LTC3728 Linear Technology, LTC3728 Datasheet - Page 12

• Manufacturer Part Number: LTC3728
• Description: 2-Phase Synchronous Step-Down Switching Regulator
• Manufacturer: Linear Technology

OPERATIO
LTC3728
Figure 3 compares the input waveforms for a representa-
tive single-phase dual switching regulator to the LTC1628
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
tion in itself, remember that the power losses are propor-
tional to I
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/con-
nector 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
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
operation are not just limited to a narrow operating range,
but in fact extend over a wide region. A good rule of thumb
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.
A final question: If 2-phase operation offers such an
advantage over single-phase operation for dual switching
12
RMS
IN
RMS
Figure 3. 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 LTC1628 2-Phase Regulator Allows
Less Expensive Input Capacitors, Reduces Shielding Requirements for EMI and Improves Efficiency
to 1.55A
(Duty Cycle = V
2
, meaning that the actual power wasted is
U
RMS
(Refer to Functional Diagram)
. While this is an impressive reduc-
OUT
I
IN(MEAS)
/V
IN
= 2.53A
(a)
). Figure 4 shows how
RMS
DC236 F03a
INPUT CURRENT
INPUT VOLTAGE
3.3V SWITCH
5V SWITCH
500mV/DIV
20V/DIV
20V/DIV
5A/DIV
regulators, why hasn’t it been done before? The answer is
that, while simple in concept, it is hard to implement.
Constant-frequency current mode switching regulators
require an oscillator derived “slope compensation” signal
to allow stable operation of each regulator at over 50%
duty cycle. This signal is relatively easy to derive in single-
phase dual switching regulators, but required the develop-
ment of a new and proprietary technique to allow 2-phase
operation. In addition, isolation between the two channels
becomes more critical with 2-phase operation because
switch transitions in one channel could potentially disrupt
the operation of the other channel.
These 2-phase parts are proof that these hurdles have
been surmounted. They offer unique advantages for the
ever-expanding number of high efficiency power supplies
required in portable electronics.
3.0
2.5
2.0
1.5
1.0
0.5
Figure 4. RMS Input Current Comparison
0
0
I
V
V
IN(MEAS)
O1
O2
= 5V/3A
= 3.3V/3A
10
= 1.55A
(b)
INPUT VOLTAGE (V)
DUAL CONTROLLER
RMS
2-PHASE
SINGLE PHASE
DUAL CONTROLLER
20
www.DataSheet4U.com
DC236 F03b
30
3728 F04
40
3728fb