LTC1705 Datasheet, PDF (11/28 Pages) Linear Technology – Dual 550kHz Synchronous Switching Regulator Controller with 5-Bit VID and 150mA LDO

LTC1705
APPLICATIO S I FOR ATIO
The LTC1705 includes two, step-down (buck), voltage
mode feedback switching regulator controllers and a low
dropout linear regulator. The three outputs are designed to
power the core, I/O and CLK supplies of an Intel Mobile
Pentium system. Each switching regulator controller em-
ploys a synchronous switching architecture with two
external N-channel MOSFETs per channel. The chip oper-
ates from a low voltage input supply (6V maximum) and
provides high power, high efficiency, precisely regulated
output voltages. Several features make the LTC1705 par-
ticularly suited for microprocessor supply regulation.
Output regulation at the core supply is extremely tight,
with initial accuracy and DC line and load regulation better
than 1.25%. Total regulation including transient response
is inside of 3.5% with a properly designed circuit. The
550kHz switching frequency and the high speed internal
feedback amplifiers allow the use of physically small, low
value external components without compromising perfor-
mance. An onboard 5-bit DAC sets the core output voltage,
consistent with the Intel Mobile VID specification (Table␣ 1).
The 800mV internal reference allows regulated output
voltages as low as 800mV without external level shifting
amplifiers. The linear regulator controls an internal P-
channel MOSFET that can provide more than 150mA of
current at an output voltage of 2.5V. A power good
(PGOOD) flag goes high when all the three outputs are in
regulation.
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 regula-
tor, such as the LTC1705. 2-step conversion eliminates the
need for a single converter to convert 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 reasonable duty cycle. In
contrast, a regulator converting in a single step from a high
input voltage to a 1.xV output must operate at a very
narrow duty cycle, mandating trade-offs in external com-
ponent values while compromising efficiency and tran-
sient response. The efficiency loss can exceed that of a
2-step solution. Further complicating the calculation 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 LTC1705
usually match or exceed the total system efficiency of
single-step solutions and provide the additional benefits
of improved transient response, reduced PCB area and
simplified power trace routing.
2-step regulation can also buy advantages in thermal
management. Power dissipation in the LTC1705 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 directly next to the
CPU. In a 1-step design, all of the power dissipated by the
core regulator is located next to the already hot CPU,
aggravating thermal management. In a 2-step LTC1705
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 LTC1705 section of the system is relatively low,
minimizing the added heat near the CPU.
Fast Transient Response
The LTC1705 core and I/O supplies use fast 20MHz GBW
op amps as error amplifiers. This allows the compensation
network to be designed with several poles and zeros in a
more flexible configuration than with typical gm feedback
amplifiers. The high bandwidth of the amplifier, coupled
with the high 550kHz switching frequency and the low
values of the external inductor and output capacitor, allow
very high loop cross-over frequencies. Additionally, a
typical LTC1705 circuit uses an inductor value on the
order of 1µH, allowing very fast di/dt slew rates. The result
is superior transient response compared with conven-
tional solutions.
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