LTC3868



3868fb

Figure 2. Input Waveforms Comparing Single-Phase (a) and 2-Phase (b) Operation for Dual Switching Regulators

Converting 12V to 5V and 3.3V at 3A Each. The Reduced Input Ripple with the 2-Phase Regulator Allows

Less Expensive Input Capacitors, Reduces Shielding Requirements for EMI and Improves Efficiency

betweenthetwointernalcontrollers,assummarizedin

Table 1. The phases are calculated relative to the zero

degreesphasebeingdefinedastherisingedgeofthetop

gatedriveroutputofcontroller1(TG1).

Table 1

CONTROLLER 2 PHASE

CLKOUT PHASE

GND

180°

60°

Floating

180°

90°

240°

120°

Output Overvoltage Protection

Anovervoltagecomparatorguardsagainsttransientover-

shootsaswellasothermoreseriousconditionsthatmay

than10%aboveitsregulationpointof0.800V,thetop

MOSFETisturnedoffandthebottomMOSFETisturned

onuntiltheovervoltageconditioniscleared.

Power Good (PGOOD1 and PGOOD2) Pins

Each PGOOD pin is connected to an open drain of an

internalN-channelMOSFET.TheMOSFETturnsonand

voltageisnotwithin±10%ofthe0.8Vreferencevoltage.

ThePGOODpinisalsopulledlowwhenthecorresponding

iswithinthe±10%requirement,theMOSFETisturned

offandthepinisallowedtobepulledupbyanexternal

resistortoasourcenogreaterthan6V.

Theory and Benefits of 2-Phase Operation

Whytheneedfor2-phaseoperation?Upuntilthe2-phase

family, constant frequency dual switching regulators

operated both channels in phase (i.e., single phase

operation).Thismeansthatbothswitchesturnedonat

thesametime,causingcurrentpulsesofuptotwicethe

amplitudeofthoseforoneregulatortobedrawnfromthe

inputcapacitorandbattery.Theselargeamplitudecurrent

pulsesincreasedthetotalRMScurrentflowingfromthe

inputcapacitor,requiringtheuseofmoreexpensiveinput

capacitorsandincreasingbothEMIandlossesintheinput

capacitorandbattery.

With 2-phase operation, the two channels of the dual

switchingregulatorareoperated180degreesoutofphase.

Thiseffectivelyinterleavesthecurrentpulsesdrawnbythe

switches,greatlyreducingtheoverlaptimewheretheyadd

together.TheresultisasignificantreductionintotalRMS

inputcurrent,whichinturnallowslessexpensiveinput

capacitorstobeused,reducesshieldingrequirementsfor

EMIandimprovesrealworldoperatingefficiency.

Figure2comparestheinputwaveformsforarepresenta-

tivesingle-phasedualswitchingregulatortotheLTC3868

2-phasedualswitchingregulator.Anactualmeasurementof

theRMSinputcurrentundertheseconditionsshowsthat

3868 F02

5V SWITCH

20V/DIV

3.3V SWITCH

20V/DIV

INPUT CURRENT

5A/DIV

INPUT VOLTAGE

500mV/DIV