アプリケーションノート 1782

All-Ceramic Capacitors Enable the Smallest Footprint for a 1MHz, 6A Step-Down Regulator


要約: This application note explains how the use of all-ceramic capacitors reduces footprint of 1MHz, 6A step-up converter. The MAX1945R step-down regulator achieves 10µs response time and 1% accuracy, and uses ceramic capacitors to reduce overall size.

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 0.68µH, 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.
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 2. Output ripple voltage and LX switching waveforms.

Figure 3 shows the load transient response. With two 47µF ceramic output capacitors, the maximum output voltage deviation can meet ±5% for 50% step load change (3A). In addition, the transient response finishes within 10µs. 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 10µs for 50% step load change.
Figure 3. Step load transient response: output current from 1A to 4.2A back to 1A. The output voltage back to regulation within 10µs for 50% step load change.

Table 1. Bill of materials
Component
Function
Part Number
Vendor
L1
Output inductor
0.68µH/12A, 6mΩ ESR max, 4783-T007
Sumida
C1
Input filtering cap.
2 x 22µF/6.3V ceramic caps. 20mΩ max each cap. LMK325BJ226MM
Taiyo Yuden
C2
Loop compensation capacitor
100pF/10V, X7R ceramic capacitor
 
C3
Bootstrap cap.
0.1µF/10V, X7R ceramic capacitor
 
C4
Reference bypass cap.
0.22F/6.3V, X7R ceramic capacitor
 
C5
Output filtering cap.
2 x 47µF/6.3V ceramic caps. 20mΩ max each cap. JMK325BJ476
Taiyo Yuden
C6
Input filtering cap
0.1µF/10V, X7R ceramic capacitor
 
R1
VCC filtering resistor
10Ω, 5%
 
R2
Loop compensation resistor
100kΩ, 1%
 
D1
Bootstrap diode
BAT54A, Shorttky diode
Central Semiconductor
IC1
6A step-down regulator
MAX1945R
Maxim

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APP 1782:
アプリケーションノート 1782,AN1782, AN 1782, APP1782, Appnote1782, Appnote 1782