LTM4613 datasheet(11/28 Pages) LINER

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LTM8001

8001f

For more information www.linear.com/8001

OPERATION

The LTM8001 consists of two major parts: the first is a

standalonenonisolatedstep-downswitchingDC/DCpower

converter that can deliver up to 5A of output current. The

second part is an array of five parallelable 1.1A LDOs. The

DC/DC converter provides a precisely regulated output

voltage programmable via one external resistor from 1.2V

to 24V. The input voltage range is 6V to 36V. Given that it

is a step-down converter, make sure that the input volt-

age is high enough to support the desired output voltage

and load current. The linear regulator array consists of

five low drop-out regulators, of which three inputs are

dedicated to the buck converter’s output (VOUT0) and two

tie to an undedicated input (VIN45). Each individual linear

regulator may be set to a unique voltage through its SET

pin, or may be paralleled with other LDOs by tying their

respective SET and VOUT pins together.

TheLTM8001step-downswitchingconverterutilizesfixed

frequency, average current mode control to accurately

regulate the output current. This results in a constant-

voltage, constant-current output characteristic, making

the LTM8001’s step-down regulator well suited for many

supercapacitorandbatterychargingapplications.Asshown

intheTypicalPerformanceCharacteristics,thecurrentlimit

works in both directions. The control loop will regulate the

current in the internal inductor. Once the VOUT0 output has

reached the regulation voltage determined by the resistor

fromtheFBOpintoground,thevoltageregulationloopwill

reduce the output current and maintain the output voltage.

The ILIM input may be used to set the maximum allowable

current output of the LTM8001. The analog control range

of the ILIM pin is from 0V to 1.5V. If the ILIM pin is raised

above 1.5V, there is little or no effect.

The RUN pin functions as a precision enable for the step-

down switching converter connected to VOUT0. As the

VOUT1-3 LDO inputs are tied to VOUT0, the RUN pin will

also implicitly enable or disable these LDOs as well, unless

some external power source is tied to VOUT0. Refer to the

ApplicationsInformationsectionShortedInputProtection

if VOUT0 is forced above VIN0. When the voltage at the RUN

pin is lower than 1.55V, switching is terminated. Below the

turn-on threshold, the RUN pin sinks 5.5μA. This current

can be used with a resistor between RUN and VIN0 to set

hysteresis.PleaserefertotheUVLOandShutdownsection

in the Applications Information for further details. During

start-up, the SS pin is held low until the part is enabled,

after which the capacitor at the soft-start pin is charged

with an 11μA current source.

The LTM8001 is equipped with thermal shutdown circuitry

to protect the device during momentary overload condi-

tions. It is set above the 125°C absolute maximum internal

temperature rating to avoid interfering with normal speci-

fied operation, so internal device temperatures will exceed

the absolute maximum rating when the overtemperature

protection is active. Thus, continuous or repeated activa-

tion of the thermal shutdown may impair device reliability.

During thermal shutdown, all switching is terminated and

the SS pin is driven low.

The switching frequency is determined by a resistor at

the RT pin. The LTM8001 may also be synchronized to an

external clock through the use of the SYNC pin. Please

see the Switching Frequency Synchronization section in

the Applications Information for further details.

The VOUT1-5 linear regulators are easy to use and have

all the protection features expected in high performance

regulators. Included are short-circuit protection and safe

operating area protection, as well as thermal shutdown.

These linear regulators are especially well suited to ap-

plications needing multiple rails. Their architecture allows

their outputs to be adjusted down to zero volts. The output

voltage is set by a single resistor, handling modern low

voltage digital ICs as well as allowing easy parallel opera-

tion and simplified thermal management.

The linear regulators can be operated in two modes. One

mode has the BIAS123 and BIAS45 pins connected to the

linear regulator power input pins (VOUT0 and VIN45) which

givesalimitationofabout1.6Vdropout.Intheothermode,

the BIAS123 and BIAS45 pins can be tied to a voltage at

least1.6Vabovetheirhighestrespectiveoutputs.Thelinear

regulator power input (VOUT0 and VOUT45) can then be set

to a lower voltage that meets the dropout requirement,

minimizing the power dissipation.

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