The MAX1734 voltage-mode buck DC-DC converter was design to work with medium ESR tantalum capacitors; however, by slightly changing the feedback scheme, small, low-ESR ceramic capacitors may be used. A schematic, design equations, and load-transient response waveforms are provided.
Many stepdown (buck) DC-DC controller ICs incorporate a voltage-mode control algorithm. As a result (for stable operation in continuous-conduction mode), the resulting application circuit's output capacitor is normally a high-ESR tantalum type. The circuit of Figure 1, however, allows use of an inexpensive ceramic output capacitor. To remove the effects of phase lag in the feedback loop, feedback is derived from the LX pin instead of the output.
A ceramic-capacitor circuit offers several benefits over the standard application circuit. First, ceramic capacitors are more readily available than tantalum types. Second, (see Figure 2) they cause less output ripple (<5mVPP vs. >20mVPP), and less load-transient overshoot (<50mVPP vs. >100mVPP). IC11 needs 20mVPP or more at the OUT pin for stable operation under load. To meet this requirement, first calculate the R1 value:
Per the MAX1734 data sheet, VOUT is 1.5V or 1.8V, L1 is 10µH, Tmin is 0.4µsec, ILOADMAX is 250mA, and IOUTSENSE is 4µA. The result is R1 = 4.3kΩ for VOUT = 1.8V, and R1 = 5.2kΩ for VOUT = 1.5V. R1 may therefore be rounded to 5kΩ. Next, calculate the feedforward-capacitor value:
If R1 = 5kΩ and VOUT = 1.5V, then Cff < 12nF. Select Cff = 10nF. Choosing a much smaller value will cause excessive load-transient overshoot, and choosing a larger value will cause instability under loaded conditions. For optimized load transients, the inductor series resistance should be
In this case the RL value should be about 200mΩ, which allows use of a small inductor and causes an approximate efficiency drop of only 3% at maximum load. Because the inductor time constant L1/RL is matched to the feedback time constant R1 × Cff, the short-term load-transient response equals the DC load regulation (Figure 2). If RL is chosen less than 200mΩ, the peak-to-peak load-transient voltage will increase but the DC load regulation will decrease.
Finally, choose COUT large enough for stability:
where ΔIL is approximately 100mA when the MAX1734 operates with a 10µH inductor. In this case, COUT should be greater than 4µF.
1The MAX1734 stepdown DC-DC converter supplies a fixed 1.8V or 1.5V output at 250mA from an input voltage range of 2.7V to 5.5V. Its 5-pin SOT23 package and internal synchronous rectifier allows a small application circuit with a minimum number of external components.
A similar version of this article appeared in the June 7, 2001 issue of EDN magazine.