© 2007-2014 Exar Corporation 18 / 27 exar.com/XR8051
Rev 1B
XR8051, XR8052, XR8054
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
(θJA) is used along with the total die power dissipation.
TJunction = TAmbient + (θJA × 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 × IRMSsupply
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 specied 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 (+150°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.
0
0.5
1
1.5
2
2.5
-40 -20 020 40 60 80 100 120
Maximum Power Dissipation (W)
Ambient Temperature (°C)
MSOP-8
SOIC-8
TSSOP-14
TSOT-5
SOIC-14
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 amplier and the load to help improve stability
and settling performance. Refer to Figure 7.
+
-
Rf
Input
Output
Rg
Rs
CLRL
Figure 7. Addition of RS for Driving Capacitive Loads
Table 1 provides the recommended RS for various capacitive
loads. The recommended RS values result in approximately
<1dB peaking in the frequency response.
CL (pF) RS (Ω) -3dB BW (MHz)
22pF 0 120
47pF 15 80
100pF 15 65
492pF 6.5 40
Table 1: Recommended RS vs. CL