The switching frequency selection has an impact on several items in the design. Two aspects which are often looked at are design size and efficiency. A higher switching frequency results in smaller, external passives, with the tradeoff of increased power dissipation.

LMR66430-EP offers two orderables (Section 4). LMR66430MB3RXBRNEP is used in this design example. The device operates at 1MHz, nominally, when operated in auto mode. The mode can be configured for FPWM to allow 1MHz, CCM operation, even at light-loads. LMR66430R3RXBRNEP allows for synchronization to an external clock as well. Refer to the MODE/SYNC section and specifications for more information.

This design requires 3A, peak load current. At 12VIN, 3.3VOUT (fix), with a 3A load, the power dissipation is 1.5W (Figure 8-5). Figure 8-12 shows that a total power dissipation of 1.5W (inductor and IC) causes the case temperature to rise approximately 55°C above ambient. Assuming minimal, additional power loss at hot temperature, the design must be operable up to 85°C, as the ambient temperature plus the case temperature rise (approximate junction temperature) is below the maximum junction rating. This does assume that the end-design has similar thermal performance to the EVM. If hypothesized that the design has poorer thermal performance to the EVM, please consult Figure 8-3. This figure shows how board thermal performance is impacted by shrinking the copper board area on all layers in a 4-layer PCB.

LMR66430MB3RXBRNEP operates at a fixed 1MHz frequency. In the case the power dissipation is found to be unsatisfactory for design requirements consider selecting LMR66430R3RXBRNEP, which can be configured for a lower switching frequency to reduce power dissipation. If neither is a feasible option, consult with TI to find an alternative power solution.