6
DEMO MANUAL DC236
DESIGN-READY SWITCHERS
that 2-phase operation lowers the input current from
2.53A
RMS
to 1.55A
RMS
.
Although this is an impressive reduction in itself, remem-
ber that the power losses are proportional to I
RMS
2
,
meaning that the actual power wasted is reduced by a
factor of 2.66. The reduced input ripple voltage also means
less power 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
voltage V
IN
(Duty Cycle = V
OUT
/V
IN
). Figure 4 shows how
the RMS input current varies for 1-phase and 2-phase
operation for 3.3V and 5V regulators over a wide input
voltage range.
It can be readily seen that the advantages of 2-phase
operation are not 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 1-phase operation for dual switching
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
1-phase dual switching regulators, but required the devel-
opment 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 poten-
tially disrupt the operation of the other channel.
The LTC1628 is proof that these hurdles have been sur-
mounted. The new device offers unique advantages for the
ever expanding number of high efficiency power supplies
required in portable electronics.
DC236 Operation
The LTC1628 switching regulator performs high effi-
ciency DC/DC voltage conversion while maintaining con-
stant frequency over a wide range of load current, using a
2-phase current mode architecture. The 2-phase approach
results in 75% less power loss (and heat generated) in the
input source resistance because dissipated power is
proportional to the square of the RMS current. The input
ripple frequency is also double the individual controller’s
switching frequency, further reducing the input capaci-
tance requirement. Reducing peak currents and doubling
the radiated frequency significantly reduces EMI related
problems.
The internal oscillator frequency is set by the voltage
applied to the FREQSET pin. The FREQ jumper on the
demonstration board allows selection of three different
voltages: 0V, 1.2V when the jumper is left off, and 5V. The
internal oscillator will run at 130kHz, 200kHz and 300kHz
respectively. The frequency can be continuously varied
over a 130kHz to 300kHz range by applying an external 0V
to 2.4V to the FREQSET pin.
High efficiency is made possible by selecting either of two
low current modes: 1) Burst Mode operation for maximum
efficiency and 2) constant frequency, burst disable mode
for slightly less efficiency. Constant frequency is desirable
in applications requiring minimal electrical noise.
INPUT VOLTAGE (V)
0
INPUT RMS CURRENT (A)
3.0
2.5
2.0
1.5
1.0
0.5
0
10
20
30
40
DC236 F04
SINGLE-PHASE
DUAL CONTROLLER
2-PHASE
DUAL CONTROLLER
V
O1
= 5V/3A
V
O2
= 3.3V/3A
Figure 4. RMS Input Current Comparison
OPERATIO
U
