AN1782

POWER-SUPPLY CIRCUITS Nov 11, 2002 Small Footprint, 10us Response Time, 10mV Output Ripple, 1MHz, 6A Step-Down Regulator One benefit of high-frequency switching is the fast loop response. This translates to smaller footprint and lower cost, because the ceramic capacitors can be used. The MAX1945 internal switch step-down regulator is designed for high-frequency switching. It can be switched at 1MHz. Its peak current mode control architecture allows simple Type II compensation (R2 and C2) to achieve high bandwidth, further reducing the external component count. As shown in Figure 1, for 6A output, only two 1210 footprint ceramic capacitors are needed to meet the 50% step load change, thus greatly reducing the output capacitor size when compared to switching at 500kHz or less. A second benefit is due to the low ESR of the ceramic capacitors. Large inductor ripple current is allowed to achieve the 1% ripple voltage requirement.The value of the output inductor is 0.68uH, resulting in a footprint of 8.2mm x 8.2mm for 6A output current, further reducing buck converter size. Figure 1. Schematic of the MAX1945 for 1MHz all ceramic capacitor application. In Figure 1, the output inductor is the largest external component with the footprint of 8.2mm x 8.2mm. However, with the new developments at TOKO and Sumida we expect the inductor size to be reduced to 7.2mm x 7.2mm, further reducing the regulator footprint size. Figure 2 shows the output ripple voltage, whose peak-to-peak value is less than 10mV for 1.8V output, well less than 1%. Figure 2. Output ripple voltage and LX switching waveforms. Figure 3 shows the load transient response. With two 47uF ceramic output capacitors, the maximum output voltage deviation can meet ±5% for 50% step load change (3A). In addition, the transient response finishes within 10us. Table 1 lists the bill of materials. Figure 3. Step load transient response: output current from 1A to 4.2A back to 1A. The output voltage back to regulation within 10us for 50% step load change. Table 1. Bill of Materials Component L1 C1 C2 C3 C4 Function Output inductor Input filtering cap. Loop compensation capacitor Bootstrap cap. Reference bypass cap. Part Number 0.68uH/12A, 6m ESR max, 4783-T007 2 x 22uF/6.3V ceramic caps. 20m max each cap. LMK325BJ226MM 100pF/10V, X7R ceramic capacitor 0.1uF/10V, X7R ceramic capacitor 0.22F/6.3V, X7R ceramic capacitor Vender Sumida Taiyo Yuden C5 C6 R1 R2 D1 IC1 Output filtering cap. Input filtering cap Vcc filtering resistor Loop compensation resistor Bootstrap diode 6A step-down regulator 2 x 47uF/6.3V ceramic caps. 20m max each cap. JMK325BJ476 0.1uF/10V, X7R ceramic capacitor 10 , 5% 100k, 1% BAT54A, Shorttky diode MAX1945R Taiyo Yuden Central Semiconductor MAXIM November 2002