AN2744 datasheet(21/56 Pages) STMICROELECTRONICS

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AN2744

Board description

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on frequency response of the filters. In simulation curves, the ideal response is drawn in

blue, while red curves indicate statistical variations generated through simulation.

5.1.1

Dual channel selection

To obtain a dual channel interface, each filter is tunable via software command. The

ST7538Q dual channel reference design has two available channel frequencies, 72 and

86 kHz.

The channel can be simply changed by including or excluding one passive component per

each filter: an inductor for the Tx filter, a capacitor for the Rx passive filter and a resistor for

the Rx active filter.

5.1.2

Dual channel Tx passive filter

The dual channel Tx passive filter is made of the following parts: DC-decoupling capacitor

C29, line transformer T2, inductors L5 and L6 and X2 safety capacitor C28, plus a shunt

branch made of R21 and C27.

The center frequency for the series resonance is calculated with good approximation as:

Equation 1

where CP = C29(C27+C28)/(C27+C28+C29) and LP is equal to: L6 for 72 kHz channel, L6 //

L5 for 86 kHz channel. To guarantee adequate filtering action on signal harmonics, it is

required to set the center frequency at a value lower than the one of the channel frequency.

See Figure 10 and Figure 11 to check the resulting measured frequency response.

L5 and L6 have been accurately chosen to have high saturation current (> 1 A) and low

equivalent series resistance (< 0.5

Ω), to limit distortion and insertion losses.

R21 is intended to damp resonance, to better control filtering action and getting desired

rejection on transmitted signal harmonics.

Resonance shape is also affected by the ratio between the two capacitors C27 and C28.

C27 must be smaller than C28 (in this case, about half the value of C28) to get lower

insertion losses when the line impedance is very low.

To optimize the coupling efficiency, particular attention must be paid to the line transformer.

The required characteristics are listed in Table 5.

In order to have a good power transfer and to minimize the insertion losses, it's

recommended to choose a transformer with a primary (magnetizing) inductance greater

than 1mH and a series resistance lower than 0.5

Ω.

Another important parameter is the leakage inductance. If it has a relevant value (10 to 50

µH), this can be used to design the coupling filter without adding series inductance (L5, L6).

The drawback is that this parameter is usually affected by poor accuracy which can lead to a

drift on the filter response and then to bad coupling.

Consequently, a low leakage inductance value (<1 µH) has been chosen. The series

inductance is fixed through discrete components, resulting in a greater accuracy.

The last parameter specified in the table, the 4 kV insulation voltage requirement, is

described and codified by the EN50065-4-2 CENELEC document.

⋅

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