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

Selecting the CFB

CFB is a capacitor for adjusting the ripple feedback and influences the load-transient response. The optimum CFB values for L values are shown in Table 10. Selecting CFB values either smaller or larger than the optimum values will deteriorate the operation stability.

Influences of CFB in the XC9220C093 have been measured in the circuit shown in Figure 26. In this circuit, when RFB1 is 82kΩ, FZFB will be 10kHz with CFB of about 390pF. Load-transient responses varied with CFB are shown in Figure 23 (CFB=39pF), Figure 24 (CFB=390pF) and Figure 25 (CFB=1000pF). With CFB=39pF, the voltage drops sharply when the load becomes heavy but the normal voltage is restored shortly. With CFB=1000pF, the voltage drop is small when the load current is increased heavily but restoration of the normal voltage takes time.

Table 10: Standard fZFB for determining optimum CFB
Item fZFB = (1/(2π x RFB1 x CFB))
( ) indicates the adjustable range
30kHz when L=10μH
20kHz when L=22μH
10kHz when L=47μH
XC9213B093 10kHz
(Adjustable between 1kHz and 50kHz)
(Adjustable between 0.5kHz and 20kHz)
(Adjustable between 0.1kHz and 20kHz)
XC9220/XC9221 5kHz
(Adjustable between 1kHz and 20kHz)
XC9223/XC9224 20kHz
(Adjustable between 1kHz and 50kHz)

Figure21: Load-transient response of XC9220C093 ( IOUT=0mA to 200mA, CFB=39pF)

Figure 22: Load-transient response of XC9220C093 ( IOUT=0mA to 200mA, CF=390pF)

Figure 23: Load-transient response of XC9220C093 ( IOUT=0mA to 200mA, CFB=1000pF)

Figure 24: Test circuit for XC9220C093 Figures 23 through 25

Figure 25 shows the standard CFB values varied with RFB1 and FZFB.

Figure 25: Relationship between RFB1 and CFB