LTC3857-1_15 Datasheet, PDF (14/38 Pages) Linear Technology – Low IQ, Dual, 2-Phase Synchronous Step-Down Controller

LTC3857-1
OPERATION (Refer to the Functional Diagram)
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 opera-
tion 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 chan-
nel 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
38571 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
38571 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
38571fc
14