AN47
Si321
X
L
I N E F E E D
P
O W E R
M
O N I T O R I N G
Introduction
The Silicon Laboratories’ ProSLIC products are
designed to continuously monitor the power dissipated
in each of the six external bipolar transistors in the
linefeed circuit. These power measurement results are
available to the user in software registers and are also
used by the ProSLIC to protect linefeed transistors from
damage due to overpower conditions. Using proper
power threshold and thermal low-pass filter settings, the
ProSLIC will either alert the user or automatically
transition the open state in the event of an overpower
condition.
The thermal resistance (R
THJA
) of the transistor is
improved when it is mounted on a PCB board. This
improvement depends on the PCB size, the material it is
made of, and the amount of the copper surface on the
PCB board. Figure 1 illustrates how the board material,
available board area, and the amount of copper present
influence the thermal resistance of the transistors. This
chart can be obtained from the transistor manufacturer if
not included in the transistor data sheet.
AND
P
ROTECTION
As the dissipated power in linefeed transistors
increases, so does the junction temperature of the
transistor die. The maximum admissible junction
temperature must not be exceeded because this could
damage or destroy the transistor die. In the Si321x, the
measured power consumed in each of the transistors is
compared to the power threshold values in the
corresponding indirect registers. If the power in any
external transistor exceeds the programmed threshold
(after passing through a user-programmable low pass
filter which will be explained in the next section), a
power alarm is triggered to indicate line fault condition.
Unless the auto-open feature is disabled (direct
register 67, bit 0), the ProSLIC automatically goes into
the open state.
The value of the power threshold is calculated based on
the characteristic of the transistors used. Transistor
manufacturers provide this information in terms of
thermal resistance for each transistor package. The
relationship
between
the
maximum
junction
temperature and the maximum power that can be
dissipated by the transistor package is defined in the
following equation:
T
JMAX
=
T
AMB
+
P
MAX
×
R
THJA
Thermal Resistance (°C/W)
Power Threshold
450
300
FR4 Min. Copper
SRBP Min. Copper
SRBP Max. Copper
150
FR4 Max. Copper
0
0.01
0.1
1
10
P.C.B. Area (Sq.Ins.)
Thermal Resistance v P.C.B. Area
Figure 1. SOT23
In practice, the transistors are normally mounted on a
PCB with several square inches area, but for illustration
purposes consider a model in which the transistor
package is mounted on 1-inch square of FR4 PCB with
0.25-inch square of copper surface. This 1-inch square
PCB model and the thermal resistance vs. PCB area
charts provide the practical thermal resistances for the
following transistor packages:
SOT23: R
THJA
= 200 °C/W
The thermal resistance can also be obtained from the
transient thermal resistance curve with D = 1 as shown
in Figure 2 and Figure 3.
SOT89: R
THJA
= 82.5 °C/W
SOT223: R
THJA
= 62.5 °C/W
where T
JMAX
is the maximum junction temperature
(usually 150 °C), T
AMB
is the ambient temperature
(70 °C for commercial rating), and P
MAX
is the
maximum power allowance on the transistor package.
R
THJA
is the junction to ambient thermal resistance of
the transistor package.
Rev. 0.2 9/02
Copyright © 2002 by Silicon Laboratories
AN47-DS02
SILABS [ SILICON LABORATORIES ]
