Datasheet AD8392A (Analog Devices) - 9

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AD8392A Data Sheet APPLICATIONS SUPPLIES, GROUNDING, AND LAYOUT
The information in Table 3 and Figure 3 is based on a standard The AD8392A can be powered from either single or dual JEDEC 4-layer board and a maximum die temperature of 150°C. supplies, with the total supply voltage ranging from 10 V to To provide additional guidance and design suggestions, a 24 V. For optimum performance, a well regulated low ripple thermal study was performed under a set of conditions more supply should be used. closely aligned with an actual ADSL/ADSL2+ application. As with all high speed amplifiers, close attention should be paid In a typical ADSL/ADSL2+ line card, component density to supply decoupling, grounding, and overall board layout. Low usual y dictates that most of the copper plane used for thermal frequency supply decoupling should be provided with 10 µF dissipation be internal. Additional y, each ADSL/ADSL2+ port tantalum capacitors from each supply to ground. In addition, all may be allotted only 1 square inch, or even less, of board space. supply pins should be decoupled with 0.1 µF quality ceramic For these reasons, a special thermal test board was constructed chip capacitors placed as close as possible to the driver. An for this study. The 4-layer board measured approximately internal low impedance ground plane should be used to provide 4 inches × 4 inches and contained two internal 1 oz copper a common ground point for al driver and decoupling capacitor ground planes, each measuring 2 inches × 3 inches. The top ground requirements. Whenever possible, separate ground layer contained signal traces and an exposed copper strip planes should be used for analog and digital circuitry. ¼ inch × 3 inches to accommodate heat sinking, with no other copper on the top or bottom of the board. High speed layout techniques should be fol owed to minimize parasitic capacitance around the inverting inputs. Some practical Three 28-lead TSSOPs were placed on the board representing examples of these techniques are keeping feedback traces as six ADSL channels, or one channel per square inch of copper, short as possible and clearing away ground plane in the area of with each channel dissipating 700 mW on-chip (1.4 W per the inverting inputs. Input and output traces should be kept package). The die temperature is then measured in stil air and short and as far apart from each other as practical to avoid in a wind tunnel with calibrated airflow of 100 LFM, 200 LFM, crosstalk. When used as a differential driver, all differential and 400 LFM. Figure 17 shows the power dissipation vs. the signal traces should be kept as symmetrical as possible. ambient temperature for each airflow condition. The figure assumes a maximum die temperature of 135°C. No heat sink
POWER MANAGEMENT
was used. The AD8392A can be configured in any of three active bias
4.5
states as wel as a shutdown state via the use of two sets of
TJ = 135°C
digitally programmable logic pins. Pin PD0 (1, 2) and Pin PD1
4.0 400LFM
(1, 2) control Amplifier 1 and Amplifier 2, while PD0 (3, 4) and
) 3.5
Pin PD1 (3, 4) control Amplifier 3 and Amplifier 4. These pins
(W N 200LFM
can be control ed directly with either 3.3 V or 5 V CMOS logic
IO T 3.0
by using the GND pins as a reference. If left unconnected, the
PA
PD pins float low, placing the amplifier in the ful bias mode.
ISSI 2.5 D
Refer to the Specifications for the per amplifier quiescent
ER STILL AIR W 100LFM 2.0
current for each of the available bias states.
PO
As is shown in Figure 13, the AD8392A exhibits low output
1.5
impedance for the three active states. The shutdown state
1.0
(PD1, PD0 = 1, 1) provides a high impedance output.
5 15 25 35 45 55 65 75 85
051
AMBIENT TEMPERATURE (°C)
06477-
THERMAL CONSIDERATIONS
Figure 17. Power Dissipation vs. Ambient Temperature and Air Flow 28-Lead TSSOP/EP When using a quad, high output current amplifier, such as the AD8392A, special consideration should be given to system level This data is only provided as guidance to assist in the thermal thermal design. In applications such as the ADSL/ADSL2+, design process. Due diligence should be performed with regards the AD8392A could be required to dissipate as much as 1.4 W to power dissipation because there are many factors that can or more on-chip. Under these conditions, particular attention affect thermal performance. should be paid to the thermal design to maintain safe operating temperatures on the die. To aid in the thermal design, the thermal information in the Thermal Resistance section can be combined with what fol ows here. Rev. A | Page 8 of 12 Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION PIN CONFIGURATIONS TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS THERMAL RESISTANCE Maximum Power Dissipation ESD CAUTION TYPICAL PERFORMANCE CHARACTERISTICS THEORY OF OPERATION APPLICATIONS SUPPLIES, GROUNDING, AND LAYOUT POWER MANAGEMENT THERMAL CONSIDERATIONS TYPICAL ADSL/ADSL2+ APPLICATION MULTITONE POWER RATIO OUTLINE DIMENSIONS ORDERING GUIDE NOTES