AD8387 Просмотр технического описания (PDF) - Analog Devices

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AD8387

700 MHz to 2700 MHz Quadrature Modulator

Analog Devices 

AD8387

APPLICATIONS

OPTIMIZED RELIABILITY WITH THE THERMAL

SWITCH

While internal current limiters provide short-term protection

against temporary shorts at the outputs, the thermal switch

provides protection against persistent shorts lasting for several

seconds. To optimize reliability with the use of the thermal

switch, the following sequence of operations is recommended.

INITIAL POWER-UP AFTER ASSEMBLY OR REPAIR

Grounded output mode is disabled, and thermal switch is

enabled. Ensure that the GSW pin is HIGH and that the

TSW pin is HIGH upon initial power-up and that they remain

unchanged throughout this procedure.

The initial power-up sequence follows:

1. Execute the initial power-up.

2. Identify any shorts at outputs. Power down, repair shorts,

and repeat the initial power-up sequence until proper system

functionality is verified.

3. Disable the thermal switch.

POWER-UP DURING NORMAL OPERATION

Grounded output mode is disabled, and thermal switch is

disabled.

If TSW = LOW and GSW = HIGH, all outputs go into normal

operating mode with the thermal switch disabled.

POWER SUPPLY SEQUENCING

As indicated under the Absolute Maximum Ratings, the voltage

at any input pin cannot exceed its supply voltage by more than

0.5 V. Power-on and power-off sequencing can be required to

comply with the absolute maximum ratings.

Failure to comply with the Absolute Maximum Ratings can

result in functional failure or damage to the internal ESD

diodes. Damaged ESD diodes can cause temporary parametric

failures, which can result in image artifacts. Damaged ESD

diodes cannot provide full ESD protection, reducing reliability.

POWER-ON SEQUENCE

1. Turn on AVCC

2. Turn on VRH

3. Turn on VRL

4. Turn on DVCC

5. Disable thermal switch: TSW = LOW

6. Turn on input signals

POWER-OFF SEQUENCE

1. Turn off input signals

2. Turn off VRL

3. Turn off VRH

4. Turn off AVCC

5. Turn off DVCC

GROUNDED OUTPUT MODE DURING POWER-OFF

Certain applications require that video outputs be held near

AGND during power-down. The following power-off sequence

ensures that the outputs are near ground during power-off and

that the Absolute Maximum Ratings are not violated.

1. Enable grounded output mode: GSW = LOW

2. Turn off input signals

3. Turn off VRL

4. Turn off VRH

5. Turn off AVCC

6. Turn off DVCC

PCB DESIGN FOR OPTIMIZED THERMAL

PERFORMANCE

Although the maximum safe operating junction temperature is

higher, the AD8387 is 100% tested at a junction temperature of

125°C. Consequently, the maximum guaranteed operating

junction temperature is 125°C. To limit the maximum junction

temperature at or below the guaranteed maximum, the package

in conjunction with the PCB must effectively conduct heat away

from the junction.

The AD8387 package is designed to provide enhanced thermal

characteristics through the exposed die paddle on the bottom

surface of the package. To take full advantage of this feature, the

exposed paddle must be in direct thermal contact with the PCB,

which then serves as a heat sink.

A thermally effective PCB must incorporate two thermal

pads and a thermal via structure. The thermal pad on the top

surface of the PCB provides a solderable contact surface on the

top surface of the PCB. The thermal pad on the bottom PCB

layer provides a surface in direct contact with the ambient. The

thermal via structure provides a thermal path to the inner and

bottom layers of the PCB to remove heat.

Rev. 0 | Page 14 of 16

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