CPC5002G datasheet(10/14 Pages) IXYS

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R01

CPC5002

4.2 Application Example

Shown below is an example of an isolated POE Controller SMBus where the SDA signal has been split into separate

SDAIN and SDOUT signals on the isolated slave side of the barrier. In this example, the low power SMBus master, not

shown, requires a buffer (U3) capable of driving the CPC5002 input LEDs. Although selection of the appropriate

buffer is determined by the product definition and the ability to drive the LED’s, it is recommended the buffer have

Schmitt trigger inputs to ensure clean bounce-free LED drive signals. A high power SMBus master with the ability to

sink 4mA of pullup current may not require a buffer to drive the CPC5002 inputs. In this example, the POE Controllers

are specified as SMBus high power and I2C compatible. This enables the POE Controllers to drive the CPC5002

LEDs directly without the need of an external buffer.

Circuit design of the SMBus physical layer using the CPC5002 consists of two parts, one being the LED input drive

current and the other being the buffered galvanically isolated logic output signals.

The following design constraints are assumed for this example:

• Supply Voltages: VDDx = 3.0V to 3.6V

• Ambient Temperature: TA = 0°C to 70°C

• VOL  0.4V for U3 and the POE Controllers

• IOL  4mA for U3 and the POE Controllers

• Resistors:

• Tolerance = 1%

• Temperature Coefficient = 100ppm

Figure 3. Optically isolated SMBus for POE Controllers with Separate SDAIN and SDAOUT Pins

To minimize pulse width distortion of the output signal, the input LED drive current needs to be set at the lower end of

it’s operational range. Because the forward voltage of the LED has a negative temperature coefficient this will occur at

the minimum operating temperature point with the minimum supply voltage. With VDD = 3.0V and VF = 1.442V at

TA = 0°C and IF = 1.4mA, the calculated maximum value for the series input resistor RS is 826.8. Taking tolerance

and value change due to temperature into account, the nearest E96 standard value sets RS = 806. Using

VOL_Nominal = 0.25V and VOL_Minimum = 0.1V and calculating for the LED current range over the specified operating

conditions with RS = 806, the LED input current IF will be 1.455mA to 3.212mA. At nominal operating conditions with

TA = 25°C, the nominal LED input current is: IF_Nominal = 2.28mA.

SCL

SDA

INT

SCL

SDA

OUT

SMBus

POE

Controllers

CPC5002

3.3V

DDS

3.3V

DDS

3.3V

DDS

3.3V

DDS

GND

3.3V

DDM

3.3V

DDM

3.3V

DDM

CPC5002

806

3.3V

DDM

3.3V

DDM

GND

3.3V

DDS

3.3V

DDS

3.3V

DDS

3.3V

DDS

0.1

μF

0.1

μF

0.1

μF

0.1

μF

3.3V

DDM

806

R9*

R10*

INT

SCL

SDA

OUT

INT

R9 and R10 are not required for this design.

See text for explanation.

511

10k

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