8.1 Layout Guidelines

8.1.4 Power System Example

The ADC12D1800RB uses continuous ground planes (except where clear areas are needed to provide appropriate impedance management for specific signals), see Figure 8-1. Power is provided on one plane, with the 1.9V ADC supply being split into multiple zones or peninsulas for the specific power buses of the ADC. Decoupling capacitors are connected between these power bus peninsulas and the adjacent ground planes using vias. The capacitors are located as close to the individual power/ground pin pairs of the ADC as possible. In most cases, this means the capacitors are located on the opposite side of the PCB to the ADC.

Figure 8-1 Power and Grounding Example

8.2 Layout Example

The following examples show layout-example plots. Figure 6-15 show a typical stack up for a 10 layer board.

Figure 8-2 ADC12D1800RF Layout Example 1 – Top side and inner layers

Figure 8-3 ADC12D1800RF Layout Example 1 – Bottom side and inner layers

Figure 8-4 ADC12D1800RF Typical Stackup – 10 Layer Board

8.3 Thermal Management

The Heat Slug Ball Grid Array (HSBGA) package is a modified version of the industry standard plastic BGA (Ball Grid Array) package. Inside the package, a copper heat spreader cap is attached to the substrate top with exposed metal in the center top area of the package. This results in a 20% improvement (typical) in thermal performance over the standard plastic BGA package.

Figure 8-5 HSBGA Conceptual Drawing

The center balls are connected to the bottom of the die by vias in the package substrate, Figure 8-5. This gives a low thermal resistance between the die and these balls. Connecting these balls to the PCB ground planes with a low thermal resistance path is the best way dissipate the heat from the ADC. These pins should also be connected to the ground plane via a low impedance path for electrical purposes. The direct connection to the ground planes is an easy method to spread heat away from the ADC. Along with the ground plane, the parallel power planes will provide additional thermal dissipation.

The center ground balls should be soldered down to the recommended ball pads (See AN-1126 [SNOA021]). These balls will have wide traces which in turn have vias which connect to the internal ground planes, and a bottom ground pad/pour if possible. This ensures a good ground is provided for these balls, and that the optimal heat transfer will occur between these balls and the PCB ground planes.

In spite of these package enhancements, analysis using the standard JEDEC JESD51-7 four-layer PCB thermal model shows that ambient temperatures must be limited to a max of 65°C to ensure a safe operating junction temperature for the ADC12D1800. However, most applications using the ADC12D1800 will have a printed circuit board which is more complex than that used in JESD51-7. Typical circuit boards will have more layers than the JESD51-7 (eight or more), several of which will be used for ground and power planes. In those applications, the thermal resistance parameters of the ADC12D1800 and the circuit board can be used to determine the actual safe ambient operating temperature up to a maximum of 85°C.

Three key parameters are provided to allow for modeling and calculations. Because there are two main thermal paths between the ADC die and external environment, the thermal resistance for each of these paths is provided. θ_JC1 represents the thermal resistance between the die and the exposed metal area on the top of the HSBGA package. θ_JC2 represents the thermal resistance between the die and the center group of balls on the bottom of the HSBGA package. The final parameter is the allowed maximum junction temperature, which is T_J.

In other applications, a heat sink or other thermally conductive path can be added to the top of the HSBGA package to remove heat. In those cases, θ_JC1 can be used along with the thermal parameters for the heat sink or other thermal coupling added. Representative heat sinks which might be used with the ADC12D1800 include the Cool Innovations p/n 3-1212XXG and similar products from other vendors. In many applications, the printed circuit board will provide the primary thermal path conducting heat away from the ADC package. In those cases, θ_JC2 can be used in conjunction with printed circuit board thermal modeling software to determine the allowed operating conditions that will maintain the die temperature below the maximum allowable limit. Additional dissipation can be achieved by coupling a heat sink to the copper pour area on the bottom side of the printed circuit board.

Typically, dissipation will occur through one predominant thermal path. In these cases, the following calculations can be used to determine the maximum safe ambient operating temperature:

T_J = T_A + P_D × (θ_JC+θ_CA)

T_J = T_A + P_C(MAX) × (θ_JC+θ_CA)

For θ_JC, the value for the primary thermal path in the given application environment should be used (θ_JC1 or θ_JC2). θ_CA is the thermal resistance from the case to ambient, which would typically be that of the heat sink used. Using this relationship and the desired ambient temperature, the required heat sink thermal resistance can be found. Alternately, the heat sink thermal resistance can be used to find the maximum ambient temperature. For more complex systems, thermal modeling software can be used to evaluate the printed circuit board system and determine the expected junction temperature given the total system dissipation and ambient temperature.