LTC1702A_15 Datasheet, PDF (8/36 Pages) Linear Technology – Dual 550kHz Synchronous 2-Phase Switching Regulator Controller

LTC1702A
APPLICATIONS INFORMATION
2-Step Conversion
“2-step” architectures use a primary regulator to convert
the input power source (batteries or AC line voltage) to an
intermediate supply voltage, often 5V. This intermediate
voltage is then converted to the low voltage, high current
supplies required by the system using a secondary regu-
lator— the LTC1702A. 2-step conversion eliminates the
need for a single converter that converts a high input
voltage to a very low output voltage, often an awkward
design challenge. It also fits naturally into systems that
continue to use the 5V supply to power portions of their
circuitry, or have excess 5V capacity available as newer
circuit designs shift the current load to lower voltage
supplies.
Each regulator in a typical 2-step system maintains a
relatively low step-down ratio (5:1 or less), running at high
efficiency while maintaining a reasonable duty cycle. In
contrast, a regulator taking a single step from a high input
voltage to a 1.xV or 2.xV output must run at a very narrow
duty cycle, mandating trade-offs in external component
values and compromising efficiency and transient
response. The efficiency loss can exceed that of using a
2-step solution (see the 2-Step Efficiency Calculation
section and Figure 14). Further complicating the calcula-
tion is the fact that many systems draw a significant
fraction of their total power off the intermediate 5V supply,
bypassing the low voltage supply. 2-step solutions using
the LTC1702A usually match or exceed the total system
efficiency of single-step solutions, and provide the addi-
tional benefits of improved transient response, reduced
PCB area and simplified power trace routing.
2-step regulation can buy advantages in thermal manage-
ment as well. Power dissipation in the LTC1702A portion
of a 2-step circuit is lower than it would be in a typical 1-
step converter, even in cases where the 1-step converter
has higher total efficiency than the 2-step system. In a
typical microprocessor core supply regulator, for ex-
ample, the regulator is usually located right next to the
CPU. In a 1-step design, all of the power dissipated by the
core regulator is right there next to the hot CPU, aggravat-
ing thermal management. In a 2-step LTC1702A design,
a significant percentage of the power lost in the core
regulation system happens in the 5V supply, which is
usually located away from the CPU. The power lost to heat
in the LTC1702A section of the system is relatively low,
minimizing the added heat near the CPU.
See the Optimizing Performance section for a detailed
explanation of how to calculate system efficiency.
2-Phase Operation
The LTC1702A dual switching regulator controller also
features the considerable benefits of 2-phase operation.
Notebook computers, hand-held terminals and automo-
tive electronics all benefit from the lower input filtering
requirement, reduced electromagnetic interference (EMI)
and increased efficiency associated with 2-phase
operation.
Why the need for 2-phase operation? Up until the
LTC1702A, constant-frequency dual switching regulators
operated both channels in phase (i.e., single-phase opera-
tion). This means that both topside MOSFETs turned on at
the same time, causing current pulses of up to twice the
amplitude of those for one regulator to be drawn from the
input capacitor. These large amplitude current pulses
increased the total RMS current flowing from the input
capacitor, requiring the use of more expensive input
capacitors and increasing both EMI and losses in the input
capacitor and input power supply.
With 2-phase operation, the two channels of the LTC1702A
are operated 180 degrees out of phase. This effectively
interleaves the current pulses coming from the switches,
greatly reducing the overlap time where they add together.
The result is a significant reduction in total RMS input
current, which in turn allows less expensive input capaci-
tors to be used, reduces shielding requirements for EMI
and improves real world operating efficiency.
Figure 7 shows example waveforms for a single switching
regulator channel versus a 2-phase LTC1702A system
with both sides switching. A single-phase dual regulator
with both sides operating would exhibit double the single
side numbers. In this example, 2-phase operation reduced
the RMS input current from 9.3A_RMS (2 × 4.66A_RMS) to
4.8A_RMS. While this is an impressive reduction in itself,
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