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LTM8065

8065fa

For more information www.linear.com/LTM8065

OPERATION

The LTM8065 is a stand-alone non-isolated step-down

switching DC/DC power supply that can deliver up to

3.5A. The continuous current is determined by the internal

operating temperature. It provides a precisely regulated

output voltage programmable via one external resistor

from 0.97V to 18V. The input voltage range is 3.4V to 40V.

Given that the LTM8065 is a step-down converter, make

sure that the input voltage is high enough to support the

desired output voltage and load current. A simplified Block

Diagram is given on the previous page.

The LTM8065 contains a current mode controller, power

switching elements, power inductor and a modest amount

of input and output capacitance. The LTM8065 is a fixed

frequency PWM regulator. The switching frequency is set

by simply connecting the appropriate resistor value from

the RT pin to GND.

An internal regulator provides power to the control cir-

cuitry. This bias regulator normally draws power from the

VIN pin, but if the BIAS pin is connected to an external

voltage higher than 3.2V, bias power is drawn from the

external source (typically the regulated output voltage).

This improves efficiency. The RUN pin is used to place

the LTM8065 in shutdown, disconnecting the output and

reducing the input current to a few µA.

Toenhanceefficiency,theLTM8065automaticallyswitches

to Burst Mode operation in light or no load situations.

Between bursts, all circuitry associated with controlling

the output switch is shut down reducing the input supply

current to just a few µA.

TheoscillatorreducestheLTM8065’soperatingfrequency

when the voltage at the FB pin is low. This frequency fold-

back helps to control the output current during start-up

and overload.

The TR/SS node acts as an auxiliary input to the error

amplifier. The voltage at FB servos to the TR/SS voltage

until TR/SS goes above 0.97V. Soft-start is implemented

by generating a voltage ramp at the TR/SS pin using an

externalcapacitorwhichischargedbyaninternalconstant

current. Alternatively, driving the TR/SS pin with a signal

source or resistive network provides a tracking function.

Do not drive the TR/SS pin with a low impedance volt-

age source. See the Applications Information section for

more details.

The LTM8065 contains a power good comparator which

trips when the FB pin is at about 90% to 110% of its

regulated value. The PG output is an open-drain transistor

that is off when the output is in regulation, allowing an

external resistor to pull the PG pin high. The PG signal

is valid when VIN is above 3.4V. If VIN is above 3.4V and

RUN is low, PG will drive low.

The LTM8065 is equipped with a thermal shutdown that

inhibits power switching at high junction temperatures.

The activation threshold of this function is above the maxi-

mum temperature rating to avoid interfering with normal

operation, so prolonged or repetitive operation under a

condition in which the thermal shutdown activates may

damage or impair the reliability of the device.

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