LTC1704 Linear Technology, LTC1704 Datasheet - Page 10

• Manufacturer Part Number: LTC1704
• Description: 550kHz Synchronous Switching Regulator Controller Plus Linear Regulator Controller
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LTC1704/LTC1704B
OVERVIEW
The LTC1704 includes a step-down (buck), voltage mode
feedback switching regulator controller and a linear regu-
lator controller. The switching regulator controller em-
ploys a synchronous switching architecture with two
external N-channel MOSFETs. The chip operates from a
low voltage input supply (6V maximum) and provides high
power, high efficiency, precisely regulated output voltage.
The switcher output regulation is extremely tight, with
initial accuracy and DC line and load regulation and better
than 1.5%. Total regulation, including transient response,
is inside of 3.5% with a properly designed circuit. The
550kHz switching frequency allows the use of physically
small, low value external components without compro-
mising performance.
The LTC1704’s internal feedback amplifier is a 20MHz gain
bandwidth op amp, allowing the use of complex multipole/
zero compensation networks. This allows the feedback
loop to maintain acceptable phase margin at higher fre-
quencies than traditional switching regulator controllers,
improving stability and maximizing transient response.
The 800mV internal reference allows regulated output
voltages as low as 800mV without external level shifting
amplifiers. The LTC1704’s synchronous switching logic
transitions automatically into Burst Mode operation, maxi-
mizing efficiency with light loads.
The linear regulator controller drives an external NPN pass
transistor to provide a programmable output voltage up to
2A of current. An external pull-up resistor programs the
current limit threshold for the linear regulator. Under
short-circuit condition, the foldback current limit circuitry
prevents excessive pass transistor heating. The switcher
and the linear regulator can be individually disabled. When
both controllers are disabled, the LTC1704 enters shut-
down mode and the supply current reduces to 75 A. An
onboard power good (PGOOD) flag goes high when both
outputs are regulating.
Small Footprint
The LTC1704 switcher supply operates at a 550kHz switch-
ing 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
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the inductor can be reduced without risking core satura-
tion, saving PCB board space. The high operating fre-
quency also means less energy is stored in the output
capacitors between cycles, minimizing their required value
and size. The remaining components, including the
LTC1704, are tiny, allowing an entire power convertor to
be constructed in 1.5in
Fast Transient Response
The LTC1704 switcher supply uses a fast 20MHz 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
amplifier. 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 crossover frequencies. The low inductor value is the
other half of the equation—with a typical value on the
order of 1 H, the inductor allows very fast di/dt slew rates.
The result is superior transient response compared with
conventional solutions.
High Efficiency
The LTC1704 switcher supply uses a synchronous step-
down (buck) architecture, with two external N-channel
MOSFETs. A floating topside driver and a simple external
charge pump provide full gate drive to the upper MOSFET.
The voltage mode feedback loop and MOSFET V
limit sensing remove the need for an external current
sense resistor, eliminating an external component and a
source of power loss in the high current path. Properly
designed circuits using low gate charge MOSFETs are
capable of efficiencies exceeding 90% over a wide range
of output voltages.
Linear Regulator Controller
The LTC1704 linear regulator controller drives an exter-
nal NPN pass transistor in emitter-follower configuration
to provide an externally adjustable output voltage. The con-
troller senses the output voltage via the REGFB pin, drives
the base of the NPN through the REGDR pin to regulate
the REGFB pin to 0.8V. REGDR is capable of sourcing more
than 30mA of base current to the external NPN.
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of PCB space.
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feedback
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