STLC3080 datasheet(16/23 Pages) STMICROELECTRONICS

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STLC3080: allowed Rfeed values vs. Ilim

The STLC3080 device has been designed in or-

der to fit in a small SMD package (TQFP44). This

target has been achieved by using a dedicated

circuit for power management based on one ex-

ternal transistor (Qext).

The particular power management circuit adopted

allows to define the percentage of power dissi-

pated on the SLIC itself and on the Qext. The

sharing percentage is defined by the Rfeed value,

in particular the higher is Rfeed, the higher is the

percentage dissipated on the SLIC.

Rfeed represents the DC feeding impedance at

TIP/RING terminals (including 2xRp) when the

SLIC is in the resistive feed region of the DC

characteristics.

Since the max. power dissipation inside the SLIC

is limited it is important to know which value of

Rfeed can be implemented without exceeding the

max power allowed in the SLIC.

In order to define the allowed Rfeed values sev-

eral other parameters should be considered, in

particular:

Pdslic:

Max allowed power dissipation on SLIC, two val-

ues are considered:

1.1W for 70

°CTamb application;

0.9W for 85

°CTamb application;

Pdqext:

Max allowed power dissipation on Qext, three val-

ues are considered:

1.0W

1.5W

2.0W

These values depend on the package and the as-

sembly of the Qext.

Ilim:

Programmed constant current value, continuous

variations are considered from 20mA to 50mA.

Vbat:

Battery voltage, three values are considered:

48V

54V

62V

The following diagrams show the allowed Rfeed

values depending on the above parameters. three

diagrams are shown each one for a particular bat-

tery (Vbat = -48V, -54V, -60V).

In each diagrams you can find an upper and a

lower limits for the Rfeed value:

The upper limit is defined by one of the two b1, b2

curves.

b1 is the limit when max. power on SLIC is

equal to 0.9W (Tamb =85

°C)

b2 is the limit when max. power on SLIC is

equal to 1.1W (Tamb = 70

°C)

The lower limit is defined by one of the three a1,

a2, a3 curves.

a1 is the limit when max. power allowed on

Qext is equal to 1.0W

a2 is the limit when max. power allowed on

Qext is equal to 1.5W

a3 is the limit when max. power allowed on

Qext is equal to 2.0W

Figure B1. Rfeed allowed values vs. Ilim (Vbat = -48V).

APPENDIX B

STLC3080

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