10.1 Layout Guidelines

Some layout guidelines must be followed to ensure proper regulation of the output voltage with minimum noise. Traces carrying the load current must be wide to reduce the amount of parasitic trace inductance. To improve PSRR, a bypass capacitor can be placed at the OUTPUT pin and must be placed as close as possible to the IC. All that is required for the typical fixed output regulator application circuit is the LM340-MIL IC and a 0.22-µF input capacitor if the regulator is placed far from the power supply filter. A 0.1-µF output capacitor is recommended to help with transient response. In cases when VIN shorts to ground, an external diode must be placed from VOUT to VIN to divert the surge current from the output capacitor and help protect the device.

10.2 Layout Example

Figure 27. Layout Example DDPAK

Figure 28. Layout Example SOT-223

10.3 Heat Sinking DDPAK/TO-263 and SOT-223 Package Parts

Both the DDPAK/TO-263 (KTT) and SOT-223 (DCY) packages use a copper plane on the PCB and the PCB itself as a heat sink. To optimize the heat sinking ability of the plane and PCB, solder the tab of the plane.

Figure 29 shows for the DDPAK/TO-263 the measured values of θ_(J–A) for different copper area sizes using a typical PCB with 1-oz copper and no solder mask over the copper area used for heat sinking.

As shown in Figure 29, increasing the copper area beyond 1 square inch produces very little improvement. It should also be observed that the minimum value of θ_(J–A) for the DDPAK/TO-263 package mounted to a PCB is 32°C/W.

As a design aid, Figure 30 shows the maximum allowable power dissipation compared to ambient temperature for the DDPAK/TO-263 device (assuming θ_(J–A) is 35°C/W and the maximum junction temperature is 125°C).

Figure 29. θ_(J–A) vs Copper (1 Ounce) Area for the DDPAK/TO-263 Package

Figure 30. Maximum Power Dissipation vs T_AMB for the DDPAK/TO-263 Package

Figure 31 and Figure 32 show the information for the SOT-223 package. Figure 31 assumes a θ_(J–A) of 74°C/W for 1-oz. copper and 51°C/W for 2-oz. copper and a maximum junction temperature of 125°C.

Figure 31. θ_(J–A) vs Copper (2 Ounce) Area
for the SOT-223 Package

Figure 32. Maximum Power Dissipation vs
T_AMB for the SOT-223 Package

See AN-1028 LMX2370 PLLatinum Dual Freq Synth for RF Pers Comm LMX2370 2.5GHz/1.2GHz for power enhancement techniques to be used with the SOT-223 package.