THS3122 Datasheet, PDF (25/43 Pages) Texas Instruments – LOW-NOISE, HIGH-SPEED, 450 mA CURRENT FEEDBACK AMPLIFIERS

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THS3122, THS3125
SLOS382E – SEPTEMBER 2001 – REVISED MAY 2015
Layout Guidelines (continued)
10.1.4 Power Dissipation And Thermal Considerations
The THS3125 and THS3122 incorporate automatic thermal shutoff protection. This protection circuitry shuts
down the amplifier if the junction temperature exceeds approximately +160°C. When the junction temperature
reduces to approximately +140°C, the amplifier turns on again. However, for maximum performance and
reliability, the designer must take care to ensure that the design does not exceed a junction temperature of
+125°C. Between +125°C and +150°C, damage does not occur, but the performance of the amplifier begins to
degrade and long-term reliability suffers. The thermal characteristics of the device are dictated by the package
and the PCB. Maximum power dissipation for a given package can be calculated using the following formula.
T -T
P_DMax =
max
A
q_JA
where:
• P_DMax is the maximum power dissipation in the amplifier (W)
• T_max is the absolute maximum junction temperature (°C)
• T_A is the ambient temperature (°C)
θ_JA = θ_JC + θ_CA
where:
• θ_JC is the thermal coefficient from the silicon junctions to the case (°C/W)
• θ_CA is the thermal coefficient from the case to ambient air (°C/W)
For systems where heat dissipation is more critical, the THS3125 and THS3122 are also available in an 8-pin
MSOP with PowerPAD package that offers even better thermal performance. The thermal coefficient for the
PowerPAD packages are substantially improved over the traditional SOIC. Maximum power dissipation levels are
depicted in Figure 50 for the available packages. The data for the PowerPAD packages assume a board layout
that follows the PowerPAD layout guidelines discussed above and detailed in the PowerPAD application note
(literature number SLMA002). Figure 50 also illustrates the effect of not soldering the PowerPAD to a PCB. The
thermal impedance increases substantially, which may cause serious heat and performance issues. Always
solder the PowerPAD to the PCB for optimum performance.
4.0
T_J = +125°C
3.5
3.0
q_JA = 58.4°C/W
2.5
2.0
q_JA = 95°C/W
1.5
1.0
0.5
q_JA = 158°C/W
0
-40 -20
0
20
40
60
80 100
T_A - Free-Air Temperature (°C)
Results shown are with no air flow and PCB size of 3 in × 3 in (76,2 mm × 76,2 mm).
• θ_JA = 58.4°C/W for 8-pin MSOP with PowerPAD (DGN package)
• θ_JA = 95°C/W for 8-pin SOIC High-K test PCB (D package)
• θ_JA = 158°C/W for 8-pin MSOP with PowerPAD without solder
Figure 50. Maximum Power Dissipation vs Ambient Temperature
When determining whether or not the device satisfies the maximum power dissipation requirement, it is important
to not only consider quiescent power dissipation, but also dynamic power dissipation. Often times, this type of
dissipation is difficult to quantify because the signal pattern is inconsistent, but an estimate of the RMS power
dissipation can provide visibility into a possible problem.
Copyright © 2001–2015, Texas Instruments Incorporated
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