What you'll learn:
- How does a current-mode regulator operate?
- Advantages of current-mode regulators.
- Disadvantages of current-mode regulators.
Thousandsofdifferentswitchingregulators are available onthemarket.Selectionis based on specifications such as input-voltage range, output-voltage capability, maximumoutputcurrent,andaslewofotherparameters. Thisarticleexplainscurrentmode, a differentiating feature commonly listed in datasheets, and its advantages anddisadvantages.
Figure1 showsthebasicworkingprincipleofacurrent-moderegulator.Here,the feedbackvoltageisnotonlycomparedwithaninternalvoltagereference,butalsowithasawtoothvoltagerampforgeneratingthenecessary pulse-width-modulation (PWM)signal for the power switch.
The slope of this ramp is fixed in voltage-mode regulators. Incurrent-moderegulators,theslopedependsontheinductorcurrentandis yielded from the current measurement shown in Figure 1 at the switching node. This is what differentiates current-mode regulators from voltage-mode regulators.
Advantages of Current-Mode Regulators
Current-mode regulators offer many advantages. One is that the inductorcurrentimmediatelyadaptstochangesintheinputvoltage(VINinFigure1).Thus, the input-voltage change information is directly fed into the control loop, even before the output voltage (VOUT in Figure 1) tracks this input-voltage change.
Anotherkeyadvantageissimplifiedcontrol-loopcompensation.TheBodeplotofa voltage-moderegulatorshowsadoublepole;acurrent-moderegulatorgenerates justonesimplepoleofthepowerstageatthispoint.Thisproducesaphaseshift of90 degrees insteadof the 180 degreeswithadoublepole. Thus,acurrent-moderegulatorcanbe much more easily compensated and subsequently stabilized. Figure 2 shows the simple transfer function of the power stage of a typical current-mode regulator.
Disadvantages to Weigh
However, a few disadvantages exist alongside the mentioned advantages. Current-mode regulators can’t immediately make the required current measurements after a switching transition because the noise will couple into the measurement strongly at this time. It takes a few nanoseconds for the noise caused by the switching to subside. This is called the blanking time. It normally results in a somewhat longer minimum on-time specification than for voltage-mode regulators.
Anotherdisadvantageofcurrent-moderegulatorsisthepossibility,inprinciple,ofasubharmonicoscillation (Fig. 3).Ifadutycycleof greater than 50% is required, a current-mode regulator may alternately execute short and long pulses. In many applications, this is considered instability, which should be avoided.
To overcome this issue, a certain ramp compensation can be added to the generated current ramp shown in Figure 1. It can shift the critical duty-cycle threshold to well above 50% so that even at higher duty cycles, there are no subharmonic oscillations.
Even these earlier mentioned restrictions, due to the blanking time and the resultant duty-cycle limitations, can be circumventedthrough the IC design. For example, one remedy is to incorporate low-side current sensing where the inductor current is measured during the off-time rather than the on-time.
Allinall,theadvantagesofcurrent-modecontrolinswitchingregulatorsoutweigh thedisadvantagesformostapplications.Andthroughvariouscircuitinnovations andmodifications,thedisadvantagescanbebypassed.That’s whymostswitching-regulator ICs today use current-mode control.