ST ST7538Q, Application Note

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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 C
P
= C29(C27+C28)/(C27+C28+C29) and L
P
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.
P P
C
C L 2
1
f
⋅ ⋅ π
=