Texas Instruments TPS3801, Отчет о приложении

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Applications
19 TPS3801/09 – Smallest SVS for Monitoring DSPs and Processors
5.4.2 Worst Case Considerations for Application 1
The following assumptions have been made:
Resistors with 1% tolerance are used and the device current can be
neglected.
PARAMETER MINIMUM TYPICAL MAXIMUM
R3 1317
Ω
1330
Ω
1343
Ω
R4 2188
Ω
2210
Ω
2232
Ω
VIT– 2.20 V 2.25 V 2.30 V
Vtrip 3.50 V 3.60 V 3.71 V
Worst case 1:
V
trip min
+
R
3 min
)
R
4 max
R
4 max
V
IT min
+
1317
W )
2232
W
2232
W
2.20
V
+
3.50
V
Worst case 2:
V
trip max
+
R
3 max
)
R
4
min
R
4
min
V
IT max
+
1343
W )
2188
W
2188
W
2.30
V
+
3.71
V
RESULT: The adjusted threshold voltage has a tolerance of –2.8% to 3.1%.
5.4.3 Threshold Voltage Change Application 2
Now, the current through R
4
is only ten times higher than the device current at
V
DD
=2.25 V, it is 100
µ
A. Therefore the device current cannot be neglected in the
calculation. The adjusted threshold voltage should be again 3.6 V. R
4
can be
calculated as follows:
R
4
+
V
IT
I
Q
+
2.25
V
100
m
A
+
22.5 k
W
To choose a E96 series value:
R
4
+
22.6
k
W
The current into the device is 10
µ
A at V
DD
=2.25 V, so that for further calculations
the resistor R
DEVICE
= 225 k
Ω
parallel to R
4
must be taken into account. The
resulting resistor value R
4equivalent
of the two parallel resistors is
R
4 equivalent
+
ǒ
R
4
ø
R
Device
Ǔ
+
225
k
W
22.6
k
W
225 k
W )
22.6 k
W +
20.54 k
W
Now R
3
can be determined to
R
3
+
V
trip
*
V
IT
I
Q
)
I
Device
+
3.6
V
*
2.25
V
100
m
A
)
10
m
A
+
12.3 k
W
Values with 1% tolerance are recommended, so the following resistor of the E96
series is well-suited:
R
3
+
12.4 k
W