Keywords: MAX8505, Internal Switch, Small Footprint
The MAX8505, a 3A internal switch step-down regulator, is designed to meet this requirement. The part is available in a QSOP 16 package, which occupies the same footprint of an SO-8 package. Its 1MHz switching frequency, allows customers to use only ceramic capacitors in their designs, resulting in small footprint power supplies.
Figure 1 shows the schematic of the MAX8505 operating at 1MHz with all ceramic capacitors.
Figure 1. Schematics of the MAX8505 step-down regulator operating at 1MHz.
The MAX8505 employs a peak current mode control architecture that simplifies the voltage feedback loop compensation. A simple Type II compensation network (R1 and C8) stabilizes its feedback loop, further reducing the external component account. The above circuit is designed for 1% initial set output voltage error and 5% maximum output voltage deviation for 50% step load change (25% to 75%), as shown in Figure 2. The output voltage is set to 3.3V, however, other output voltages (from 0.8V to 3.3V) can be set by changing the feedback resistor divider (R2, R3, and R4). The above circuit is stable with any output voltage from 0.8V to 3.3V. Table 1 shows the bill of materials.
Table 1. Bill of Materials
|C1, C2||2||10µF ±20% 10V X5R ceramic capacitor (1206) TDK: C3216X5R0J106M|
|C3, C5, C6, C7||4||0.1µF ±10% 50V X7R ceramic capacitor (0603) TDK: C1608X7R1H104KT|
|C4||1||47µF ±20% 6.3V X5R ceramic capacitor (1210) TDK: C3225X5R0J476M|
|C8||1||220pF ±10% 50V X7R ceramic capacitor (0603) TDK: C1608X7R1H221K|
|C9||1||3300pF ±10% X7R 50V ceramic capacitor (0603) Murata: GRM188R71H332K or TDK: C1608X7R1H332K|
|D1||1||100mA, 30V Schottky diode (SOD-523) Central: CMOSH-3 (Top Mark 53)|
|L1||1||1.0µH Inductor Coilcraft: DO3316P-152 or DO3316P-102|
|R1||1||51.1kΩ, 1% resistor (0603)|
|R2||1||33.2kΩ, 1% resistor (0603)|
|R3||1||10kΩ, 1% resister (0603)|
|R4||1||536kΩ, 1% resistor (0603)|
|R5||1||10Ω, 5% resistor (0603)|
|R6||1||2kΩ, 5% resistor (0603)|
|U1||1||MAX8505EEE (16 pin QSOP)|
Figure 2. Shows the transient response for 50% step load change.
A layout example of the circuit shown in Figure 1 is given in Figure 3. The board has four 1oz copper layers. The total area is about 0.58in2 excluding the jumpers and the POK pullup resistor.