CDC960 datasheet(7/10 Pages) TI

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CDC960

200MHz CLOCK SYNTHESIZER/DRIVER

WITH SPREAD SPECTRUM CAPABILITY AND DEVICE CONTROL INTERFACE

SCAS675 – APRIL 2002

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

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 < VZ < 3.5 V can be used for this application. In this case R1 is

recommended as 150

Ω.

The worst IOH in equation (1) is 16 mA when VOH = 5.5 V, RS = 0 Ω and VIO = 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.

Clamping Diode to VDD

150 k

Ω

SCLK

SDATA

Driver

VIO

SCLK

SDATA

CDC960

3.3 V

Figure 2. SCLK SDATA Connection to 5-V SMBus Using Clamping Diode to VDD

Diode D1 in Figure 2 should have a small forward voltage (VF). Ideally, we want VF 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 VF of 350 mV at 1.0 A. Using the 10BQ015 with R1 = 150 Ω, the worst-case IOH is

calculated using equation (4).

OH +

5.5 V

* (3.0 * 0.35) V

150

+ 14 mA

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) + VF(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.

(4)

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