1.5A High Efficiency Switching Regulator

features 8 individually con-trolled 1.3A rated floating NMOS switch channels and can support up to

Demonstration circuit 2719A is a high performance bidirectional buck-boost converter featuring the LT8708/ LT8708-1 that can operate from input voltages above, below or equal to the output voltage. The demo board input range is 10.5V to 14.5V (25VMAX). The output voltage is set at 14.5V and the output current limit at 40A. The board can operate in both forward and reverse mode. The controller has integrated input voltage and output voltage regulators and two sets of input and output current regulators that control current flow in forward or reverse direction. Features are included that simplify bidirectional power conversion in battery/capacitor backup systems and other applications that may need regulation of VIN, VOUT, IIN and/or IOUT. While the current mode control limits the inductor current both in normal and in reverse direction these current limits have some variation as input/output voltage changes. The forward and reverse input and output current regulators offer four accurate current limits that can be set individually. The input voltage regulator is often used in applications with high impedance power sources and will reduce the forward current if the input voltage drops below the set point. When operating with reverse current flow, the input voltage regulator regulates the voltage at the input side. The operating mode of the controller is determined through the MODE pin (jumper JP9 Pins 5 to 12) and can be set to discontinuous mode, hybrid discontinuous mode, forced continuous mode and Burst Mode® operation. The LT8708 is capable of bidirectional operation when operating in the continuous conduction mode (CCM). DCM, HCM and Burst Mode operation only allow power to flow in one direction. Additional circuitry may be needed depending on the application.

Demonstration circuit 1031A-B is a 36V-72Vin, synchronous forward converter featuring the LTC3725 forward converter and the LTC3726 secondary-side controller. This circuit was designed specifically to attain a high current, low ripple, synchronously rectified forward converter to efficiently power 3.3V loads at up to 20A from a typical telecom input voltage range. This circuit features secondary side control of the supply eliminating the need for an optocoupler.

Demonstration circuit 2596A is a high performance bidirectional buck-boost converter featuring the LT®8708 that can operate from input voltages above, below or equal to the output voltage. The demo board input range is 36V to 75V. The output voltage is set at 48V and the output current limit at 6A. The controller has integrated input voltage and output voltage regulators and two sets of input and output current regulators that control current flow in forward or reverse direction. Features are included that simplify bidirectional power conversion in battery/capacitor backup systems and other applications that may need regulation of VIN, VOUT, IIN and/or IOUT. While the current mode control limits the inductor current both in normal and in reverse direction these current limits have some variation as input/output voltage changes. The forward and reverse input and output current regulators offer four accurate current limits that can be set individually. The input voltage regulator is often used in applications with high impedance power sources and will reduce the forward current if the input voltage drops below the set point. When operating with reverse current flow the input voltage regulator regulates the voltage at the input side. The operating mode of the controller is determined through the MODE pin (jumper JP7 Pins 5 to 12) and can be set to discontinuous mode, hybrid discontinuous mode, forced continuous mode and Burst Mode® Operation. The LT8708 is capable of bidirectional operation when operating in the continuous conduction mode (CCM). DCM, HCM and Burst Mode operation only allow power to flow in one direction. Additional circuitry may be needed depending on the application. By feeding the LT8708 from a separate low voltage supply the power dissipation can be reduced. The LT8708 will start when >5.5V voltage is applied at the input (VIN pin), and when it is running it will draw current from the EXTVCC pin if the voltage at the pin is > 6.4V. Typical efficiency with 5A load is above 96.3% across a 36V to 75V input range. The LT8708 data sheet gives a complete description of the part, operation and application information. The data sheet should be read in conjunction with this quick start guide for DC2596A. The input voltage range of the LT8708 itself is 2.8V (need EXTVCC > 6.4V) to 80V and the output range is 1.3V to 80V. The LT8708EUHG is assembled in a 40-Lead (5mm × 7mm) plastic QFN package with a thermal pad underneath the chip. Proper board layout is essential for maximum thermal and electrical performance. See the data sheet Circuit Board Layout Checklist section.

Demonstration circuit DC2626A is a 4-switch synchronous buck-boost regulator that demonstrates the high power capability of the LT8392. The output is 12V and the maximum output current is 12A for up to 144W power delivery. The switching frequency is 200kHz and efficiency can exceed 97%. The steady-state operating input voltage range of DC2626A in which the temperature of the components is less than 90°C is from 9V to 18V. The transient operating input voltage range of DC2626A is from 3V to 36V. The output voltage and EN/UVLO are all programmed by resistor dividers. EN/UVLO is set so the circuit will turn off when the input voltage falls below 3V and will turn on when the input voltage rises above 4V. The PCB has large copper planes and extensive vias for excellent high power thermal performance. DC2626A features MOSFETs that complement the 5V gate drive of the LT8392 to achieve high efficiency. 40V AEC-Q101 MOSFETs are used on the input and output side of the four-switch topology. Ceramic capacitors are used at both the circuit input and output because of their small size and high ripple current capability. In addition to ceramic capacitors, there are bulk aluminum polymer capacitors on the input and output to make input and output stable during transient period. The CTRL input is pulled up to the VREF pin through a 0Ω resistor to set the output current limit to its maximum, and an external voltage on CTRL can be used to lower the current limit if the resistor is removed. A capacitor at the SS pin programs soft-start. To improve the EMI performance, the LT8392 has spread spectrum frequency modulation. With the SYNC/SPRD pin tied to INTVCC, LT8392 spreads its switching frequency ±15% around the programmed oscillator frequency. The PGOOD status flag indicates when output voltage is within ±10% of the final regulation voltage. The LT8392’s proprietary peak current mode buck-boost architecture ensures DC2626A runs either in discontinuous conduction mode (DCM) or pulse-skipping mode (PSM) without reverse inductor current. Both modes enhance the light load efficiency. The demo circuit is designed to be easily reconfigured to suit other applications, including the example schematics in the data sheet. Consult the factory for assistance High power operation, 3V input voltage operation, 4-switch buck-boost topology, proprietary peak current mode architecture, fault protection and output current monitoring make the LT8392 attractive for high power voltage regulator circuits and also circuits whose input voltage drops to 3V such as cold crank of car battery. It is also suitable for output current regulation such as battery chargers. The LT8392JFE is available in a thermally enhanced 28 lead TSSOP package. The LT8392 data sheet must be read in conjunction with the demo manual to properly use or modify demo circuit DC2626A.

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