Functional description

STLC3085

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3.2.5

Layout recommendation

A properly designed PCB layout is a basic issue to guarantee a correct behaviour and good

noise performances.

Particular care must be taken on the ground connection and in this case the star configuration

allows surely to avoid possible problems (see Application Diagram Figure 7 and Figure 8).

The ground of the power supply (VPOS) has to be connected to the center of the star, let’s call

this point SYSTEM-GND. This point should show a resistance as low as possible, that means it

should be a ground plane.

In particular to avoid noise problems the layout should prevent any coupling between the DC/

DC converter components and analog pins that are referred to AGND (ex: RD, IREF, RTH,

RLIM, VF). As a first reccomendation the components CV, L, T1, D1, CVPOS, RSENSE should

be kept as close as possible to each other and isolated from the other components.

Additional improvements can be obtained:

●

decoupling the center of the star from the analog ground of STLC3085 using small chokes.

●

adding a capacitor in the range of 100nF between VPOS and AGND in order to filter the

switch frequency on VPOS.

3.2.6

External components list

In order to properly define the external components value the following system parameters

have to be defined:

●

The AC input impedance shown by the SLIC at the line terminals "Zs" to which the return

loss measurement is referred. It can be real (typ. 600

Ω) or complex.

●

The AC balance impedance, it is the equivalent impedance of the line "Zl" used for

evaluation of the trans-hybrid loss performances (2/4 wire conversion). It is usually a

complex impedance.

●

The value of the two protection resistors Rp in series with the line termination.

●

The slope of the ringing waveform "

∆V

●

The value of the constant current limit current "Ilim".

●

●

●

is 64V). This value can be obtained from the desired ring peak level + 5V.

●

The value of the maximum current peak drawn from Vpos "IPK".

Table 10.

External Components

Name

Function

Formula

Typ. Value

BUCKBOOST CONFIGURATION

RREF

Bias setting current

RREF = 1.3/Ibias

Ibias = 50

µA

26k

Ω 1%

CSVR

Negative Battery Filter

CSVR = 1/(2

π ⋅ fp ⋅ 1.8MΩ)

fp = 50Hz

1.5nF 10%

100V