6

Revision 2 / October 21, 2002

www .semtech.com

TEST AND MEASUREMENT PRODUCTS

Edge646

Application Information

PRELIMINARY

Power Supplies Decoupling

A .1

µF capacitor is recommended between VCC and VEE.

In addition, solid VCC and VEE planes are recommended

to provide a low inductance path for the power supply

currents. These planes will reduce any inductive supply

drops associated with swtiching currents on the power

supply pins. If solid planes are not possible, then wide

power busses are preferable.

VH, VL, and VTT Decoupling

As the VH, VL, and VTT inputs are unbuffered and must

supply the driver output current, decoupling capacitors for

these inputs are recommended in proportion to the amount

of output current the application requires. In general, a

surge current of 50 mA (5V swings series terminated with

50 Ohms into a 50 Ohm transmission line) are the maxi-

mum dynamic output currents the driver should see. The

decoupling capacitors should be able to provide this cur-

rent for the duration of the round trip time between the

pin electronics and the DUT, and then recharge themselves

before the next such transition would occur. Once this

condition is satisfied, the VH, VL, and VTT supply voltages

are more responsible for establishing the DC levels asso-

ciated with each function and recharging the capacitors,

rather than providing the actual dynamic currents required

to drive the DUT transmission line.

Ideally, VH, VL, and VTT would each have a dedicated power

layer on the PC board for the lowest possible inductance

power supply distribution.

VCC

VEE

1N5820 or

Equivalent

Power Supplies

The Edge646 uses two power supplies for circuit opera-

tion; VCC and VEE. In order to protect the Edge646 and

avoid damaging it, the following power supply requirements

must be satisifed at all times:

VEE

≤ All Inputs ≤ VCC

The sequence below can be used as a guideline when

operating the Edge646:

Power-On Sequencing

Power-Off Sequencing

1. VCC (substrate)

1. Inputs

2. VEE

2. VEE

3. Inputs

3. VCC

The two diode configuration shown in Figure 3 should be

used on a once-per-board basis to ensure power supply

sequence and fault tolerance.

Figure 3. Power Supply Protection Scheme

Warning: It is extremely important that the voltage

on any device pin does not exceed the range of VEE

–0.5V to VCC +0.5V at any time, either during power

up, normal operation, or during power down. Failure

to adhere to this requirement could result in latchup

of the device, which could be destructive if the sys-

tem power supplies are capable of supplying large

amounts of current. Even if the device is not imme-

diately destroyed, the cumulative damage caused by

the stress of repeated latchup may affect device re-

liability.