XR8051ASO8X View Datasheet(PDF) - Exar Corporation

Part Name

Description

MFG CO.

XR8051ASO8X

Low Cost, High Speed Rail-to-Rail Amplifers

Exar Corporation 

Data Sheet

Power Dissipation

Power dissipation should not be a factor when operating

under the stated 2kΩ load condition. However, applications

with low impedance, DC coupled loads should be analyzed

to ensure that maximum allowed junction temperature

is not exceeded. Guidelines listed below can be used to

verify that the particular application will not cause the

device to operate beyond it’s intended operating range.

Maximum power levels are set by the absolute maximum

junction rating of 170°C. To calculate the junction

temperature, the package thermal resistance value

ThetaJA (ӨJA) is used along with the total die power

dissipation.

TJunction = TAmbient + (ӨJA × PD)

Where TAmbient is the temperature of the working environment.

In order to determine PD, the power dissipated in the load

needs to be subtracted from the total power delivered by

the supplies.

PD = Psupply - Pload

Supply power is calculated by the standard power

equation.

Psupply = Vsupply × IRMS supply

Vsupply = VS+ - VS-

Power delivered to a purely resistive load is:

Pload = ((VLOAD)RMS2)/Rloadeff

The effective load resistor (Rloadeff) will need to include

the effect of the feedback network. For instance,

Rloadeff in Figure 3 would be calculated as:

RL || (Rf + Rg)

These measurements are basic and are relatively easy to

perform with standard lab equipment. For design purposes

however, prior knowledge of actual signal levels and load

impedance is needed to determine the dissipated power.

Here, PD can be found from

PD = PQuiescent + PDynamic - PLoad

Quiescent power can be derived from the specified IS

values along with known supply voltage, VSupply. Load

power can be calculated as above with the desired signal

amplitudes using:

(VLOAD)RMS = VPEAK / √2

( ILOAD)RMS = ( VLOAD)RMS / Rloadeff

The dynamic power is focused primarily within the output

stage driving the load. This value can be calculated as:

PDYNAMIC = (VS+ - VLOAD)RMS × ( ILOAD)RMS

Assuming the load is referenced in the middle of the

power rails or Vsupply/2.

The XR8051 is short circuit protected. However, this may

not guarantee that the maximum junction temperature

(+170°C) is not exceeded under all conditions. Figure 6

shows the maximum safe power dissipation in the package

vs. the ambient temperature for the packages available.

2.5

2

1.5

SOIC-8

1 MSOP-8

TSSOP-14

SOIC-14

0.5

0

-40

SOT23-5

-20

0

20

40

60

80

Ambient Temperature (°C)

100 120

Figure 6. Maximum Power Derating

Driving Capacitive Loads

Increased phase delay at the output due to capacitive

loading can cause ringing, peaking in the frequency

response, and possible unstable behavior. Use a series

resistance, RS, between the amplifier and the load to help

improve stability and settling performance. Refer to Figure 7.

Input

+

-

Rf

Rg

Rs

Output

CL

RL

Figure 7. Addition of RS for Driving Capacitive Loads

©2013 Exar Corporation

18/23

Rev 1A

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