Circuit Design Guide for DC/DC Converters（4/7）

Selecting the C_L

If a larger C_L value is selected, the output ripple becomes smaller. However, an unnecessarily large C_L value increases the dimensions of the capacitor, increasing the cost. Determine the C_L value based on the targeted ripple level. If the targeted ripple level is to be in the range of 10mV to 40mV, you may begin by using the C_L values shown in Table 8 and Table 9 for a step-up and for a step-down, respectively. Note: If your DC/DC converter is not compatible with low ESR capacitors, using these C_L values may cause abnormal switching. If a low ESR capacitor is to be used in the continuous mode, check the load-transient response to confirm that the output voltage is rapidly stabilized (converges within two switching cycles).

Figure 19 shows the variation of output ripple measured by changing only the C_L in the XC9104D093 circuit shown in Figure 20. The ripple increases in proportion to the ESR value and in inverse proportion to the C_L value. In the case of an aluminum electrolytic capacitor, the ESR value is so large that a ceramic capacitor connected in parallel is required for getting output current.

Actual values to be used are obtained by multiplying the above values by the step-up ratio (=V_OUT/V_IN).

Actual values to be used are obtained by multiplying the above values by the step-up ratio (=V_OUT/V_IN).

Figure 19: Output ripple varied with C_L value (XC9104D093)

Figure 20: Test circuit for XC9104D093 shown in Figure 19