Electronic Design

Stable I<sub>Q</sub> Reduces DC-DC Losses

Inexpensive, low-power dc-dc converters can suffer power inefficiencies if the system isn't designed properly. In a low-power converter, the system efficiency is greatly affected by the controller's quiescent current.

The MC34063A is constructed as either a buck-type or boost-type flexible dc-dc converter, or one that's able to output a negative voltage. Many companies offer the MC34063A in their portfolios. There are differences between designs and IC fabs within and between companies, however. Normal process shifts can make a stable system unstable from lot to lot. To ensure a good design, a system designer must account for all of these factors.

When manufacturing a dc-dc step-down converter, it's best to design the system to operate in discontinuous mode. A step-down converter in continuous mode has zeros in the system loop that must be compensated. In low-power systems, a shift in quiescent current of 1 mA makes a 5% difference in efficiency for a 20-mA input current.

Unstable systems, those whose duty cycle varies more than 10% from cycle to cycle, have higher quiescent currents. The instability in duty cycle causes the controller's internal circuitry to change rapidly, resulting in a higher current draw. Efficiency also is lost in the external circuit components due to higher peak currents. These higher peak currents produce a larger ripple voltage on the input and output.

Figures 1 and 2 illustrate the difference in stable and unstable waveforms.

Table 1 shows the differences in the critical parameters of a system with an 18-V input, a 9.5-V output, and a 30-mA output.

To correct the instability, it's necessary to slow the system loop. The first design approach would be to slow the feedback via RC compensation across the feedback op amp. But the MC34063A doesn't provide access to the output of the feedback op amp. Another choice is to maximize the inductance, the current sense resistor, or the output capacitor. Increasing either the resistor or inductor will limit the amount of variation in duty cycle, thereby decreasing the quiescent current.

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