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

Selecting the CL

If a larger CL value is selected, the output ripple becomes smaller. However, an unnecessarily large CL value increases the dimensions of the capacitor, increasing the cost. Determine the CL 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 CL 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 CL 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 CL in the XC9104D093 circuit shown in Figure 20. The ripple increases in proportion to the ESR value and in inverse proportion to the CL 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.

Table 8: Recommended CL values for a step-up
Output current Ceramic OS Tantalum Aluminum electrolytic
0mA-300mA 20µF 22µF 47µF 100µF+2.2µF(with ceramic capacitor)
300mA-600mA 30µF 47µF 94µF 150µF+2.2µF(with ceramic capacitor)
600mA-900mA 40µF 100µF 150µF 220µF+4.7µF(with ceramic capacitor)
900mA-1.2A 50µF 150µF 220µF 470µF+4.7µF(with ceramic capacitor)

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

Table 9: Recommended CL values for a step-down
Output current Ceramic OS Tantalum Aluminum electrolytic
0mA-500mA 10µF 15µF 22µF 47µF+2.2µF(with ceramic capacitor)
500mA-1.5A 20µF 22µF 33µF 100µF+2.2µF(with ceramic capacitor)
1.5A-3A 20µF 33µF 47µF 100µF+4.7µF(with ceramic capacitor)
3A-5A 30µF 47µF 68µF 220µF+4.7µF(with ceramic capacitor)

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

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

Figure 20: Test circuit for XC9104D093 shown in Figure 19

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