ltc3890 Linear Technology Corporation, ltc3890 Datasheet - Page 14

• Manufacturer Part Number: ltc3890
• Description: 60v Low Iq, Dual, 2-phase Synchronous Step-down Dc/dc Controller
• Manufacturer: Linear Technology Corporation

OPERATION
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 signifi cant 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 effi ciency.
Figure 1 compares the input waveforms for a representa-
tive single-phase dual switching regulator to the LTC3890
2-phase dual switching regulator. An actual measurement of
the RMS input current under these conditions shows that
2-phase operation dropped the input current from 2.53A
to 1.55A
remember that the power losses are proportional to I
meaning that the actual power wasted is reduced by a fac-
tor of 2.66. The reduced input ripple voltage also means
less power is lost in the input power path, which could
include batteries, 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
LTC3890
14
RMS
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 Effi ciency
. While this is an impressive reduction in itself,
(Refer to the Functional Diagram)
I
IN(MEAS)
= 2.53A
RMS
RMS
RMS
INPUT CURRENT
INPUT VOLTAGE
3.3V SWITCH
2
5V SWITCH
500mV/DIV
,
20V/DIV
20V/DIV
5A/DIV
voltage V
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.
IN
Figure 2. RMS Input Current Comparison
3.0
2.5
2.0
1.5
1.0
0.5
(Duty Cycle = V
0
0
I
IN(MEAS)
V
V
O1
O2
= 5V/3A
= 3.3V/3A
= 1.55A
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INPUT VOLTAGE (V)
DUAL CONTROLLER
RMS
OUT
2-PHASE
SINGLE PHASE
DUAL CONTROLLER
20
/V
IN
). Figure 2 shows how
3890 F01
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3890 F02
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