LTC3857 Datasheet, PDF (14/38 Pages) Linear Technology – Low IQ, Dual, 2-Phase Synchronous Step-Down Controller

LTC3857
Operation (Refer to the Functional Diagram)
pulses increased the total RMS current flowing from the
input capacitor, requiring the use of more expensive input
capacitors and increasing both EMI and losses in the input
capacitor and battery.
With 2-phase operation, the two channels of the dual
switching regulator are operated 180 degrees out of phase.
This effectively interleaves the current pulses drawn by the
switches, greatly reducing the overlap time where they add
together. The result is a significant reduction in total RMS
input current, which in turn allows less expensive input
capacitors to be used, reduces shielding requirements for
EMI and improves real world operating efficiency.
Figure 1 compares the input waveforms for a single-phase
dual switching regulator to a 2-phase dual switching
regulator. An actual measurement of the RMS input cur-
rent under these conditions shows that 2-phase operation
dropped the input current from 2.53A_RMS to 1.55A_RMS.
While this is an impressive reduction in itself, remember
that the power losses are proportional to I_RMS2, meaning
that the actual power wasted is reduced by a factor of 2.66.
The reduced input ripple voltage also means less power is
lost in the input power path, which could include batter-
ies, switches, trace/connector resistances and protection
circuitry. Improvements in both conducted and radiated
EMI also directly accrue as a result of the reduced RMS
input current and voltage.
Of course, the improvement afforded by 2-phase operation
is a function of the dual switching regulator’s relative duty
cycles which, in turn, are dependent upon the input voltage
V_IN (Duty Cycle = V_OUT/V_IN). Figure 2 shows how the RMS
input current varies for single-phase and 2-phase operation
for 3.3V and 5V regulators over a wide input voltage range.
It can readily be seen that the advantages of 2-phase op-
eration are not just limited to a narrow operating range,
for most applications is that 2-phase operation will reduce
the input capacitor requirement to that for just one channel
operating at maximum current and 50% duty cycle.
3.0
SINGLE PHASE
2.5
DUAL CONTROLLER
2.0
1.5
2-PHASE
DUAL CONTROLLER
1.0
0.5 _{VO1 = 5V/3A}
V_O2 = 3.3V/3A
0
0
10
20
30
INPUT VOLTAGE (V)
40
3857 F02
Figure 2. RMS Input Current Comparison
5V SWITCH
20V/DIV
3.3V SWITCH
20V/DIV
INPUT CURRENT
5A/DIV
INPUT VOLTAGE
500mV/DIV
I_IN(MEAS) = 2.53A_RMS
I_IN(MEAS) = 1.55A_RMS
3857 F01
Figure 1. 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
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