LT8550

Demonstration circuit 2331A-A/2331A-B is a 4-phase DC/DC expander with internal gate drivers for buck converters that features the LT8550. The demonstration circuit expands (increases) the number of power sections for a LT3763 60V step-down controller to provide higher output power. The increased number of power sections allowed by the LT8550 results in more available output power without a corresponding increase in the difficulty of the design or a sacrifice of LT3763 features. The demo board uses the multi-chip (master-slave) feature of the LT8550 so the total number of power sections can be increased in four power section increments:

• DC2331A-A has one power section for the LT3763 primary controller and an additional four power sections for its LT8550 which is configured as a master
• DC2331A-B does not have a primary controller, and its LT8550 is configured as a slave with four power sections

The DC2331A-A and DC2331A-B use the same PCB and the PCB is an example of how ground plane layers should be configured and traces routed for reliable performance.

The input voltage range of DC2331A-A/DC2331A-B is from 14V to 56V and the output is 12V. The maximum output current is 14A per power section, so the maximum output current of DC2331A-A is 70A and each slave DC2331A-B adds an additional 56A of output current capability. The switching frequency is 250kHz and at 24V input to 12V_OUT at full load, the efficiency of DC2331A-A is 97.5%.

The DC2331A-A/DC2331A-B is designed so it is easy to add power sections and increase output power. The master and slave assemblies are programmed using jumpers and adjustment of regulation loop compensation is usually not necessary as power sections are added. The control signal interface between master and slave assemblies is a common ribbon cable. The power cabling between master and slave assemblies is similarly simple.

The DC2331A-A/DC2331A-B's jumpers program the phase relationship between the power sections and turn phase shedding on and off. Jumper JP5 programs the LT8550 SYNC pin for master or slave operation and allows DC2331A-A to be synchronized to an external clock. The VCC and RSYNC JP5 settings - and the DC2331A assemblies in general - are forward compatible with the next generation of phase expander ICs.

A ribbon cable provided with DC2331A-B interfaces the control logic between a master DC2331A-A and a single or multiple slave DC2331A-B(s). Each DC2331A-A/DC2331A-B has two identical ribbon cable headers, so daisy-chain ribbon cables are not required, even for systems with multiple slave assemblies. The DC2331A-A/DC2331A-B uses high current #10 bolt-on terminals for the input supply and load connections; and a sufficient quantity of #10 ring lugs is provided to parallel master and slaves with minimal DC offset voltage between assemblies. VIN−, VIN+, VO− and VO+ terminals allow convenient monitoring of input and output voltages while minimizing the measured I • R PCB voltage drops caused by high currents.

The DC2331A-A/DC2331A-B has terminals for SHDN and SYNC inputs to the LT8550. There are also terminals that make it easy to monitor the control logic between master and slave DC2331s. Removal of jumper resistors can cleanly separate the electrical interface between the LT3763 primary controller and the remainder of DC2331A-A, making it possible to connect an alternate primary controller in place of the LT3763. Noise immune signal and ground via pairs are available to re-establish the connections between DC2331A-A and twisted wire pairs sensing the new primary controller.

A combination of capacitors is used at the DC2331A-A/DC2331A-B power input: 1210 ceramics, 2220 ceramics in a metal frame, hybrids and bulk electrolytic capacitors.

The 2220 ceramics in a metal frame are robust and convenient but may be replaced with an equivalent amount of 1210 capacitance to minimize electrical noise. The hybrid and bulk electrolytic capacitors stabilize the input power. A combination of ceramic and aluminum polymer capacitors are used at the power output in accordance with design practice for the LT3763 primary controller.

DC2331A-A/DC2331A-B includes filters and filter options for inputs to the LT8550 in accordance with data sheet recommendations. SHDN voltage, switching frequency and LT8550 ILIM current limit levels can all be adjusted by resistors or resistor jumpers. DC2331A-A/DC2331A-B comes with a housekeeping circuit that provides an 11.5V input voltage to the LT8550. The housekeeping circuit reduces the power dissipation that would otherwise occur at high input voltages in regulator transistor Q21. DC2331A-A/DC2331A-B uses resistors to sense inductor current, but can be configured for DCR sensing of inductor current.

DC2331A-A/DC2331A-B retains the features of the LT3763 primary controller including:

• two inputs for analog control of the output current regulation level
• output current monitoring
• fault indication.

A resistor divider combined with the CTRL1 output current regulation level control makes the LT3763 current limit occur before the LT8550 current limit to ensure that the power sections share equally during output current overload conditions. A resistor-NTC voltage divider combined with the CTRL2 control reduces output current when the PCB temperature reaches approximately 100°C. A sense resistor for input current can be added and used for input current regulation and monitoring. DC2331A-A has placeholders for optional components that will allow a voltage applied to the CTLV terminal to make modest changes to the output voltage.

A high level of available output power without a corresponding high level of design complexity or loss of controller features makes the LT8550 attractive for high cur-rent DC bus and battery systems in commercial, industrial and automotive settings. DC2331A-A/DC2331A-B features the LT8550EUKG in a thermally enhanced 52-pin 7mm × 8mm QFN package. The LT8550 and LT3763 data sheets must be read in conjunction with the demo manual to properly use or modify DC2331A-A/DC2331A-B.