IRDC3856 TM SupIRBuck USER GUIDE FOR IR3856 EVALUATION BOARD DESCRIPTION An output over-current protection function is The IR3856 is a synchronous buck implemented by sensing the voltage developed converter, providing a compact, high across the on-resistance of the synchronous performance and flexible solution in a small rectifier MOSFET for optimum cost and 4mmx5mm Power QFN package. performance. Key features offered by the IR3856 include This user guide contains the schematic and bill programmable soft-start ramp, precision of materials for the IR3856 evaluation board. 0.7V reference voltage, Power Good, The guide describes operation and use of the thermal protection, over voltage protection, evaluation board itself. Detailed application programmable switching frequency, information for IR3856 is available in the synchronization to external clock, Sequence IR3856 data sheet. input, Enable input, input under-voltage lockout for proper start-up, and pre-bias start-up. BOARD FEATURES V = +12V (13.2V Max) in V =+5V (5.5V Max) cc V = +1.8V 0- 6A out F =600kHz s L= 1.0uH C = 3x10uF (ceramic 1206) + 330uF (electrolytic) in C = 4x22uF (ceramic 0805) out 07/21/10 1IRDC3856 CONNECTIONS and OPERATING INSTRUCTIONS A well regulated +12V input supply should be connected to VIN+ and VIN-. A maximum 6A 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. IR3856 has two input supplies, one for biasing (Vcc) and the other as input voltage (Vin). Separate supplies should be applied to these inputs. Vcc input should be a well regulated 4.5V-5.5V supply and it would be connected to Vcc+ and Vcc-. If single 12V application is required connect R7 (zero Ohm resistor) which enables the on board bias regulator (see schematic). In this case there is no need of external Vcc supply. The output can track a sequencing input at the start-up. For sequencing application, R16 should be removed and the external sequencing source should be applied between Seq. and Agnd. The value of R14 and R28 can be selected to provide the desired ratio between the output voltage and the tracking input. For proper operation of IR3856, the voltage at Seq. pin should not exceed Vcc. Table I. Connections Connection Signal Name VIN+ V (+12V) in VIN- Ground of V in Vcc+ Vcc input Vcc- Ground for Vcc input VOUT- Ground of V out VOUT+ V (+1.8V) out Enable Enable Seq. Sequence Input PGood Power Good Signal SYNC External Synchronization Clock LAYOUT The PCB is a 4-layer board. All of layers are 2 Oz. copper. The IR3856 SupIRBuck and all of the passive components are mounted on the top side of the board. Power supply decoupling capacitors, the Bootstrap capacitor and feedback components are located close to IR3856. The feedback resistors are connected to the output voltage at the point of regulation and are located close to the SupIRBuck. To improve efficiency, the circuit board is designed to minimize the length of the on-board power ground current path. 07/21/10 2