DC2123B: 24V @ 5A, 8 ~ 56V in, Buck/Boost

The DC2123B is a 60V four-switch synchronous buck-boost controller that demonstrates the high power capability of the LT®3790. The output is 24V and the maximum output current is 5A. The switching frequency is 200kHz and efficiency is as high as 98% for a 24V input.

The LT3790 is capable of high power operation. DC2123B can be altered for increased output power with higher minimum input voltage. If VIN remains always above 20V, IOUT can be increased to 12A+ (for 290W output) by changing RS2 to 4mΩ.

The operating input voltage range of DC2123B is from 8V to 56V. The output voltage, EN/UVLO, and OVLO are all programmed by resistor dividers. EN/UVLO is set so the circuit will turn off when the input voltage falls below 8V and will turn on when the input voltage rises above 9.6V. OVLO is set to engage for input voltages above 57V.

Current sense resistors program input and output current limits and also determine the monitoring voltages that indicate input and output current. IVINMON provides a 30mV/1A voltage that is used to monitor the input current. Input current limit occurs at 33.3A and is set by RIN1. ISMON provides a 160mV/1A voltage that is used to monitor the output current. The output current limit is 7.5A and is set by RS2.

The demo circuit features MOSFETs that compliment the 5V gate drive of the LT3790 to achieve high efficiency. 60V MOSFETs are used on the input side of the four-switch topology while 40V MOSFETs are used on the output side. 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 is an aluminum polymer capacitor on the output that assures feedback loop stability, even at low temperatures. The input has an aluminum electrolytic capacitor in addition to ceramic capacitors. A two-stage L-C input filter can easily be added for electro-magnetic compatibility. The PCB has large copper planes and extensive vias for thermal performance.

The CLKOUT output and the SYNC input can be used to synchronize switching between two or more DC2123B circuits. A resistor from SYNC to ground must be removed prior to using the SYNC input.SHORT and C/10 are open-collector status flag outputs and are pulled up to the INTVCC pin voltage. A resistor shorts the CCM pin to C/10 and causes the circuit to change to discontinuous conduction mode (DCM) when C/10 is active at light load currents. CCM can also be connected with a resistor to INTVCC instead of C/10 for continuous conduction mode (CCM) operation over the entire load range.

The CTRL input is pulled up to the VREF pin through a 0? 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 and additionally SS is pulled up to the VREF pin through a 100k resistor. The switching frequency is adjustable with a resistor.

The demo circuit is designed to be easily reconfigured to many other applications, including the example schematics in the data sheet. Consult the factory for assistance.

High power operation, four-switch buck-boost topology, fault protection and full monitoring make the LT3790 attractive for high power voltage regulator circuits and also circuits that require output current regulation such as battery chargers. The LT3790EFE is available in a thermally enhanced 38-lead TSSOP package. The LT3790 data sheet must be read in conjunction with this demo manual to properly use or modify demo circuit DC2123B.