LT3150_15 Datasheet, PDF (9/20 Pages) Linear Technology – Fast Transient Response, Low Input Voltage, Very Low Dropout Linear Regulator Controller

LT3150
APPLICATIO S I FOR ATIO
INTRODUCTION
With each new generation of computing systems, total
power increases while system voltages fall. CPU core,
logic and termination supplies below 1.8V are now com-
mon. Power supplies must not only regulate low output
voltages, but must also operate from low input voltages.
A low voltage, very low dropout linear regulator is an
attractive conversion option for applications with output
current in the range of several amperes. Component count
and cost are low in comparison with switching regulator
solutions and with low input-to-output differential volt-
ages, efficiencies are comparable.
In addition to low input-to-output voltage conversion,
these systems require stringent output voltage regulation.
The output voltage specification includes input voltage
change, output load current change, temperature change
and output load current transient response. Total toler-
ances as low as ±2% are now required. For a 1.5V output
voltage, this amounts to a mere ±30mV. Transient load
current response is the most critical component as output
current can cycle from zero to amps in tens of nanosec-
onds. These requirements mandate the need for a very
accurate, very high speed regulator.
Historically employed solutions include monolithic
3-terminal linear regulators, PNP transistors driven by
low cost control circuits and simple buck converter
switching regulators. The 3-terminal regulator provides
high integration, the PNP driven regulator provides low
dropout performance and the switching regulator pro-
vides high electrical efficiency.
However, these solutions manifest a common trait of
transient response measured in many microseconds. This
fact translates to a regulator output decoupling capacitor
scheme requiring several hundred microfarads of very low
ESR bulk capacitance using multiple capacitors in parallel.
This required bulk capacitance is in addition to the ceramic
decoupling capacitor network that handles the transient
load response during the first few hundred nanoseconds
as well as providing high frequency noise immunity. The
combined cost of all capacitors is a significant percentage
of the total power supply cost.
The LT3150 controller IC is a unique, easy-to-use device
that drives an external N-channel MOSFET as a source
follower and realizes an extremely low dropout, ultrafast
transient response regulator. The circuit achieves supe-
rior regulator bandwidth and transient load performance
by eliminating expensive special polymer, tantalum or
bulk electrolytic capacitors in the most demanding appli-
cations. Performance is optimized around the latest gen-
eration of low cost, low ESR, readily available ceramic
capacitors. Users benefit directly by saving significant
cost as all bulk capacitance is removed. Additional savings
include insertion cost, purchasing/inventory cost and
board space.
The precision-trimmed adjustable voltage LT3150 ac-
commodates most power supply voltages. Proper selec-
tion of the N-channel MOSFET R_DS(ON) allows user-settable
dropout voltage performance. Transient load step perfor-
mance is optimized for ceramic output capacitor networks
allowing the regulator to respond to transient load changes
in a few hundred nanoseconds. The output capacitor
network typically consists of multiple 1µF to 10µF ceramic
capacitors in parallel depending on the power supply
requirements. The LT3150 also incorporates current lim-
iting, on/off control for power supply sequencing and
overvoltage protection or thermal shutdown with simple
external components.
The LT3150 combines the benefits of low input voltage
operation, very low dropout voltage performance, preci-
sion regulation and fast transient response. With low
input/output differential voltage applications becoming
the norm, an LT3150-based solution is a practical alterna-
tive to switching regulators providing comparable effi-
ciency performance at an appreciable cost savings.
BLOCK DIAGRAM OPERATION
Gate drive for the external N-channel MOSFET in the linear
regulator loop is provided by a current mode, internally
compensated, fixed frequency step-up switching regula-
tor. Referring to the Block Diagram, Q1 and Q2 form a
bandgap reference core whose loop is closed around the
output of the regulator. The voltage drop across R5 and R6
3150f
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