AN1714 datasheet(23/46 Pages) STMICROELECTRONICS

23 / 46 page

AN1714

Demonstration board for ST7538Q

23/46

In the demonstration board it is possible to link the feedback signal (top of R12 resistor) to

the ATOP1 or to the ATOP2 pin through jumper J36. The choice of the feedback connection

point depends on the network coupling circuit topology.

If a big noise coming from the mains is present disturbing the voltage control loop, a

possible solution is to connect the feedback to the ATO pin. In this case the output signal

has half the value with respect to the ATOP pins, so the R12 resistor has a half value (or the

R14 resistor has to be doubled).

In the demonstration board it is possible to change the output signal voltage level acting on

the R12 trimmer. Table 2 gives the values of the trimmer to assume some standard output

values.

The resistor connected to the CL pin (the trimmer R13 in the demonstration board) has the

function to program the current protection threshold. The capacitor C37 in parallel to the

resistor has a filtering function for noise and spikes.

A mirrored current (ratio 1:5000) of the p channel power Mos of the powerline interface of

the device (both ATOP1 and ATOP2) is present on the CL pin. The voltage on the CL pin is

proportional to the output current and to the resistor connected to the pin.

The peak value of this voltage is compared with an internal reference of the device. If the

signal overcomes the threshold, the loop acts on the VCA reducing the transmitted signal

and therefore the output current.

The resistor value determines the output signal that the interface is able to supply. In

conjunction with the programmed output voltage V0 the maximum current level fixes the

minimum value of driving impedances (Z0).

Table 2.

Voltage regulation loop (divider and R12 resistors values)

Vout (Vrms)

(1)

The regulated Vout voltage is the point linked to the voltage feedback divider (top of R12).

Vout (dBuV)

(R14+R12)/R14

R12 (KΩ)

(2)

The rate of R14 takes in account the input resistance on the VSENSE pin (36 KΩ). The decoupling capacitor

(C17) has been neglected.

0.150

103.5

1.1

0.1

0.250

108.0

1.9

0.9

0.350

110.9

2.7

1.7

0.500

114.0

3.7

2.6

0.625

115.9

4.7

3.6

0.750

117.5

5.8

4.7

0.875

118.8

6.6

5.4

1.000

120.0

7.6

6.4

1.250

121.9

9.5

8.3

1.500

123.5

10.8

9.5

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