Integrates high-efficiency switching power conversion and low-noise linear regulation in a micromodule package

Devices with high-speed or high-resolution functions require clean power. Switching regulators provide high efficiency over a wide range of input/output conditions, but typical switching power supplies have difficulty delivering the clean, low output noise and fast transient response characteristics required for high-data-rate FPGA I/O channels or high-bit-count data converters. In contrast, high-performance linear regulators have low output noise and fast transient response, but they heat up quickly.

The LTM8028 combines the best features of both, a high efficiency synchronous switching converter controlled by an UltraFast™ linear regulator, all in a small 15mm x 15mm µModule® package. The package is available in LGA (4.32mm high) and BGA (4.92mm high) lead styles, both of which are RoHS compliant.

The linear regulator is responsible for controlling the output of the switching power supply to a value 300mV higher than the desired output voltage to provide the best combination of margin, efficiency and transient response performance. The LTM8028 accepts an input of up to 40V and can generate an output voltage between 0.8V and 1.8V at a current of up to 5A. Figure 1 shows a typical 1.2V output application.

Figure 1: LTM8028 in a 36V input, UltraFast, low output noise 5A µModule regulator.

The output voltage of the LTM8028 is set by controlling three three-state inputs (VO0, VO1 and VO2). Applying a voltage to the MARGA pin allows the user to margin the output up to ±10%. The maximum 5A current limit can be reduced using the IMAX pin, while a PGOOD signal is used to indicate that the output is within 10% of the target voltage.

A design using a traditional linear regulator to generate a 1.2V/5A output from a 12V supply would consume over 50W of power and would likely require an expensive heat sink. As shown in Figure 2, the LTM8028 dissipates less than 1/12 of that (less than 4W), resulting in a typical junction temperature rise of only 45°C.

Figure 2: In a 12V input to 1.2V/5A output application, the LTM8028 dissipates less than 4W and heats up only 45°C.

The core of the LTM8028 is a high-performance linear regulator. Its overall line and load regulation is less than 0.2% at room temperature and 1% over the entire –40°C to 125°C temperature range. The UltraFast bandwidth of this linear regulator provides the LTM8028 with a transient response of only 2% for a 10% to 90% load step. Figures 3 and 4 show the transient response of the LTM8028 when the load steps from 0.5A to 5A at a slew rate of 1A/µs when the device is configured to provide 1V and 1.8V outputs, respectively.

Figure 3: At 1.0V output, the LTM8028's transient response is less than 20mV.

Figure 4: The LTM8028 has a transient response of only 38mV. High power supply rejection and integrated noise reduction circuitry produce low output noise, despite the linear regulator and synchronous switching converter being packaged together. As shown in Figure 5, peak-to-peak noise is less than 1mV.*

Figure 5: The peak-to-peak switching noise at the LTM8028 output is less than 1mV. (The schematic shows the setup used to achieve these results.)

In the frequency domain, the spectral noise content is very low, with a peak of 4µV/√Hz at the switching converter's 300kHz fundamental frequency, as shown in Figure 6. This is important when powering high-bit-count data conversion circuits.

Figure 6: Output noise spectral density of only 4µV/√Hz peak makes the LTM8028 an excellent choice for highly sensitive data conversion circuits.

in conclusion

When system designs require low power loss, tight regulation, fast transient response and low output noise, the LTM8028 µModule regulator can be used. The device combines the best features of a high performance switching regulator and a linear regulator in a single space-efficient package.

Product brochures, demo boards and other application information are available at www.linear.com/LTM8028.

Note

* Measuring low amplitude noise can be tricky. This measurement was done with coaxial cable, impedance matching, and a 150MHz HP461A amplifier. This is similar to the setup described in Linear Technology's Application Note 70, "A Monolithic Switching Regulator with 100μV Output Noise" (by Jim Williams), the only difference being that the measurement here does not limit the bandwidth to 10MHz.