LTC1703CG Linear Technology, LTC1703CG Datasheet - Page 9

• Manufacturer Part Number: LTC1703CG
• Description: IC REG SW DUAL SYNC VID 28SSOP
• Manufacturer: Linear Technology

Specifications of LTC1703CG

• Applications: Controller, Mobile Intel Pentium® III
• Voltage - Input: 3 ~ 7 V
• Number Of Outputs: 2
• Voltage - Output: 0.9 ~ 2 V
• Operating Temperature: 0°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 28-SSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

APPLICATIO S I FOR ATIO
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 example, 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, aggravating
thermal management. In a 2-step LTC1703 design, a
significant percentage of the power lost in the core regu-
lation system happens in the 5V supply, which is usually
located away from the CPU. The power lost to heat in the
LTC1703 section of the system is relatively low, minimiz-
ing 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 LTC1703 dual switching regulator controller also
features the considerable benefits of 2-phase operation.
Notebook computers, handheld terminals and automotive
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 LTC1703,
constant-frequency dual switching regulators operated
both channels in phase (i.e., single-phase operation). 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 LTC1703
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.
U
U
W
U
Figure 7 shows example waveforms for a single switching
regulator channel versus a 2-phase LTC1703 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
4.8A
remember that the power losses are proportional to I
meaning that the actual power wasted is reduced by a
factor of 3.75. 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.
Small Footprint
The LTC1703 operates at a 550kHz switching frequency,
allowing it to use low value inductors without generating
excessive ripple currents. Because the inductor stores
less energy per cycle, the physical size of the inductor can
be reduced without risking core saturation, saving PCB
board space. The high operating frequency also means
less energy is stored in the output capacitors between
cycles, minimizing their required value and size. The
remaining components, including the SSOP-28 LTC1703,
are tiny, allowing an entire dual-output LTC1703 circuit to
be constructed in 1.5in
is generally located right next to the microprocessor or in
some similarly congested area, where PCB real estate is at
a premium. The fact that the LTC1703 runs off the 5V
supply, often available from a power plane, is an added
benefit in portable systems —it does not require a dedi-
cated supply line running from the battery.
Fast Transient Response
The LTC1703 uses a fast 25MHz GBW op amp as an error
amplifier. This allows the compensation network to be
designed with several poles and zeros in a more flexible
configuration than with a typical g
The high bandwidth of the amplifier, coupled with the high
switching frequency and the low values of the external
inductor and output capacitor, allow very high loop cross-
over frequencies. The low inductor value is the other half
RMS
. While this is an impressive reduction in itself,
2
of PCB space. Further, this space
RMS
m
(2 × 4.66A
feedback amplifier.
LTC1703
RMS
RMS
) to
1703fa
9
2
,