CPC5002
INTEGRATED
C
IRCUITS
D
IVISION
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 and SD signals on the isolated slave side of the barrier. In this example, the low power SMBus master, not
IN
OUT
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
2
are specified as SMBus high power and I C 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
= 3.0V to 3.6V
• I 4mA for U3 and the POE Controllers
DDx
OL
• Ambient Temperature: T = 0°C to 70°C
• Resistors:
A
• Tolerance = 1%
• Temperature Coefficient = 100ppm
• V 0.4V for U3 and the POE Controllers
OL
Figure 3. Optically isolated SMBus for POE Controllers with Separate SDA and SDA
Pins
IN
OUT
U1
3.3VDDS
3.3VDDM
CPC5002
1
2
3
4
8
SMBus
POE
3.3VDDS
R5
U3
R1
806Ω
Controllers
7
511Ω
SCLM
SDAM
SCL
SDAIN
INT
3.3VDDS
R2
806Ω
R6
SDAOUT
511Ω
6
5
3.3VDDS
0.1μF
GNDS
3.3VDDM
0.1μF
GNDM
U2
3.3VDDS
3.3VDDM
CPC5002
8
7
1
2
3
4
SCL
SDAIN
INT
3.3VDDS
3.3VDDM
R7
R3
806Ω
R9*
SDAOUT
10k
INTM
3.3VDDS
3.3VDDM
R4
806Ω
R10*
R8
10k
6
5
R9 and R10 are not required for this design.
See text for explanation.
*
3.3VDDS
3.3VDDM
0.1μF
GNDM
0.1μF
GNDS
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 = 3.0V and V = 1.442V at
DD
F
T = 0°C and I = 1.4mA, the calculated maximum value for the series input resistor R is 826.8. Taking tolerance
A
F
S
and value change due to temperature into account, the nearest E96 standard value sets R = 806. Using
S
V
= 0.25V and V
= 0.1V and calculating for the LED current range over the specified operating
OL_Nominal
OL_Minimum
conditions with R = 806, the LED input current I will be 1.455mA to 3.212mA. At nominal operating conditions with
S
F
T = 25°C, the nominal LED input current is: I
= 2.28mA.
A
F_Nominal
10
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