Circuit Design Guide for DC/DC Converters(2/7)

How to select the Switching Frequency

DC/DC converter circuits have their unique switching frequencies. In general, they affect the circuit properties as shown in Table 2 below:

Table 2. Relationships between switching frequency and properties
Properties Low High
Maximum efficiency High Low
Output current at maximum efficiency Light load Heavy load
Ripple Large Small
Response speed Slow Fast

Figure 7 and Figure 8 show the relationships between switching frequencies and efficiencies of the step-down models XC9235/XC9236 (1.2MHz) and XC9235/XC9236 (3MHz), respectively, as examples. As you see, the Influences of switching frequency on efficiency as indicated in Table 2 are apparent. With two models, electric current values at the maximum efficiency are different. This is because if switching frequencies differ, complying inductance values differ too. With coils of the same structure, the larger the inductance is, the larger the direct-current resistance becomes, increasing the loss at times of heavy-load. Thus, if the switching frequency becomes lower, the current value at the maximum efficiency moves toward the light-load side. On the contrary, if the switching frequency becomes higher, the charge/discharge frequency of the FET and the IC's unique quiescent current increase: On the 3MHz model, the efficiency at the light-load condition is substantially reduced compared to the 1.2MHz model. When totally reviewing these influences, we can see that the 1.2MHz model has a higher maximum efficiency (the peak value is higher than the 3MHz model on the graph) and the output current at the maximum efficiency is small (the peak is to the leftward of the 3 MHz model on the graph). Also, when PFM is actuated, the frequencies at the light-load time are lowered in both models, substantially improving the efficiencies.

Figure 7: XC9235/XC9236, VOUT=1.8V (with switching frequency of 1.2 MHz)

CIN:10µF CL:10µF L=4.7µH(NR3015T-4R7M) Topr=25℃

Figure 8: XC9235/XC9236, VOUT=1.8V (with switching frequency of 3 MHz)

CIN:10µF CL:10µF L=4.7µH(NR3015T-4R7M) Ta=25℃

Figure 9: Test circuit for XC9235/XC9236 illustrated in. Figures 7 and 8