance.

When choosing a transformer, it is best to pick one with

less parallel winding capacitance when possible.

LTC6820 Datasheet(PDF) 84 Page - Analog Devices