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DS1780
The worse such accident would be connecting -12V to +12V; a total of 24V difference, with the series
resistors this would draw a maximum of approximately 24 ma.
The internal scaling factor depends upon the particular input. The +12V IN , +5V IN , +3.3V IN , and +2.5V IN
inputs are internally scaled such that the nominal value of the respective supply corresponds to 3/4 of full
range, or a decimal count of 192. The approximate resolution is thus equal to:
LSb (V NOM ) @
4/3 V NOM
256
This is depicted below in Table 5.
VOLTAGE/DATA RELATIONSHIPS FOR POSITIVE ONLY VOLTAGE INPUTS
(+12V IN , +5V IN , +3.3V IN , and +2.5V IN ) Table 5
INPUT PIN +12V IN +5V IN
LSb WEIGHTING 62.5 26.0
(mV)
+3.3V IN
17.2
+2.5V IN
13.0
ADC RESULT (BASE
PIN VOLTAGE (V)
PIN VOLTAGE (V)
PIN VOLTAGE (V)
PIN VOLTAGE (V)
10)
0
1
2
3
4
?
?
?
190
191
192
193
?
?
?
252
253
254
255
0
0.063
0.125
0.188
0.25
?
?
?
11.875
11.938
12.0
12.063
?
?
?
15.75
15.813
15.875
15.938
0
0.026
0.052
0.078
0.104
?
?
?
4.948
4.974
5.0
5.026
?
?
?
6.563
6.589
6.615
6.641
0
0.017
0.034
0.052
0.069
?
?
?
3.266
3.283
3.3
3.317
?
?
?
4.331
4.348
4.366
4.383
0
0.013
0.026
0.039
0.052
?
?
?
2.474
2.487
2.5
2.513
?
?
?
3.281
3.294
3.307
3.32
The other two voltage inputs use a slightly different scaling technique, due to the nature of the PC voltage
they are monitoring. Because processor voltage (V CCP ) can vary to 3.6V, the +V CCP1 and +2.5V S /+V CCP1
inputs are internally scaled such that the ADC result is 0h for a 0V input and the maximum value of FFh
is returned for a voltage of 3.60V. This corresponds to an LSb weighting of 14.1 mV.
The inputs can also be used to monitor a negative supply, such as -12V. However, a resistor ladder and
positive reference voltage (V REF ) must be used (see Figure 1) such that input voltage to the DS1780
swings between OV and +3.6V. Assuming the DS1780 +V CCP1 and +2.5V S /+V CCP2 pins have infinite
input impedance and the V REF is a perfect supply, then the resolution and range of -12V input are:
ê ú
?
?
LSb (R 1 , R 2 ) @
3.6
255
é R 1 + R 2 ù
R 1
9 of 28
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