LT1577CS-3.3/ADJ Linear Technology, LT1577CS-3.3/ADJ Datasheet - Page 10

• Manufacturer Part Number: LT1577CS-3.3/ADJ
• Description: IC LDO REG 3.3V/ADJ DUAL 16-SOIC
• Manufacturer: Linear Technology
• Type: Positive Fixed and Adjustabler

Specifications of LT1577CS-3.3/ADJ

• Number Of Outputs: 2
• Voltage - Output: 3.3V, Adjustable
• Current - Supply: 12mA
• Voltage - Input: 5 ~ 22 V
• Operating Temperature: 0°C ~ 70°C
• Package / Case: 16-SOIC (3.9mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

APPLICATIONS
LT1575/LT1577
Introduction
The current generation of microprocessors place strin-
gent demands on the power supply that powers the
processor core. These microprocessors cycle load cur-
rent from near zero to amps in tens of nanoseconds.
Output voltage tolerances as low as 100mV include
transient response as part of the specification. Some
microprocessors require only a single output voltage from
which the core and I/O circuitry operate. Other higher
performance processors require a separate power supply
voltage for the processor core and the I/O circuitry. These
requirements mandate the need for very accurate, very
high speed regulator circuits.
Previously employed solutions included monolithic
3-terminal linear regulators, PNP transistors driven by low
cost control circuits and simple buck converter switching
regulators. The 3-terminal regulator achieves a high level
of integration, the PNP driven regulator achieves very low
dropout performance and the switching regulator achieves
high electrical efficiency.
However, the common trait manifested by these solutions
is that transient response is measured in many microsec-
onds. This fact translates to a regulator output decoupling
capacitor scheme that requires several hundred microfar-
ads of very low ESR bulk capacitance using multiple
capacitors surrounding the CPU. This required bulk ca-
pacitance is in addition to the ceramic decoupling capaci-
tor network that handles the transient load response
during the first few hundred nanoseconds as well as
providing microprocessor clock frequency noise immu-
nity. The combined cost of all capacitors is a significant
percentage of the total power supply cost.
The LT1575/LT1577 family of single/dual controller ICs
are unique, easy to use devices that drive external
N-channel MOSFETs as source followers and permit a user
to realize an extremely low dropout, ultrafast transient
response regulator. These circuits achieve superior regu-
lator bandwidth and transient load performance by com-
pletely eliminating expensive tantalum or bulk electrolytic
capacitors in the most modern and demanding micropro-
cessor applications. For example, a 200MHz Pentium
processor can operate with only the recommended 24 1 F
ceramic capacitors. Users benefit directly by saving sig-
10
U
INFORMATION
U
W
U
nificant cost as all additional bulk capacitance is removed.
The additional savings of insertion cost, purchasing/in-
ventory cost and board space are readily apparent.
Precision-trimmed adjustable and fixed output voltage
versions accommodate any required microprocessor
power supply voltage. Proper selection of the N-channel
MOSFET R
performance. The only output capacitors required are the
high frequency ceramic decoupling capacitors. This regu-
lator design provides ample bandwidth and responds to
transient load changes in a few hundred nanoseconds
versus regulators that respond in many microseconds.
The ceramic capacitor network generally consists of 10 to
24 1uF capacitors for individual microprocessor require-
ments. The LT1575/LT1577 family also incorporates cur-
rent limiting for no additional system cost, provides on/off
control and overvoltage protection or thermal shutdown
with simple external components.
Therefore, the unique design of these new ICs combines
the benefits of low dropout voltage, high functional inte-
gration, precision performance and ultrafast transient
response, as well as providing significant cost savings on
the output capacitance needed in fast load transient appli-
cations. As lower input/output differential voltage applica-
tions become increasingly prevalent, an LT1575-based
solution achieves comparable efficiency performance with
a switching regulator at an appreciable cost savings.
The new LT1575/LT1577 family of low dropout regulator
controller ICs step to the next level of performance re-
quired by system designers for the latest generation
motherboards and microprocessors. The simple versatil-
ity and benefits derived from these circuits allow the
power supply needs of today’s high performance micro-
processors to be met with ease.
Block Diagram Operation
The primary block diagram elements consist of a simple
feedback control loop and the secondary block diagram
elements consist of multiple protection functions. Exam-
ining the block diagram for the LT1575, a start-up circuit
provides controlled start-up for the IC, including the
precision-trimmed bandgap reference, and establishes all
internal current and voltage biasing.
DS(ON)
allows user-settable dropout voltage