Design Attention DN572: Introduction


LT8672 is an active rectifier controller (along with MOSFET) that provides reverse current protection and correction for power supply in automotive environments. This work has traditionally been carried out by Schottky diodes, and the active protection of LT8672 has many advantages:

Minimum power consumption

Small, predictable, and regulated 20mV voltage drop.

LT8672 also includes many features to meet the supply railway needs of automotive environments:

Reverse input protection up to -40V.

Wide input operating range: 3V to 42V.

Ultra fast transient response

Rectify 6VP-P to 50kHz; Calibrate 2VP-P to 100kHz.

The performance of the integrated boost regulator driven by FET is superior to that of the charge pump device.

Figure 1 shows a complete protection solution.

Fast response to input ripple rectification.

Automotive standards - ISO 16750 or LV124- specify that automotive electronic control units (ECUs) may face an additional AC ripple, up to 6VP-P and up to 30kHz. The gate drive of LT8672, which controls the external MOSFET, is powerful enough to handle fluctuating frequencies up to 100kHz, minimizing reverse current. Figure 2 shows an example of AC ripple rectification.


Compared to Schottky diodes, it has lower power consumption.

The performance of LT8672 (using IPD100N06S4-03 as an external MOSFET) can be compared to Schottky diode (CSHD10-45L), as shown in Figure 3. Here, a 12V power supply is used to simulate the voltage supply of a car and output a constant load current of 10A. The thermal performance of the two solutions in a stable state is shown in Figure 4. Without cooling, the thermal performance of the LT8672 solution is much higher, reaching a peak temperature of only 36, while the Schottky diode solution achieves a higher 95.1.

Additional low input voltage operation capability.

The key circuits of a car must be able to operate under cold crankshaft conditions, when the car battery voltage can collapse to 3.2V. Considering this, many automotive grade electronic products are designed to operate up to 3V input. The variable positive voltage drop of Schottky can cause problems in cold cranks, where the downstream voltage generated by this drop is 2.5V to 3V, which is too low for some systems. In contrast, the LT8672 solution ensures the required 3V as it regulates a voltage drop of 20mV, making circuit design easier and system robustness improved.

When VBATT drops to 3.2 V, the LT8672 control system (a) maintains VIN>3 V, allowing the LT8650S VSYS to maintain its output stable at 1.8 V. However, in the Schottky diode system (b), the input voltage VIN LT8650S is lower than its minimum operating voltage, preventing maintenance of the VSYS output at 1.8 V.



Comprehensive improvement of regulatory agencies

Many alternative active rectifier controllers use a charge pump to drive gate drive. These solutions often fail to provide strong gate charging current and regulated output voltage, limiting the frequency range and performance of continuous rectification. The integrated boost regulator of LT8672 provides a strictly controlled gate drive voltage and powerful gate drive current.


Conclusion

LT8672 can correct the impact of high-frequency AC on automotive supply. It uses an integrated boost regulator to drive MOSFETs for ultrafast reactions during continuous rectification, improving the charge pump solution. It provides low-power rectification and reverse input protection, as well as a very wide operating range (suitable for cold cranks) in a small 10 leading MSOP package.