LTM8003 datasheet(12/26 Pages) LINER

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LTM8073

8073fa

For more information www.linear.com/LTM8073

operaTion

The LTM8073 is a standalone nonisolated step-down

switching DC/DC power supply that can deliver up to 5A.

The continuous current is determined by the internal oper-

ating temperature. It provides a precisely regulated output

voltage programmable via one external resistor from 0.8V

to 15V. The input voltage range is 3.4V to 60V. Given that

the LTM8073 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 LTM8073 contains a current mode controller, power

switching elements, power inductor and a modest amount

of input and output capacitance. The LTM8073 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 LTM8073 in shutdown, disconnecting the output and

reducing the input current to a few μA.

To enhance efficiency, the LTM8073 automatically

switches 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.

The oscillator reduces the LTM8073’s operating frequency

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 about 0.8V. Soft-start is imple-

mented by generating a voltage ramp at the TR/SS pin

using an external capacitor which is charged by an internal

constant 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 imped-

ance voltage source. See the Applications Information

section for more details.

The LTM8073 contains a power good comparator which

trips when the FB pin is between 0.73V and 0.88V, typical.

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 LTM8073 is equipped with a thermal shutdown that

inhibits power switching at high junction temperatures.

The activation threshold of this function is above 125°C 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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