Si3220/Si3225
Transistor Power Equations
(Using Discrete Transistors)
Power Filter and Alarms
The power calculated during each A/D sample period
When using the Si3220 or Si3225 along with discrete must be filtered before being compared to a user-
bipolar transistors, it is possible to control the total programmable maximum power threshold. A simple
power of the solution by individually regulating the digital low-pass filter is used to approximate the
power in each discrete transistor. Figure 18 illustrates transient thermal behavior of the package, with the
the basic transistor-based linefeed circuit for one output of the filter representing the effective peak power
channel. The power dissipation of each external within the package or, equivalently, the peak junction
transistor is estimated based on the A/D sample values. temperature.
The approximate power equations for each external
BJT are as follows:
For Q1, Q2, Q3, and Q4 in SOT23 and Q5 and Q6 in
SOT223 packages, the settings for thermal low-pass
filter poles and power threshold settings are (for an
ambient temperature of 70 °C) calculated as follows: If
P
P
P
P
P
P
≅ V
≅ V
≅ V
≅ V
≅ V
≅ V
x I ≅ (|V | + 0.75 V) x (I
)
Q1
Q2
Q3
Q4
Q5
Q6
CE1
CE2
CE3
CE4
CE5
CE6
Q1
TIP
Q1
x I ≅ (|V
| + 0.75 V) x (I
)
)
)
Q2
RING
Q2
the thermal time constant of the package is τ
, the
thermal
x I ≅ (|V | – R7 x I ) x (I
Q3
BAT
Q5
Q3
decimal values of RAM locations PLPF12, PLPF34, and
PLPF56 are given by rounding to the next integer the
value given by the following equation:
x I ≅ (|V | – R6 x I ) x (I
Q4
BAT
Q6
Q4
x I ≅ (|V | – |V
| – R7 x I ) x (I
)
Q5
BAT
RING
Q5
Q5
x I ≅ (|V | – |V | – R6 x I ) x (I )
Q6
BAT
TIP
Q6
Q6
4096
3
------------------------------------
PLPFxx (decimal value) =
× 2
The maximum power threshold for each device is
software-programmable and should be set based on the
characteristics of the transistor package, PCB design,
and available airflow. If the peak power exceeds the
programmed threshold for any device, the power-alarm
bit is set for that device. Each external bipolar has its
own register bit (PQ1S–PQ6S bits of the IRQVEC3
register), which goes high on a rising edge of the
comparator output and remains high until the user
clears it. Each transistor power alarm bit is also
maskable by setting the PQ1E–PQ6E bits in the
IRQEN3 register.
800 × τthermal
Where 4096 is the maximum value of the 12-bit plus
sign RAM locations PLPF12, PLPF34, and PLPF56,
and 800 is the power calculation clock rate in Hz. The
equation is an excellent approximation of the exact
equation for τ
= 1.25 ms … 5.12 s. With the
thermal
above equations in mind, example values of the RAM
locations, PTH12, PTH34, PTH56, PLPF12, PLPF34,
and PLPF56 are as follows:
PTH12 = power threshold for Q1, Q2 = 0.3 W (0x25A)
PTH34 = power threshold for Q3, Q4 = 0.22 W
(0x1B5E)
Si3200 Power Calculation
PTH56 = power threshold for Q5, Q6 = 1 W (0x7D8)
When using the Si3200, it is also possible to detect the
thermal conditions of the linefeed circuit by calculating
the total power dissipated within the Si3200. This case
is similar to the transistor power equations case, with
the exception that the total power from all transistor
devices is dissipated within the same package
enclosure, and the total power result is placed in the
PSUM RAM location. The power calculation is derived
using the following set of equations:
PLPF12 = Q1/Q2 thermal LPF pole = 0x0012
(for SOT–89 package)
PLPF34 = Q3/Q4 thermal LPF pole = 0x008C
(for SOT–23 package)
PLPF56 = Q5/Q6 thermal LPF pole = 0x000E
(for SOT–223 package)
In the case where the Si3200 is used, thermal filtering
needs to be performed only on the total power reflected
in the PSUM RAM location. When the filter output
exceeds the total power threshold, an interrupt is
issued. The PTH12 RAM location is used to preset the
total power threshold for the Si3200, and the PLPF12
RAM location is used to preset the thermal low-pass
filter pole.
P
P
P
P
P
P
≅ (|V | + 0.75 V) x I
Q1
Q2
Q3
Q4
Q5
Q6
TIP Q1
≅ (|V
≅ (|V
| + 0.75 V) x I
Q2
|+ 0.75 V) x I
RING
BAT
Q3
≅ (|V | + 0.75 V) x I
BAT
Q4
≅ (|V | – |V
|) x I
BAT
RING
Q5
≅ (|V | – |V |) x I
BAT
TIP
Q6
PSUM = total dissipated power = P
Q4
+ P
+ P
+
Q3
Q1
Q2
P
+ P + P
Q5 Q6
Note: The Si3200 THERM pin must be connected to the
THERM a/b pin of the Si3220/Si3225 in order for the
Si3200 power calculation method to work correctly.
Rev. 1.0
37
SILICONIMAGE [ Silicon image ]
