CDC960 Datasheet, PDF (7/38 Pages) Texas Instruments – 200-MHz CLOCK SYNTHESIZER/DRIVER WITH SPREAD SPECTURM CAPABILITY AND DEVICE CONTROL INTERFACE

CDC960
200ĆMHz CLOCK SYNTHESIZER/DRIVER
WITH SPREAD SPECTRUM CAPABILITY AND DEVICE CONTROL INTERFACE
SCAS675 – APRIL 2002
connecting SCLK and SDATA to 5-V SMBus signals (continued)
There are many manufacturers making Zener diodes that can be used for this application. Panasonic MA8033
and Vishay BZX84C3V3 that have 3.1 V < V_Z < 3.5 V can be used for this application. In this case R1 is
recommended as 150 Ω.
The worst I_OH in equation (1) is 16 mA when V_OH = 5.5 V, R_S = 0 Ω and V_IO = 3.1 V.
The current in equation (2), between 4 mA and 4.6 mA, satisfies the requirement.
Equation (3) is also satisfied with the selected R1 and diode.
2. Clamping Diode to V_DD
3.3 V
3.3 V
SCLK
OR
VO
SDATA
Driver
D1
R1
150 kΩ
VIO
SCLK
or
SDATA
CDC960
Figure 2. SCLK SDATA Connection to 5-V SMBus Using Clamping Diode to V_DD
Diode D1 in Figure 2 should have a small forward voltage (V_F). Ideally, we want V_F to be less than 300 mV to
meet the input voltage requirement of the CDC960. International IOR Rectifier has a device (part number
10BQ015) with maximum V_F of 350 mV at 1.0 A. Using the 10BQ015 with R1 = 150 Ω, the worst-case I_OH is
calculated using equation (4).
IOH
+
5.5
V
*
(3.0
150
*
W
0.35)
V
+
14
mA
(4)
The calculation for equations (2) and (3) is the same as in part 1.
When using the configuration in Figure 2, the power supply is required to have a capability of sinking current.
The total amount of sinking current is dependent on the overall load connected to that power supply.
Using the above interface circuitry with a high-impedance source, the available high-level voltage on the SMBus
is limited to about (Vzk) for the configuration in Figure 1 and (VDD_(CDC960) + V_F(D1)) for the configuration in
Figure 2. One has to choose which option best fits a given SMBus configuration.
Actually, the typical SMBus configuration is an open-drain configuration with pullup resistors to the
corresponding power supply. It does not require a 5-V SMBus driver that has a low impedance to drive the
CDC960 SMBus ports with its additional components as shown in Figure 1 and Figure 2. The external
components are not needed if the pullup resistors of the SMBus are directly connected to a voltage equal to
the supply voltage of the CDC960 (typically 3.3 V). This pullup resistor connection is strongly recommended.
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