User Guide For IR3821 Evaluation Board

By wordpainter Aug 7th, 2009

The IR3821 is a synchronous buck converter, providing a compact, high performance and flexible solution in a small 5mmx6mm Power QFN package. Key features offered by the IR3821 include programmable soft-start ramp, precision 0.6V reference voltage, programmable Power Good, thermal protection, fixed 600kHz switching frequency requiring no external component, input under-voltage lockout for proper start-up, and pre-bias start-up. An output over-current protection function is implemented by sensing the voltage developed across the on-resistance of the synchronous rectifier MOSFET for optimum cost and performance. This user guide contains the schematic and bill of materials for the IR3821 evaluation board. The guide describes operation and use of the evaluation board itself. Detailed application information for IR3821 is available in the IR3821 data sheet.

BOARD FEATURES

• Vin = +12V (13.2V Max)

• Vout = +1.8V @ 0- 7A

• L=1.0uH

• Cin=3x10uF (ceramic 1206) + 330uF (electrolytic)

• Cout=6x22uF (ceramic 0805)

CONNECTIONS and OPERATING INSTRUCTIONS
A well regulated +12V input supply should be connected to VIN+ and VIN-. A maximum 7A load should be connected to VOUT+ and VOUT-. The connection diagram is shown in Fig. 1 and inputs and outputs of the board are listed in Table I. IR3821 has two input supplies, one for biasing (Vcc) and the other as input voltage (Vin). These inputs are connected on the board with a zero ohm resistor (R15). Separate supplies can be applied to these inputs. Vcc input cannot be connected unless R15 is removed. Vcc input should be a well regulated 5V-12V supply and it would be connected to Vcc+ and Vcc-.

LAYOUT
The PCB is a 4-layer board. All of layers are 2Oz. copper. The IR3821 SupIRBuck and all of the passive components are mounted on the top side of the board. Power supply decoupling capacitors, the charge-pump capacitor and feedback components are located close to IR3821. The feedback resistors are connected to the output voltage at the point of regulation and are located close to SupIRBuck. To improve efficiency, the circuit board is designed to minimize the length of the on-board power ground current path.