PULL-UP RESISTOR SELECTION
Figure 1 shows the internal section of I2C used in FPS series. There are 3 I
2
C ICs. Only one is shown as an example.
Maximum Pull up resistor can be between 12k ohm to 5k ohm depending on how many units are connected in the rack.
Ex - If one unit is connected, pull up resistor value can be 12k ohm max
If eight units are connected, pull up resistor value can be 5k ohm max
(Total capacitance of eight supplies will be 25 x 8 =200 pF + 50pF = 250pF).
Input voltage < 85Vac
< 80% +/-5% of Vo rated
For minimum pullup resistor selection, use the following equation,
V
DD
in FPS is 5V. V
OL
(max) is 0.4V & I
OL
is 3mA. So R
P (min)
is 1.53 kOhm
Each of the 3 ICs adds some capacitance to the bus. The total capacitance generated by the 3 ICs in each power supply is about 25pF.
Considering about 50pF extra capacitance for parasitic capacitance, wire capacitance and capacitance of master, the total capacitance
seen with 1 supply can be about 75pF.
Please use figure 2 for selecting the external pull up resistor.
IC
FPS
SDA
PIN #17
PIN #18
-S
PIN #13
22Ω
200Ω
Fig. 1
Fig. 2
SCL
R
P (min)
=
V
DD
-V
OL
(max)
I
OL
