The switching DC-to-DC converters are a widely used solution for portable consumer applications to generate a voltage output that is either higher than the input voltage (i.e., a boost or step-up configuration) or lower (buck or step down configuration). In addition, the switching supplies exhibit high efficiency of power conversion from input to output.
The basic principle of operation of this kind of circuit is based on the fact that in a cyclic mode of operation, performed by opening and closing one or more electronic switches periodically (i.e., repeating every T seconds; see Figure 1), the current that flows into the inductor is periodic:
IL (0) = IL (T) = IL (2T).
Hence, utilizing the typical equation that describes the relationship between the voltage and the current in an inductor:
We see that the integral on a period of the voltage across the inductor is zero:
We can evaluate the integral of the voltage across the inductor for the particular configuration considered (step up, step down, etc.). When we do that, we get the typical voltage conversion ratio for the continuous mode of operation as shown below:
To regulate the output voltage, it’s enough to regulate the duty cycle of the switch. But one has to be sure that the timing for the switch (or switches) is operating in continuous mode (i.e., the inductor current never drops all the way to zero during the switching period; see IL waveform in Figure 1).
I have been in charge of the designing of an integrated voltage switching regulator. Here, all the components of the DC:DC switching converter were integrated into silicon with the exception of the inductor and the bypass and filter capacitors (see Figure 2).
The voltage regulator that I had to qualify had some additional features. There was an Under Voltage Lock Out block (accessible from the UVLO pin) and the Enable Switching block (accessible from the EN pin). These blocks enable (or disable) the switching of the power MOSFETs when the input voltage rises above (or drops below) a threshold; or by a logic control signal. The threshold value can be set by a network of resistors that, in the first release of the product, was external to the integrated device. A logic sequence of bits (closing the proper switches) can be programmed by the customer, setting the resistor value seen from the internal voltage reference (see Figure 2).
This solution was not acceptable to me, because, from my point of view, the customer’s need is to have a finalized solution for the particular application of interest, so I asked to the design team to integrate a trimming network of resistors and to adopt for the switches a fusible solution (see Figure 3).
The design team performed the task and generated a trimming table containing the correspondence between the logic sequences and the percentage of trimming of the reference voltage:
Utilizing the trimming info from Table 1, I programmed the trimming procedure by a special code that I inserted into the testing program at final test stage. This solution was really appreciated by the customer — they had the product exactly as it was requested. Moreover, I had the possibility of modifying the product for other customers, satisfying each request, just by fixing my testing program.
Have you ever experienced an integrating of a functional block that was customer oriented? Do you believe that integration can enhance the effectiveness of an engineer in charge of the start-up of the mass production, making him able to address the customers’ requirements?