Datasheet

LT3071
15
3071fc
For more information www.linear.com/LT3071
APPLICATIONS INFORMATION
Introduction
Current generation FPGA and ASIC processors place
stringent demands on the power supplies that power the
core, I/O and transceiver channels. These microprocessors
may cycle load current from near zero to amps in tens of
nanoseconds. Output voltage specifications, especially in
the 1V range, require tight tolerances including transient
response as part of the requirement. Some ASIC processors
require only a single output voltage from which the core
and I/O circuitry operate. Some high performance FPGA
processors require separate power supply voltages for the
processor core, the I/O, and the transceivers. Often, these
supply voltages must be low noise and high bandwidth
to achieve the lowest bit-error rates. These requirements
mandate the need for very accurate, low noise, high cur
-
rent, very high speed regulator circuits that operate at low
input and output voltages.
The LT3071 is a low voltage, UltraFast transient response
linear regulator. The device supplies up to 5A of output
current with a typical dropout voltage of 85mV. A 0.01µF
reference bypass capacitor decreases output voltage noise
to 25µV
RMS
(BW = 10Hz to 100kHz). The LT3071’s high
bandwidth provides UltraFast transient response using low
ESR ceramic output capacitors (15µF
minimum), saving
bulk capacitance, PCB area and cost.
Th
e LT3071’s features permit state-of-the-art linear regula
-
tor performance. The LT3071 is ideal for high performance
FPGAs, microprocessors, sensitive communication sup-
plies, and
high current logic applications that also operate
over low input and output voltages.
Output
voltage for the LT3071 is digitally selectable in
50mV increments over a 0.8V to 1.8V range. An analog
margining function allows the user to adjust system output
voltage over a continuous ±10% range.
The LT3071 provides an output current monitor that
typically sources a current of I
OUT
/2500 or 400µA per
amp of I
OUT
at its I
MON
pin. Terminating the I
MON
pin to
GND with a resistor produces a voltage proportional to
output current. This permits a user to measure system
performance such as output power or if output current
exceeds or falls below some threshold.
The IC incorporates a unique tracking function, which if
enabled by the user, controls an upsteam regulator power
-
ing the
LT3071
’s input (see Figure 8). This tracking function
drives the buck regulator to maintain the LT3071’s input
voltage to V
OUT
+ 300mV. This input-to-output voltage
control allows the user to change the regulator output
voltage,
and have the switching regulator powering the
LT3071’s input to track to the optimum input voltage with
no component changes.
This combines the efficiency of a switching regulator
with superior linear regulator response. It also permits
thermal management of the system even with a maximum
5A output load.
LT3071 internal protection includes input undervoltage
lockout (UVLO), reverse-current protection, precision cur
-
rent limiting with power foldback and thermal shutdown.
The
LT3071 regulator is available in a thermally enhanced
28-lead, 4mm × 5mm QFN package.
The LT3071’s architecture drives an internal N-channel
power MOSFET as a source follower. This configuration
permits a user to obtain an extremely low dropout, Ultra
-
Fast transient response regulator with excellent high fre-
quency PSRR p
erformance. The LT3071 achieves superior
regulator bandwidth and transient load performance by
eliminating expensive bulk tantalum or electrolytic capaci
-
tors in the most modern and demanding microprocessor
applications. Users realize significant cost savings as all
additional bulk capacitance is removed. The additional
savings of insertion cost, purchasing/inventory cost and
board space are readily apparent. Precision incremental
output voltage control accommodates legacy and future
microprocessor power supply voltages.
Output capacitor
networks simplify to direct parallel com-
binations of ceramic capacitors. Often, the high frequency
ceramic
decoupling capacitors required by these various
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