9.1.1 PC Board Layout

The LM3100 regulation, over-voltage, and current limit comparators are very fast, and will respond to short duration noise pulses. Layout considerations are therefore critical for optimum performance. The layout must be as neat and compact as possible, and all external components must be as close as possible to their associated pins. Refer to the functional block diagram, the loop formed by C_IN, the high and low-side switches internal to the IC, and the PGND pin should be as small as possible. The PGND connection to Cin should be as short and direct as possible. There should be several vias connecting the Cin ground terminal to the ground plane placed as close to the capacitor as possible. The boost capacitor should be connected as close to the SW and BST pins as possible. The feedback divider resistors and the C_FB capacitor should be located close to the FB pin. A long trace run from the top of the divider to the output is generally acceptable since this is a low impedance node. Ground the bottom of the divider directly to the GND (pin 7). The output capacitor, C_OUT, should be connected close to the load and tied directly into the ground plane. The inductor should connect close to the SW pin with as short a trace as possible to help reduce the potential for EMI (electro-magnetic interference) generation.

If it is expected that the internal dissipation of the LM3100 will produce excessive junction temperatures during normal operation, good use of the PC board’s ground plane can help considerably to dissipate heat. The exposed pad on the bottom of the IC package can be soldered to a ground plane and that plane should extend out from beneath the IC to help dissipate the heat. The exposed pad is internally connected to the IC substrate. Additionally the use of thick copper traces, where possible, can help conduct heat away from the IC. Using numerous vias to connect the die attach pad to an internal ground plane is a good practice. Judicious positioning of the PC board within the end product, along with the use of any available air flow (forced or natural convection) can help reduce the junction temperature.