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Why Does Current-Mode Control in Switching Regulators Matter?

Oct. 18, 2022
When it comes to the control aspect in switching regulators, current mode is often the preferred choice over voltage mode. This article looks at the multiple advantages—and some rectifiable disadvantages of this important specification.

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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.

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