The user selects the desired type based on different parameters, such as input voltage range, output voltage range, maximum output current, and many others. This article introduces current mode, an important feature commonly found in datasheets, and describes the advantages and disadvantages of this mode.
Current Mode Regulators Explained
Figure 1 shows the basic operating principle of a current-mode regulator. Here, not only is the feedback voltage compared to the internal reference voltage, but it is also compared to the sawtooth voltage ramp used to generate the PWM signal required for power switching. In a voltage-mode regulator, the slope of this ramp is fixed. In current-mode regulators, the slope depends on the inductor current and is calculated from the current measurement at the switch node shown in Figure 1. Herein lies the difference between current mode regulators and voltage mode regulators. Current-mode regulators offer several advantages. The first is that the inductor current adjusts instantly with changes in the input voltage (VIN in Figure 1). Therefore, information about the input voltage change is fed directly to the control loop, even before the output voltage (VOUT in Figure 1) tracks the detection of such a change in the input voltage.
Figure 1. The basic operating principle of a current-mode regulator.
The advantages of current-mode control technology are so obvious that most switching regulator ICs on the market use this current-mode control operating principle.
Another key advantage is the simplified control loop compensation. The baud diagram of a voltage mode regulator shows a double pole; in contrast, a current mode regulator generates only a single pole in the power stage, producing a 90° phase shift instead of the 180° phase shift of a double pole. Therefore, it is easier to compensate for a current-mode regulator, and it is more stable. Figure 2 shows a simple conversion function for the power stage of a typical current-mode regulator.
Figure 2. Simplified control loop compensation implemented by current-mode control using a Porter diagram showing only a single pole in the power stage.
However, in addition to the advantages mentioned, the regulator also has disadvantages. After the switching transition, the current-mode regulator cannot immediately implement the required current measurement, because if the measurement is performed at this point, the measurement results will contain a lot of noise. It is necessary to wait a few nS for the switching-induced noise to abate. This time is called the fading time. This usually results in a slightly longer minimum on time than the minimum on time of voltage mode regulators. Another disadvantage of current-mode regulators is their potential to generate sub-harmonic oscillations. This is shown in Figure 3. If the required duty cycle is greater than 50%, the current-mode regulator may alternate between short and long pulses. In many applications, this is considered to be unstable and needs to be avoided. To avoid this instability, some ramp compensation can be added to the generated current ramp shown in Figure 1. This allows the critical duty cycle threshold to be adjusted to well above 50%, ensuring that subharmonic oscillations do not occur even at higher duty cycles.
Figure 3. Switching node voltage: subharmonic oscillations with a current-mode regulator.
Even these previously mentioned limitations (caused by the fading time and its resulting duty cycle limitations) can be circumvented by IC design. One remedy, for example, is to use low-side current detection, measuring the inductor current during turn-off, rather than during turn-on.
Conclusion
In summary, the advantages of current-mode switching regulators outweigh their disadvantages in most applications. Moreover, the disadvantages can be circumvented by various circuit innovations and improvements. Therefore, today, most switching regulator ICs use current-mode control.
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