2 Performance

Figure 2-1 to Figure 2-2 show some benchmark data taken from the circuit above on the LM3477 evaluation board. This evaluation board can also be used to evaluate a buck regulator circuit optimized for a different operating point or to evaluate a trade-off between cost and some performance parameter. For example, the conversion efficiency can be increased by using a lower R_DS(ON) MOSFET, ripple voltage can be lowered with lower ESR output capacitors, and the hysteretic threshold can be changed as a function of the R_SN and R_SL resistors.

The conversion efficiency can be increased by using a lower R_DS(ON) MOSFET, however, it drops as input voltage increases. The efficiency reduces because of increased diode conduction time and increased switching losses. Switching losses are due to the Vds × Id transition losses and to the gate charge losses, both of which can be lowered by using a FET with low gate capacitance. At low duty cycles, where most of the power loss in the FET is from the switching losses, trading off higher R_DS(ON) for lower gate capacitance will increase efficiency.

Figure 2-1 Efficiency vs Load V_OUT = 3.3 V

Figure 2-2 Efficiency vs V_IN V_OUT = 3.3 V, I_OUT = 2 A

Figure 3-1 shows a bode plot of LM3477 open loop frequency response using the external components listed in Table 1-1.

Magnitude = 20 dB/Decade, Bandwidth = 39.8 kHz, Phase = 45°/Decade, Phase Margin = 41°

Figure 2-3 Open Loop Frequency Response V_IN = 5 V, V_OUT = 3.3 V, I_OUT = 1.5 A