Datasheet LTC3701 (Analog Devices) - 9

LTC3701
U OPERATIO (Refer to Functional Diagram)
Why the need for 2-phase operation? Until recently, con- both channels switching. A single phase dual regulator stant frequency dual switching regulators operated both system with both sides switching would exhibit twice the channels in phase (i.e., single phase operation). This single side numbers. In this example, 2-phase operation means that both topside MOSFETs are turned on at the reduced the RMS input current from 1.79ARMS to same time, causing current pulses of up to twice the 0.91ARMS. While this is an impressive reduction in itself, amplitude of those from a single regulator to be drawn remember that power losses are proportional to I 2 RMS , from the input capacitor. These large amplitude pulses meaning that actual power wasted is reduced by a factor increase the total RMS current flowing into the input of 3.86. The reduced input ripple current also means that capacitor, requiring the use of more expensive input less power is lost in the input power path, which could capacitors, and increasing both EMI and losses in the include batteries, switches, trace/connector resistances, input capacitor and input power supply. and protection circuitry. Improvements in both conducted and radiated EMI also directly accrue as a result of the With 2-phase operation, the two channels of the LTC3701 reduced RMS input current and voltage. are operated 180 degrees out of phase. This effectively interleaves the current pulses coming from the switches, Of course, the improvement afforded by 2-phase opera- greatly reducing the overlap time where they add tion is a function of the dual switching regulator’s relative together. The result is a significant reduction in the total duty cycles, which in turn are dependent upon the input RMS input current, which in turn allows for use of less voltage VIN. Figure 4 shows how the RMS input current expensive input capacitors, reduces shielding requirements varies for 1-phase and 2-phase operation for 2.5V and for EMI and improves real world operating efficiency. 1.8V regulators over a wide input voltage range. Figure 3 shows example waveforms for a single switching It can be readily seen that the advantages of 2-phase regulator channel versus a 2-phase LTC3701 system with operation are not limited to a narrow operating range, but in fact extend over a wide region. A good rule of thumb for
Single Phase 2-Phase
most applications is that 2-phase operation will reduce the
Dual Controller Dual Controller
input capacitor requirement to that for just one channel operating at maximum current and 50% duty cycle. SW1 (V) 2.0 SW2 (V) 1.8 1.6 SINGLE PHASE DUAL CONTROLER 1.4 IL1 1.2 2-PHASE DUAL CONTROLER 1.0 0.8 I 0.6 L2 0.4 INPUT CAPACITOR RMS CURRENT 0.2 VOUT1 = 2.5V/2A VOUT2 = 1.8V/2A 0 2 3 4 5 6 7 8 9 10 IIN INPUT VOLTAGE (V) 3701 F04 3701 F03
Figure 3. Example Waveforms for a Single Switching Figure 4. RMS Input Current Comparison Regulator Channel vs 2-Phase LTC3701 System with Both Channels Switching
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