应用笔记 5674

MAX5860/MAX5862 Thermal Model Considerations

By: Shannon Horwitz

摘要 : This guide describes the package thermal model and discusses design considerations for the MAX5860 and the MAX5862, high-density downstream cable QAM modulator, digital upconverter (DUC), and RF digital-to-analog converter (RF-DAC) devices.


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Table of Contents

Thermal Considerations

The MAX5860 and the MAX5862 are high-performance, highly integrated devices housed in compact 12mm x 17mm packages. It is very important that the thermal system is designed properly to maintain junction temperature in the safe operating range. There are two circuits in the package: the digital upconverter (DUC) and the digital-to-analog converter (DAC). This guide describes a method to measure the internal junction temperatures of both circuits.
The thermal system design must meet both of the following conditions for safe operation:
  • The junction temperature of the DUC (TJDUC) must be kept below 110°C.
  • The junction temperature of the DAC (TJDAC) must be kept below 110°C.
Depending on the system application, meeting these two conditions may require heatsinks and/or cooling fans. The power for both the DUC and the DAC are specified in the MAX5860 data sheet and the MAX5862 data sheet.

Package Thermal Model

In order to do a thorough thermal analysis, one needs to understand the package construction and its material composition. See Table 1 for the package construction and material composition and Table 2 for the substrate construction and material composition.

Table 1. Package Construction and Materials
Package Description
Package type SBSMFC LFBGA
Ball counts 280
Package size 17 x 12 x 1.4 mm3
Ball pitch 0.8mm
Die size of DUC 8.201 x 7.261 x 0.356 mm3
Die size of DAC 5.700 x 5.500 x 0.356 mm3
Bump counts of DUC 1001
Bump counts of DAC 406
Bump diameter 0.085mm
Mold thickness 0.53mm
Substrate layers 2+2+2 layer
Substrate thickness 0.35mm
Standoff 0.41mm
Table 2. Substrate (PCB) Construction and Materials
Custom 3s3p PCB (6-layer)
PCB dimensions 101.5 x 114.3 x 1.6 mm3
Core thickness 0.60mm
Prepreg thickness 0.17mm
Top/bottom thickness 0.07mm
Inner plane thickness 0.035mm
Solder mask thickness 0.02mm
Diameter of PCB vias 0.25mm
Number of PCB vias 103
Top Cu coverage ratio 20%
L2/L5 Cu coverage ratio 50%
L3/L4 Cu coverage ratio 95%
Bottom Cu coverage ratio 20%
The thermal properties of the package and substrate component materials are listed in Table 3.


Table 3. Thermal Properties of the Component Materials
Component Material or Material Name Thermal Conductivity (W/mK)
Die Silicon 148 at 25°C
98.9 at 125°C
Underfill UA26 0.5
Substrate core CCL-HL832NX 0.53
Substrate and PCB planes Copper 389
Solder ball SAC305 58.0
Solder mask AUS320 0.23
Mold compound G760SW 0.92
This data can be used to create a package thermal model for thermal simulations. Upon request, Maxim Integrated will provide the FloTHERM® thermal model for users that have access to the FloTHERM simulator tool. Contact your Maxim Integrated representative.

DUC Junction Temperature Measurement

The DUC junction temperature (TJDUC) is measured via a thermal diode and is described in the MAX5860 data sheet and the MAX5862 data sheet. The DUC temperature can be measured continuously in system operation. See Figure 1 for the recommended method of measuring the DUC temperature, which uses the MAX6642 temperature sensor.
Figure 1. The MAX5860/MAX5862 junction temperature measurement.
Figure 1. The MAX5860/MAX5862 junction temperature measurement.

DAC Junction Temperature Measurement

The DAC junction temperature (TJDAC) is measured via a ESD diode using the SE pin circuit shown in Figure 1. The DAC temperature measurement should only be made during the thermal system design phase. The measurement should not be made during normal system operation.
The temperature of the DAC is measured pulling a 1mA current out of the SE pin while measuring the voltage of the SE pin. If the voltage at the SE pin is < -0.6V (relative to GND), the temperature of the DAC is < 110°C and the DAC is operating in the safe TJDAC range. To maintain a temperature within the allowable range of 0°C to 110°C, the voltage at the SE pin should be in the following range: -0.78V < SE < -0.6V.

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