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 10-2 for circuits running at 400 kHz and Figure 10-3 for circuits running at 2.1 MHz. These curves show how much output current can be supported at a given ambient temperature given these switching frequencies. 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 10-2 and Figure 10-3. The maximum temperature ratings are based on a 100-mm x 80-mm, 4-layer EVM PCB design, LM61460EVM. Unless a larger copper area or cooling is provided to reduce the effective R_θJA, if ambient temperature is 105°C and the switching frequency is set to 2.1 MHz, the load current must typically be limited to 4 A.

fSW = 400 kHz

PCB RθJA = 25°C/W

VOUT = 5 V

Figure 10-2 Maximum Ambient Temperature versus Output Current

fSW = 2100 kHz

PCB RθJA = 25°C/W

VOUT = 5 V

Figure 10-3 Maximum Ambient Temperature versus Output Current

Two other considerations are what maximum and minimum input voltage the part must maintain its frequency setting. Since the LM61460-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 Section 8.5
• R_{DS(ON)_HS}(MAX) = see Section 8.5

The fourth constraint is the rated frequency range of the IC. See f_ADJ in Section 8.5. 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.