The performance of any switching regulator depends as much upon the layout of the PCB as the component selection. Following a few simple guidelines allows maximum noise rejection and minimal generation of EMI within the circuit.
Discontinuous currents are the most likely to generate EMI, therefore care should be taken when routing these paths. The main path for discontinuous current in the LM34xx-Q1 buck regulator contains the input capacitor (CIN), the recirculating diode (D1), the N-channel MOSFET (Q1), and the sense resistor (RLIM). In the LM34xx-Q1 boost regulator, the discontinuous current flows through the output capacitor (CO), D1, Q1, and RLIM. In the buck-boost regulator, both loops are discontinuous and should be carefully layed out. These loops should be kept as small as possible and the connections between all the components should be short and thick to minimize parasitic inductance. In particular, the switch node (where L1, D1 and Q1 connect) should be just large enough to connect the components. To minimize excessive heating, large copper pours can be placed adjacent to the short current path of the switch node.
The RT, COMP, CSH, IS, HSP and HSN pins are all high-impedance inputs which couple external noise easily; therefore, the loops containing these nodes should be minimized whenever possible.
In some applications the LED or LED array can be far away (several inches or more) from the controller or on a separate PCB connected by a wiring harness. When an output capacitor is used and the LED array is large or separated from the rest of the regulator, the output capacitor should be placed close to the LEDs to reduce the effects of parasitic inductance on the AC impedance of the capacitor.