11.1 Layout Guidelines

1. Place the 100-nF bypass capacitor near the IN and GND pins, and make the connections using a low inductance trace.
2. Place at least 10-µF low ESR ceramic capacitor near the OUT and GND pins, and make the connections using a low inductance trace.
3. The PowerPAD must be directly connected to PCB ground plane using wide and short copper trace.

11.2 Layout Example

Figure 42. Recommended Layout

11.3 Power Dissipation and Junction Temperature

It is good design practice to estimate power dissipation and maximum expected junction temperature of the TPS20xxC and TPS20xxC-2. The system designer can control choices of package, proximity to other power dissipating devices, and printed-circuit board (PCB) design based on these calculations. These have a direct influence on maximum junction temperature. Other factors, such as airflow and maximum ambient temperature, are often determined by system considerations. It is important to remember that these calculations do not include the effects of adjacent heat sources, and enhanced or restricted air flow.

Addition of extra PCB copper area around these devices is recommended to reduce the thermal impedance and maintain the junction temperature as low as practical. The lower junction temperatures achieved by soldering the pad improve the efficiency and reliability of both TPS20xxC and TPS20xxC-2 parts and the system. The following examples were used to determine the θ_JACustom thermal impedances noted in Thermal Information: SOT-23 and Thermal Information: MSOP-PowerPAD. They were based on use of the JEDEC high-k circuit board construction (2 signal and 2 plane) with 4, 1-oz. copper weight, layers.

While TI recommends that the DGN package PAD be soldered to circuit board copper fill and vias for low thermal impedance, there may be cases where this is not desired. For example, use of routing area under the IC. Some devices are available in packages without the PowerPad (DGK) specifically for this purpose. The θ_JA for the DGN package with the pad not soldered and no extra copper, is approximately 141°C/W for 0.5-A and 1-A rated parts, and 139°C/W for the 1.5-A and 2-A rated parts. The θ_JA for the DGK mounted per Figure 45 is 110.3°C/W. These values may be used in Equation 1 to determine the maximum junction temperature.

Figure 43. DBV Package PCB Layout Example

Figure 44. DGN Package PCB Layout Example

Figure 45. DGK Package PCB Layout Example

As shown in Equation 1, the following procedure requires iteration because power loss is due to the internal MOSFET I² × R_DS(ON), and R_DS(ON) is a function of the junction temperature. As an initial estimate, use the R_DS(ON) at 125°C from the Typical Characteristics, and the preferred package thermal resistance for the preferred board construction from the Thermal Information: SOT-23 table.

If the calculated T_J is substantially different from the original assumption, estimate a new value of R_DS(ON) using the typical characteristic plot and recalculate.

If the resulting T_J is not less than 125°C, try a PCB construction or a package with lower θ_JA.