Applications
17
TPS3801/09 – Smallest SVS for Monitoring DSPs and Processors
5.4
Supervising a Customer Defined Threshold Voltage
Both families, TPS3801 and TPS3809, are available with four different fixed
threshold voltages, which are 2.25 V, 2.64 V, 2.93 V, and 4.55 V typically. If the
customer needs another threshold voltage, it can be adjusted with two external
resistors. The new threshold voltage has to be higher than the device specific
threshold voltage. There are two possibilities to realize the supervision. The first
possibility is explained in application 1, which is a solution with a higher accuracy
and a higher current consumption. Application 2 shows a solution with less
current consumption and less accuracy. The difference between both
applications is the resistance of the external voltage divider. Both application
examples supervise an adjusted threshold voltage of 3.6 V.
5.4.1
Threshold Voltage Change Application 1
In the following descriptions and calculations, the threshold voltage of V
in
is called
V
trip
. The fixed threshold voltages of the devices are named as V
IT
.
Resistors of the E96 series with a tolerance of 1% should be taken to achieve an
optimum accuracy for the threshold voltage.
A hundred times higher current through the voltage divider than the current into
the device has been chosen, so that the device current can be neglected. The
device current, which flows at V
DD
= V
IT
into the supervisor is taken, because this
current influences the threshold voltage of V
in
. For a TPS3809 I
DEVICE
at
V
DD
= 2.25 V is typically about 10
µ
A, what results in a current through the voltage
divider of I
Q
= 1 mA.
So R
4
and R
3
can be calculated as follows:
R
4
+
V
IT
I
Q
+
2.25
V
1
mA
+
2250
W
R
3
+
V
trip
–V
IT
I
Q
+
3.6
V–2.25 V
1
mA
+
1350
W
The following resistor values are from the E96 series. They are well-suited,
because they have nearly the same ratio like the calculated values.
R
3
+
1330
W
R
4
+
2210
W
The calculations lead to the application of Figure 21.
Summary of Contents for TPS3801
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