CPC5002

R01

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mixed type devices on a bus, the weakest driver on that bus determines the minimum value of the pullup resistor.

When the CPC5002 is the only device driving the bus as shown with U1, the minimum E96 standard value for pullup

resistors R5 and R6 will be 511

. For bus loading up to 400pF, this pullup resistor value will provide for Fast-mode

can be higher.

When the CPC5002 shares a bus with another device as is the case with U2, the weakest driver sets the conditions

for selecting the correct resistor value. As stated earlier, the SMBus master is rated as a Low-power device and

therefore is only capable of sinking 350uA to an output low voltage level of 0.4V. A pullup resistor attached to the

maximum supply voltage level of 3.6V and pulled down by this low power driver limits the minimum pullup resistor

value to 9.14k

. After considering tolerance and temperature effects the nearest E96 standard value is 9.31k. Most

applications will typically select the more common 10k

 value for R7 and R8, which allows for a 5% resistor tolerance.

Although shown but not needed in this example are pullup resistors R9 and R10. These resistors, not needed by the

CPC5002 at U2, are utilized whenever the busses they are attached to are also connected to device(s) having logic

level inputs. With heavy loading or excessive leakage on the bus the resistors provide supplementary bias to improve

pullup transition performance and to increase the output logic high level without impacting the LED input current bias

level.

The CPC5002 can be utilized to provide digital isolated buffering in a variety of unique applications. Design support is

available by contacting IXYS Integrated Circuits Division’s Applications.