ltc3858-2 Linear Technology Corporation, ltc3858-2 Datasheet - Page 15

• Manufacturer Part Number: ltc3858-2
• Description: Ltc3858-2 - Low Iq, Dual 2-phase Synchronous Step-down Controller
• Manufacturer: Linear Technology Corporation

OPERATION
Figure 1 compares the input waveforms for a representative
single-phase dual switching regulator to the LTC3858-2
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
factor 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.
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 Efficiency
. While this is an impressive reduction in itself,
(Refer to the Functional Diagram)
I
IN(MEAS)
= 2.53A
RMS
3.0
2.5
2.0
1.5
1.0
0.5
Figure 2. RMS Input Current Comparison
0
0
V
V
O1
O2
= 5V/3A
= 3.3V/3A
10
RMS
RMS
INPUT VOLTAGE (V)
INPUT CURRENT
INPUT VOLTAGE
DUAL CONTROLLER
3.3V SWITCH
5V SWITCH
500mV/DIV
2
20V/DIV
20V/DIV
,
5A/DIV
SINGLE PHASE
DUAL CONTROLLER
2-PHASE
20
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
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.
30
IN
38582 F02
(Duty Cycle = V
40
I
IN(MEAS)
= 1.55A
RMS
OUT
/V
IN
38582 F01
). Figure 2 shows how
LTC3858-2
15
38582f