DEMO MANUAL DC1924A LT8705 80V V and V Synchronous IN OUT 4-Switch Buck-Boost DC/DC Controller DESCRIPTION Demonstration circuit 1924A is a high performance buck- The CLKOUT output and the SYNC input can be used boost converter featuring the LT 8705 that can operate to synchronize two DC1924A circuits with 180 degree from input voltages above, below or equal to the output phase shift. voltage. The demo board input range is 36V to 80V. The By feeding the LT8705 from a separate low voltage sup- output is optimized for 48V at 5A, with the output current ply, the power dissipation can be reduced. To supply the limit set at 7A. (The circuit will operate with lower input LT8705 chip from an external voltage supply (> 6.4V), cut voltage than 36V if load current is reduced). the trace as marked on the board to disconnect the EXTVCC The controller has integrated input current, input voltage, pin from V . The LT8705 will start when voltage is ap- OUT output current and output voltage regulators. The one plied at the input (VIN pin), and when it is running it will regulator that wants to decrease current gets control over draw current from the EXTVCC pin if the voltage is > 6.4V. the compensation pin VC. The inductor current is controlled Typical efficiency with 5A load is above 97% across a 36V by the VC signal that is fed into a current comparator to 72V input range using the supplied inductor. Lower together with a ramp compensation signal. core loss can be achieved by using a ferrite core inductor. While the current mode control limits the inductor current The LT8705 data sheet gives a complete description of both in normal and in reverse direction, these current the part, operation and application information. The data limits have some variation as input voltage changes. The sheet should be read in conjunction with this quick start input and output current regulators offer more accurate guide for demo circuit 1924A. The input voltage range of current limits. the LT8705 itself is 2.8V (need EXTVCC > 6.4V) to 80V The input voltage regulator is typically used in applica- and the output range is 1.3V to 80V. tions with solar panels or other high impedance power The LT8705EUHF is assembled in a 38-Lead (5mm sources, and will reduce the current if the input voltage 7mm) plastic QFN package with a thermal pad underneath drops below the set point. the chip. Proper board layout is essential for maximum The operating mode of the controller is determined thermal and electrical performance. See the data sheet through the MODE pin (jumper JP1) and can be set to section Circuit Board Layout Checklist. discontinuous mode, forced continuous mode and Burst Design files for this circuit board are available at Mode operation. POWER LOSS (W) DISSIPATION (W) DEMO MANUAL DC1924A PERFORMANCE SUMMARY Specifications are at T = 25C A SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS V Input Supply Range Note: Output Power Is Limited When V < 30V 36 48 80 V IN IN V Output Voltage 48 V OUT I Maximum Load Current 5 A IN I Output Current Limit 7 A OUT f Switching Frequency 200 kHz SW EFF Efficiency at DC Input V = 36V, V = 48V, I = 5A 97.5 % IN OUT OUT V = 48V, V = 48V, I = 5A 97.2 % IN OUT OUT V = 72V, V = 48V, I = 5A 97.5 % IN OUT OUT 100 12 100 6 99 11 95 98 10 90 5 97 9 85 96 8 80 4 95 7 75 94 6 70 3 93 5 65 60 2 55 50 1 45 88 0 40 0 30 36 42 48 54 60 66 72 78 0.01 0.1 1 10 V (V) LOAD CURRENT (A) IN EFFICIENCY EFFICIENCY EFFICIENCY FERRITE EFFICIENCY FERRITE DISSIPATION POWER LOSS DISSIPATION FERRITE POWER LOSS FERRITE DC1924a F01a DC1924a F01b Figure 1a. Efficiency and Power Loss as Function of Input Figure 1b. Efficiency and Power Loss in CCM Buck Mode. Voltage at 5A Load. Dotted Lines Show Increased Efficiency V = 56V. Dotted Lines Show Increased Efficiency When IN When the Inductor Is Replaced by a Ferrite Core Inductor the Inductor Is Replaced by a Ferrite Core Inductor (Coiltronics SER2918H-223KL) (Coiltronics SER2918H-223KL) dc1924afa 2 92 4 91 3 90 2 89 1 EFFICIENCY (%) EFFICIENCY (%)