The choice of switching frequency is a compromise between conversion efficiency and overall solution size. Lower switching frequency implies reduced switching losses and usually results in higher system efficiency. However, higher switching frequency allows for the use of smaller inductors and output capacitors, hence, a more compact design.

When choosing operating frequency, the most important consideration is thermal limitations. This constraint typically dominates frequency selection. See Figure 9-2 for circuits running at 2.1 MHz. This curves shows how much output current can be supported at a given ambient temperature given a 2.1 MHz. Note that power dissipation is layout dependent so while these curves are a good starting point, thermal resistance in any design will be different from the estimates used to generate Figure 9-2. The maximum temperature ratings are based on a 100-mm × 80-mm, 4-layer EVM PCB design.

Two other considerations are what maximum and minimum input voltage the part must maintain its frequency setting. Since the LM61430-Q1 adjusts its frequency under conditions in which regulation would normally be prevented by minimum on-time or minimum off time, these constraints are only important for input voltages requiring constant frequency operation.

If foldback is undesirable at high input voltage, then use Equation 7:

Equation 7.

If foldback at low input voltage is a concern, use Equation 8:

Equation 8.

where:

• DCR(MAX) = Maximum DCR of the inductor
• t_{OFF_MIN}(MAX) = See the Electrical Characteristics.
• R_{DS(ON)_HS}(MAX) = See the Electrical Characteristics.

The fourth constraint is the rated frequency range of the IC. See f_ADJ in the Electrical Characteristics. All previously stated constraints (thermal, V_IN(MAX2), V_IN(MIN2), and device-specified frequency range) must be considered when selecting frequency.

Many applications require that the AM band can be avoided. These applications tend to operate at either 400 kHz below the AM band or 2.1 MHz above the AM band. In this example, 400 kHz is chosen.