Thermal Design Considerations for Video Filters

General Thermal Aspects and Some Definitions

All semiconductor devices dissipate heat to some degree. There are three paths through which heat is dissipated: package top to air, package bottom to board, and package leads to board. The dominant path for heat dissipation in the MAX11500, MAX11501/MAX11502, MAX11504/MAX11505 video filters is through the leads to the board.

Thermal resistance is a value for the thermal transfer properties of a material. It is referred to using the Greek letter Θ (theta), the first letter of the Greek word for heat, Θερµος (thermos).

The thermal resistance from the junction to case is, therefore, known as Θ_JC. The thermal resistance from case to ambient is known as Θ_CA, and the thermal resistance from junction to ambient is known as Θ_JA. Thermal models are analogous to electrical models (Figure 1), and capacitance plays a part in both. Temperature is analogous to voltage, and heat is analogous to current.

In the electrical world, if current flows through a resistance, a voltage differential will be generated. Similarly, if heat flows through a thermal resistance, a temperature differential will be generated. In addition, if there is a short transient heat flow, the thermal capacitances will shunt the heat and there will be only a small temperature rise. Objects need time to heat up.


Figure 1. Electrical analogy of heat dissipation through device leads.

In most cases, Θ_JC is smaller than Θ_CA and CT_JC is smaller than CT_JC. This can be expressed mathematically as below. From now on we will ignore the capacitances and address steady-state situations, since these characteristics are more relevant to video filters.

Θ_JA is generally used because packages are specified using a standard PCB defined in the relevant JEDEC standards, JES51-3 or JES51-7. (See more on the JEDEC standards in the Package Specification section below.)



Knowing the thermal resistance of a package allows us to calculate the junction temperature for a given power dissipation and ambient temperature. The problem in calculating junction temperature in ICs which rely on the PCB for heat dissipation, is knowing what the thermal resistance from junction to ambient actually is.

Consider a simple example. A device has V_CC = 5.0V and draws 100mA. The maximum ambient temperature is 40°C and the maximum allowed junction temperature is 150°C. The power dissipation is expressed as:



Assume that Θ_JA = 150°C/W and that the equipment must operate at a very benign ambient temperature of 40°C. The junction temperature is then calculated as:



This temperature is well below the maximum die temperature, so the device will have no thermal problems. This process looks easy, but it is deceptively complex. With small ICs, we cannot be sure what the Θ_JA actually is because it is dependant on the PCB layout.

Package Specifications

Absolute Maximum Ratings

The key specifications for this discussion are Continuous Power Dissipation and Junction Temperature. The maximum junction temperature is 150°C and the maximum power that can be dissipated is 470mW at 70°C ambient. These conditions would result in a die temperature of 150°C.

Therefore, we can calculate Θ_JA as:



This can be confirmed because Θ_JA is simply the reciprocal of the derating factor.



All of this data comes from the JEDEC specification for the package. There are three specifications of interest for Maxim video filters:

1. EIA/JESD51-3, Low Effective Thermal Conductivity Test Board for Leaded Surface Mount Packages
2. EIA/JESD51-5, Extension of Thermal Test Board Standards for Packages with Direct Thermal Attachment Mechanisms
3. EIA/JESD51-7, High Effective Thermal Conductivity Test Board for Leaded Surface Mount Packages

The Problem with Video Signals and ICs in Practical Situations

Calculating Power Dissipation

Power dissipation in these video filters comes from two sources: quiescent and signal. Calculating the quiescent dissipation is easy. The data is available directly from the data sheet.

The data sheet for the MAX11501 states the supply current at V_CC = 5.0V: 18mA typical and 24mA maximum. Therefore, the quiescent power dissipation is 120mW maximum.

The power dissipation caused by the signal is defined by the equation:



Where:

• Pdo = power dissipated in the channel
• Vorms = rms voltage at the output of the channel
• Rl = load resistance of the channel
• V_CC = device supply voltage

Worst-Case Video Signals

There are several signals that must be considered: CVBS, Y/C, RGB, and YUV.

The worst-case scenario is RGB with a 100% white picture driving double loads on each channel. This is the signal that we will consider.

The line-based signal will look like Figure 3.


Figure 3. Video line waveform.

The PAL and NTSC vertical blanking intervals are represented in Figures 4 and 5, respectively.


Figure 4. PAL vertical blanking and field synchronization pulses.


Figure 5. NTSC vertical blanking and field synchronization pulses. Note: NTSC is defined as having a minimum of 19H vertical blanking. It can, however, be longer.

Given the above information, we can calculate the RMS voltage of this signal. This will be done first for PAL.

The RMS voltage of a line is calculated below. This is normalized for a sync to a peak white level of 1V.



This value is valid for the 575 lines outside the vertical blanking period. We can calculate the RMS voltages of the lines inside the vertical blanking interval similarly. The results are shown below in Table 2, together with the final RMS calculation.

Table 2. RMS Voltage Calculations for PAL

A Practical Example

Consider the worst-case example of a MAX11501 3-channel video filter used for a 625-line RGB signal.

The inputs and outputs are DC-coupled so that peak white at the input is 1.4V. This is the maximum allowed input voltage. We allow a 5% overhead so that the maximum sync tip to peak white at the input is 1.05V. Hence, we have the voltage levels as shown in Figure 6.


Figure 6. Example input and output voltages.

Now we can easily calculate the typical and maximum power dissipations:

1. Calculate the RMS output voltages.
2. Calculate the power dissipations per channel.
3. Calculate the total power dissipation.

Now assuming that we have a good heatsink and are achieving close to the optimal heat dissipation, we can calculate the die temperature:



Clearly, the maximum of 137°C is unlikely to occur in practice. However we can now be satisfied that the die will not overheat even under worst-case conditions.

Practical Measurements

Setup

• V_CC = 5.0V
• Two video loads on each output
• Signal set to 525-line RGB, 100% white with sync on all channels

Theoretical Calculations

These calculations assume that the PCB thermal characteristics approximate those used to specify the package performance:

Results

• Quiescent Current I_CC: 18mA
• Device Temperature measured at the ground pin: 63°C
• Ambient Temperature: 25°C

Calculations

We can now calculate the Θ_JA value for the system. This value will tell us how close our board is to the ideal used in specifying the package:



We see that our PCB is performing slightly less efficiently than the ideal. We can now use the total Θ_JA for our system to calculate die temperature at the maximum ambient, say 70°C.

Conclusions