Core power supply that must generate 200A load current
LTC7852/LTC7852-1 dual output buck controller,
Supports design of 6-phase converters up to 240A


ASICs, FPGAs, and processors used in communications, servers, and computing systems require core power supplies that generate voltages directly from 12V, or from an intermediate bus, at 1V and sub-1V, sometimes with load currents exceeding 200A.

These power supplies must meet stringent efficiency and performance requirements, while also requiring a small PCB footprint. Analog Devices (ADI)'s 6-phase dual-output buck controllers, the LTC7852 and LTC7852-1, meet these requirements.

Neither controller uses internal gate drivers. Each phase generates a PWM output that interfaces with the power block and DrMOS, or with an external gate driver and discrete MOSFET.

DrMOS devices integrate the gate driver and MOSFET, contributing to solution miniaturization and efficiency, and are typically designed for a 12V input voltage. External gate drivers and MOSFETs ensure superior thermal performance and operation at higher input voltages.

The LTC7852 utilizes a sub-mΩ DCR sensing architecture to enable accurate current sensing with DCR values as low as 0.2mΩ, significantly reducing conduction losses. The LTC7852-1 is designed for use with DrMOS devices that provide their own current sense signal.

Each output is differentially sensed. The output range is 0.5 V to 2.0 V, and the total regulation accuracy is ±0.5%. The LTC7852 and LTC7852-1 are biased using an external 5V supply rather than the input voltage, so there are no input voltage restrictions on the converter. The switching frequency range is 250kHz to 1.25MHz, and the minimum on-time is 40ns.

The two controllers can use the PHCFG pin to select between 3+3, 4+2, or 5+1 phase configurations for their two outputs. In the 3+3 configuration, two outputs can be connected in parallel to achieve a 6-phase converter with a maximum load current of 240 A.

Design and layout are significantly simplified by using a single 6-phase controller instead of two 3-phase controllers or three 2-phase controllers. Up to 12-phase operation is possible when two controllers are used. The LTC7852 is available in a 5mm x 6mm GQFN package, and the LTC7852-1 is available in a 4mm x 5mm QFN package.

◇ 6-phase high-efficiency core power supply

[Figure 1] is a 6-phase 1.0V/200A converter using LTC7852. It operates at a 400kHz switching frequency and 12V input, and the power stage of each phase consists of a 5mm×5mm DrMOS and a 0.25μH ferrite inductor with a rated DCR of 0.325mΩ.

[Figure 2] shows that the resulting full-load efficiency is 90.0%. [Figure 3] shows that with 200LFM airflow at full load, the hotspot temperature is 78℃ at room temperature. Due to seamless current sharing, the temperature difference between the inductors is less than 6°C.

◇ Sub-mΩ DCR detection

The LTC7852 uses a proprietary peak current-mode sub-mΩ DCR detection architecture to improve the signal-to-noise ratio (SNR) of the current sense signal. The DCR detection filter on the inductor provides an amplified AC signal to the SNSP/SNSN pin, and a secondary filter, cascaded to the primary filter, provides a DC signal to the SNSP/SNSAVG pin. The LTC7852 amplifies the DC signal and sums it with the AC signal to reconstruct the reconstructed signal, which is 5 times the original signal. This allows clean, stable operation with a DCR value of approximately 0.2mΩ.

◇ Output current monitoring and overcurrent protection

The LTC7852's IMON1 and IMON2 pins generate signals proportional to the load current of the corresponding channel and are referenced to the V1P5 pin. These signals are used for power supply monitoring or load sensing by ADCs and MCUs.

Cycle-by-cycle current limiting is a fundamental advantage of the peak current mode architecture. Hiccup mode current limiting provides additional protection. If an overcurrent fault persists for more than 32 cycles, the converter stops switching for a period of time set by the soft-start capacitor. When this interval expires, switching resumes with a soft start.

As shown in [Figure 4], in case of a fault, the converter switches at relatively short intervals, significantly reducing the thermal stress on the MOSFETs and inductors.

In summary, the LTC7852 and LTC7852-1 are ideal for high-efficiency, high-reliability power supplies using DrMOS and power blocks, or external gate drivers and MOSFETs. They also feature selectable phase configurations, ±0.5% total regulation accuracy with a 0.5V reference, differential output sensing, a 250kHz to 1.25MHz switching frequency range, and hiccup mode current limiting. The LTC7852 also supports sub-mΩ sensing.



This article is a summary of the article titled “Flexible, Dual Output, 6-Phase Buck Controller Drives 12 V to 1.0 V/200 A Core Supply with 90.0% Efficiency” co-authored by Mike Shriver, Senior Application Engineer, and Yingyi Yan, Senior IC Design Engineer, Analog Devices (ADI).