PC755 datasheet(46/50 Pages) ATMEL

46 / 50 page

PC755/745

2138D–HIREL–06/03

In addition, it is recommended that there be several bulk storage capacitors distributed

around the PCB, feeding the V

DD, L2OVDD, and OV vplanes, to enable quick recharging

of the smaller chip capacitors. These bulk capacitors should have a low ESR (equivalent

series resistance) rating to ensure the quick response time necessary. They should also

be connected to the power and ground planes through two vias to minimize inductance.

Suggested bulk capacitors – 100-330 µF (AVX TPS tantalum or Sanyo OSCON).

Connection

Recommendations

To ensure reliable operation, it is highly recommended to connect unused inputs to an

appropriate signal level through a resistor. Unused active low inputs should be tied to

DD. Unused active high inputs should be connected to GND. All NC (no-connect) sig-

nals must remain unconnected.

Power and ground connections must be made to all external V

DD, OVDD, L2OVDD, and

GND pins of the PC755.

Output Buffer DC Impedance

The PC755 60x and L2 I/O drivers are characterized over process, voltage, and temper-

ature. To measure Z

0, an external resistor is connected from the chip pad to (L2)OVDD or

GND. Then, the value of each resistor is varied until the pad voltage is (L2)OV

DD/2 (See

Figure 33).

The output impedance is the average of two components, the resistances of the pull-up

and pull-down devices. When Data is held low, SW2 is closed (SW1 is open), and R

N is

trimmed until the voltage at the pad equals (L2)OV

DD/2. R

N then becomes the resistance

of the pull-down devices. When Data is held high, SW1 is closed (SW2 is open), and R

is trimmed until the voltage at the pad equals (L2)OV

DD/2. R

P then becomes the resis-

tance of the pull-up devices.

NO TAG describes the driver impedance measurement circuit described above.

Figure 32. Driver Impedance Measurement Circuit

(L2)OVDD

OGND

Pad

Data

SW1

SW2

(L2)OVDD

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