Figure 5 shows a high efficiency solution for 3.8 V/5 A output from 12 V input for automotive applications using the LT8636. The circuit runs at 400 kHz for very high efficiency, and an XAL7050-103 10 µH inductor is used. It maintains efficiency above 90% with loads as light as 4 mA and as high as 5 A. The peak efficiency is 96% at 1 A.
Figure 6 shows the efficiency from µA to 5 A for this solution. The internal regulator is supplied from the 5 V output through the BIAS pin to minimize power dissipation. Peak efficiency reaches 95%; full load efficiency is 92% for a 5 V output from 13.5 V input. Light load efficiency remains at or above 89% for loads down to 30 mA for the 5 V application. The converter runs at 2 MHz and the inductor used for the test is an XEL6060-222 to optimize the efficiency in both heavy and light loads, in a relatively compact solution. Light load efficiency can be further improved—to above 90%—by using a larger inductor. The current in the feedback resistor divider is minimized as it appears to the output as load current.
Figure 7 shows thermal performance for this solution under a 4 A constant load plus a 4 A pulsed load (8 A total at pulse) with a duty cycle of 10% (of 2.5 ms)— from a 13.5 V input and still air at ambient room temperature. Even at a 40 W pulsed power and 2 MHz switching frequency, the LT8636 case temperature remains below 40°C, enabling the circuit to run safely up to 8 A in short periods with no fans or heat sinks. This is possible with a 3 mm × 4 mm LQFN package because of enhanced thermal packaging technology and the LT8636’s high efficiency at high frequency.
Shrink Solution Size with High Frequency Operation
Space is an increasing premium in automotive applications, necessitating that power supplies shrink to fit costly board footprints. Increasing a power supply’s switching frequency enables the use of smaller external components such as capacitors and inductors. Plus, as previously mentioned, in automotive applications, switching frequencies above 2 MHz (or below 400 kHz) keep the fundamental out of the AM radio band. Let’s compare a commonly used 400 kHz design to a 2 MHz design. In this case, quintupling the switching frequency to 2 MHz reduces the required inductance and output capacitance to one-fifth of the 400 kHz design. Seems easy. Nevertheless, even ICs that are high frequency capable may not be usable in many applications because of some of the trade-offs inherent in using a high frequency solution.
For instance, high frequency operation in high step-down ratio applications requires a low minimum on time. According to the equation VOUT = TON × fSW × VIN, at a 2 MHz operating frequency, an on time for the top switch (TON) of about 50 ns is required to produce 3.3 V from 24 V input. If the power IC cannot achieve this low on time, pulses must be skipped to maintain the low regulated output—essentially defeating the purpose of the high switching frequency. That is, the equivalent switching frequency (due to pulse skipping) is likely in the AM band. With minimum top switch on time of 30 ns, the LT8636 allows direct high VIN to low VOUT conversion at 2 MHz. In contrast, many devices are limited to a >75 ns minimum, requiring they be operated at low frequency, 400 kHz, for higher step-down ratios to avoid skipping pulses.