Post a CommentStepdown converter uses a ceramic output capacitor
Karl R Volk, Maxim Integrated Products, Sunnyvale, CA -- EDN, 6/7/2001
Many stepdown (buck) dc/dc-converter ICs incorporate a voltage-mode-control algorithm. As a result, for stable operation in continuous-conduction mode, the application circuit's output capacitor is normally a high-ESR tantalum type for two reasons. The portion of output ripple due to ESR provides the current-mode signal that's necessary for cycle-to-cycle stability. In the frequency domain, this capacitor also provides a zero that cancels a pole in the buck converter's second-order LC filter, thereby shifting operation back to the stable region by reducing the ripple's phase shift to less than 90°.
The circuit in Figure 1, however, allows the use of an inexpensive ceramic output capacitor. To remove the effects of phase lag in the feedback loop, the circuit derives feedback from the LX pin, via the first-order RC filter comprising R1 and CFF instead of the output. Connecting the tail of CFF to the output node instead of to ground, as you would for a normal filter, provides a fast "feedforward" load-transient response.
A ceramic-capacitor circuit offers several benefits over a standard application circuit. First, ceramic capacitors are more reliable than tantalum capacitors. Second, ceramic capacitors are more readily available than tantalum types. Third, ceramic capacitors cause output ripple of less than 5 mV p-p versus more than 20 mV p-p (Figure 2). For this circuit, the load-transient overshoot is also lower: less than 50 mV p-p versus more than 100 mV p-p.
IC1 , a stepdown dc/dc converter with an internal synchronous rectifier that supplies a fixed 1.8 or 1.5V output at 250 mA from an input range of 2.7 to 5.5V, needs 20 mV p-p or more at its output pin for stable operation under load. To meet this requirement, calculate the value of R1 :
Per the data sheet for the MAX1734, VOUT is 1.5 or 1.8V, L1 is 10 µH, TMIN is 0.4 µsec, ILOADMAX is 250 mA, and IOUTSENSE is 4 µA. The result is R1 =4.3 kW for VOUT =1.8V, and R1 =5.2 kW for VOUT =1.5V. You can therefore round R1 to 5 kW.
Next you calculate the feedforward capacitor value:
If R1 =5 kW and VOUT =1.5V, then CFF 
12 nF. Select CFF =10 nF. Choosing a much smaller value causes excessive load-transient overshoot, and choosing a larger value causes instability under loaded conditions. For optimized load transients, the inductor series resistance should be as follows:
Note that this expression is the typical, not the maximum, inductor resistance. In this case, the value of RL should be approximately 200 mW, which allows you to use a small inductor and causes an approximate efficiency drop of only 3% at maximum loads and much less at lighter loads. Because the inductor time constant, L1 /RL , matches the feedback time constant, R1 ´CFF , the short-term load-transient response equals the dc load regulation (Figure 2). If RL is less than 200 mW, the peak-to-peak load-transient voltage increases, but the dc-load regulation decreases.
Finally, choose COUT large enough for stability:
where DIL is approximately 100 mA when the MAX1734 operates with a 10-µH inductor. In this case, COUT should be greater than 4 µF.
