CPC5002 Datasheet, PDF (10/14 Pages) IXYS Corporation – Dual High-Speed Open-Drain

INTEGRATED CIRCUITS DIVISION
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
SDA_IN and SD_OUT 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: V_DDx = 3.0V to 3.6V
• Ambient Temperature: T_A = 0°C to 70°C
• V_OL  0.4V for U3 and the POE Controllers
• I_OL  4mA for U3 and the POE Controllers
• Resistors:
• Tolerance = 1%
• Temperature Coefficient = 100ppm
Figure 3. Optically isolated SMBus for POE Controllers with Separate SDA_IN and SDA_OUT Pins
3.3V_DDM
1
U1
CPC5002
3.3V_DDS
8
SCL_M
SDA_M
U3
R1
806Ω ₂
R2
806Ω ₃
3.3V_DDS
R5
7
511Ω
3.3V_DDS
R6
6
511Ω
SMBus
POE
Controllers
SCL
SDA_IN
INT
SDA_OUT
3.3V_{DDM 4}
0.1μF
GND_M
3.3V_DDS
5
0.1μF
GND_S
INT_M
3.3V_DDM
8
3.3V_DDM
R7
10k
7
3.3V_DDM
R8
10k
6
3.3V_DDM
0.1μF
5
GND_M
U2
CPC5002
3.3V_DDS
1
3.3V_DDS
R3
2 ^806Ω
R9*
3.3V_DDS
R4
3 ^806Ω
R10*
SCL
SDA_IN
INT
SDA_OUT
3.3V_DDS
4
* R9 and R10 are not required for this design.
See text for explanation.
0.1μF
GND_S
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 V_DD = 3.0V and V_F = 1.442V at
T_A = 0°C and I_F = 1.4mA, the calculated maximum value for the series input resistor R_S is 826.8. Taking tolerance
and value change due to temperature into account, the nearest E96 standard value sets R_S = 806. Using
V_{OL_Nominal} = 0.25V and V_{OL_Minimum} = 0.1V and calculating for the LED current range over the specified operating
conditions with R_S = 806, the LED input current I_F will be 1.455mA to 3.212mA. At nominal operating conditions with
T_A = 25°C, the nominal LED input current is: I_{F_Nominal} = 2.28mA.
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