Feature Power Electronics A blend of speed and efficiency


David Ng at Linear Technology Corp. explores the working dynamics of a high efficiency switching power conversion device that combines low noise linear regulation in a µModule package, offering the best of both worlds


Figure 1 Figure 2


evices with high speed or high resolution functions require clean power. Switching regula- tors offer efficiency across a variety of input/output conditions, but the typical switcher is hard pressed to deliver the clean, low output noise and fast transient response needed by high data rate FPGA I/O channels or high bit count data converters. In contrast, high performance linear regulators feature low output noise and fast transient response, but can quickly heat up. Linear Technology Corp. has devel- oped the LTM8028 that combines the best of both worlds - a high efficiency synchronous switching converter con- trolled by an UltraFast linear regulator, both integrated into a small, 15mm by 15mm µModule package. The device is available in LGA (4.32mm tall) and BGA (4.92mm tall) lead styles, both of which are RoHS compliant.


D


The linear regulator controls the output of the switcher to 300mV above the desired output voltage to provide the optimum combination of head- room, efficiency and transient response. This device accepts inputs as high as 40V and produces output voltages between 0.8V and 1.8V at up to 5A. A typical 1.2V output applica- tion is shown in Figure 1. The output voltage is set by control- ling three 3-state inputs, VO0 VO2


, VO1 and


. Applying a voltage to the MARGA pin allows the user to margin the


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


Figure 2:


In a 12V input to 1.2V output, 5A application, this device dissipates less than 4W and heats up by only 45°C


Figure 3


output by as much as ±10 percent. The current limit may be reduced from the 5A maximum through the IMAX pin, and a PGOOD signal indicates that the output is within 10 percent of the target voltage.


A design using a traditional linear regulator providing 1.2V at 5A from a 12V source would burn over 50W and might require expensive heat sinking.


Figure 3


At 1.0V output, its transient response is less


than 20mV


The LTM8028, as shown in Figure 2, dissipates a twelfth of that, less at 4W, yielding a typical junction temperature rise of only 45°C.


The heart of this device is the high


performance linear regulator. Its total line and load regulation is below 0.2 percent at room temp and 1 percent over its full -40°C to 125°C tempera- ture range. The UltraFast bandwidth gives this device a 10 percent – 90 percent load step transient response of only 2 percent. Figures 3 and 4 show the transient response when the load steps from 0.5A to 5A at a slew rate of 1A/µs when the device is configured to deliver 1V and 1.8V, respectively. Even though the linear regulator and the synchronous switching converter are packaged together, high power supply rejection and integrated noise mitigation result in low output noise.


In the frequency domain, the spectral noise content is very low, peaking at


4µV/√Hz at the switching converter’s fun- damental frequency of 300kHz. This is important when powering high bit count data conversion circuits. Ultimately this regulator is ideal for when a system design requires low power loss, tight regulation, fast transient response, and low output noise. It combines the best features


of high performance


Figure 4. The LTM8028 transient response is only 38mV


Visit www.linear.com/LTM8028 for data sheets, demo boards and other applications information 22 DECEMBER/JANUARY 2014 Electronics


switching and linear regulators into a single, space efficient package. Linear Technology Corporation www.linear.com


Figure 4


David Ng is Power Module Development Manager at Linear Technology Corporation


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