What kind of controller is needed for a small power supply that meets strict efficiency and performance specifications?

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What kind of controller is needed for a small power supply that meets strict efficiency and performance specifications?

February 10 , 2023 0

Asics, FPGas, and processors in high-performance communications, servers, and computing systems require core power supplies that can generate 1.0 V (or less) voltage directly from 12 V or an intermediate bus -- maximum load currents can sometimes be higher than 200 A. These power supplies must meet strict efficiency and performance specifications and usually have relatively small PCB sizes. The LTC7852/LTC7852-1 6-phase dual output step-down controller provides a high-performance flexible solution for these power supplies.

Designed to achieve high efficiency, the LTC7852/LTC7852-1 does not use an internal grid-level driver for each phase and can generate a PWM output connected to a power module, DrMOS, or an external gate driver and a separate MOSFET. DrMOS devices integrate gate drivers and MosFETs in a single package for a smaller overall size solution and higher efficiency. They are mainly suitable for 12 V input voltage. External gate drivers and MOSFETs have excellent heat dissipation and can operate at higher input voltages. The LTC7852 uses a subMω DCR detector architecture to accurately detect current. Its DCR value is only 0.2 mΩ, which significantly reduces conduction losses. The LTC7852-1 is designed for use with DrMOS devices that provide their own current detection signals.

Each output is differentially tested for a voltage range of 0.5V to 2.0V (2.0V limit applies only to the LTC7852), with an overall adjustment accuracy of ±0.5%. Since the LTC7852 and LTC7852-1 are biased from an external 5 V power supply and not from the input voltage, the converter's input voltage range is not IC limited. Its switching frequency ranges from 250 kHz to 1.25 MHz, but its 40 ns minimum on-off time allows for a high step-down ratio.

The 3+3, 4+2, and 5+1 dual output phase configurations can be selected through the PHCFG pin. In A 3+3 configuration, the two outputs can be used in parallel for a 6-phase converter with a maximum load current of 240 A. Using one 6-phase controller instead of two 3-phase controllers or three 2-phase controllers greatly simplifies the design and layout. Only two controllers are needed for up to 12 phase operation.

The LTC7852 comes in a 5 mm x 6 mm GQFN package and the LTC7852-1 comes in a 4 mm x 5 mm QFN package.

6 phase high efficiency core power supply

The figure below shows A 6-phase 1.0V /200 A LTC7852 converter with a switching frequency of 400 kHz and a 12 V input. The power stage of each phase consists of a 5 mm × 5 mm DrMOS and a 0.25 µH ferrite inductor with a typical DCR value of 0.325 mΩ and a final full load efficiency of 90.0% (Figure 2). At full load at room temperature, the hot spot temperature of 200 LFM flow is 78°C (Figure 3). Due to strict current sharing, the temperature difference between inductors is less than 6°C.

Schematic diagram of 6-phase 1.0V /200 A LTC7852 converter using FDMF5820DC DrMos FSWITCH = 400 kHz

Circuit efficiency diagram at V IN = 7 V, 10 V, 12 V and 14 V

Thermal image of the circuit at V IN = 12 V, full load, 24°C ambient temperature and 200 LFM airflow

The LTC7852 uses a proprietary peak-current mode submω DCR detection architecture to improve the signal-to-noise ratio of current detection signals. The DCR detection filter in the inductance provides an amplified AC signal for the SNSP and SNSN pins. The second filter is cascaded with the first to provide a DC signal for the SNSP and SNSAVG pins. The LTC7852 amplifies the DC signal and sums with the AC signal to reconstruct the signal. The rebuilt signal is five times larger than the original signal, so it can operate stably and cleanly at DCR values as low as 0.2 mΩ.

The LTC7852's IMON1 and IMON2 signals generate a signal proportional to the load current in the corresponding channel and use the V1P5 pin as a voltage reference. This signal can be used by the power monitor or ADC and microcontroller for load detection.

Weekly term flow is an inherent advantage of the peak current mode architecture. Belch mode current limiting provides additional protection. If an overcurrent failure occurs for more than 32 cycles, the converter stops the switch for a period of time as set by the soft start capacitor. After this interval is over, the switching operation resumes switching with a soft start. As shown in Figure 4, during the failure period, the converter switches on and off at relatively short periods, which results in greatly reduced thermal stress on the MOSFETs and inductors.

Conclusion

The LTC7852/LTC7852-1 is a flexible, high-performance 6-phase dual-output step-down controller designed to deliver high efficiency and highly reliable power supply via DrMOS, power modules, or external gate drivers and MosFETs. Features include subMω DCR detection (LTC7852), optional phase configuration, a 0.5V reference voltage source with total adjustment accuracy of ±0.5%, differential output detection, switching frequency range of 250 kHz to 1.25 MHz, and belch mode current limiting protection.