Shutdown

VOUT

3.3V/500mA

COUT 1QQ|iF

C|N = AVX TPSC226M016R0375 COUT = AVX TPSE107M016R0125 ~ Lext = SUMIDA CD54-470

Figure 34. Improved Transient Response

VOUT

3.3V/500mA

COUT 1QQ|iF

C|N = AVX TPSC226M016R0375 COUT = AVX TPSE107M016R0125 ~ Lext = SUMIDA CD54-470

Figure 34. Improved Transient Response

50mA and 200mA, peaking at 92% at 100mA and remaining above 82% all the way to the maximum 500mA load. Current limit is set at 500mA in this example; it can be reduced by lowering the value of RIMAX. CSS sets the startup time at approximately 25ms.

The circuit in Figure 33 relies on the ESR of the output capacitor to maintain loop stability with just a single capacitor at the COMP pin. Figure 33 uses a surface mount electrolytic capacitor with about 400mQ ESR. A low ESR tantalum output capacitor can improve the transient response at the output but requires a more complex compensation network at the COMP pin (Figure 34). There is a tradeoff to be made here: the minimum component count solution is the simplest and uses the least expensive components but pays a penalty in transient response. The low ESR circuit in Figure 34 has improved transient response and actually uses less board space: the tantalum output capacitor is smaller than the electrolytic device used in Figure 33 and the additional compensation components are tiny 0603 surface mount devices.

Note that the input bypass capacitor in both Figures 33 and 34 is an AVX TPS type, a relatively costly surge-tested tantalum capacitor. This is a small, surface mount device that has a surge current rating adequate to support the 500mA maximum load current of the LTC1504. Buck regulators (like the LTC1504) inherently draw large RMS currents from the input bypass capacitor, and the capacitor type chosen must be capable of withstanding this current

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