LTM8003 datasheet(14/28 Pages) AD

14 / 28 page

LTM8051

Rev. 0

For more information www.analog.com

OPERATION

The LTM8051 is a quad standalone non-isolated step-

down switching DC/DC power supply that can deliver a

peak current of up to 2.5A per channel. The continuous

current is determined by the internal operating tempera-

ture. It provides a precisely regulated output voltage pro-

grammable via one external resistor from 0.8V to 8V. The

input voltage range for VIN1, VIN23 is 3V to 40V, while the

input voltage range for VIN4 is 2V to 40V.

Given that the LTM8051 is a step-down converter, make

sure that the input voltage is high enough to support the

desired output voltage and load current. See simplified

Block Diagram.

The LTM8051 contains current mode controllers, power

switching elements, power inductors and a modest

amount of input and output capacitance. The LTM8051 is

a fixed frequency PWM regulator. The switching frequency

is set by simply connecting the appropriate resistor value

from the RT

n pin to GND.

Internal regulators provide power to the control circuit-

ries. Bias regulators normally draw power from the VINn

pin, but if the BIAS

n pin is connected to an external volt-

age higher than 3.2V, bias power is drawn from the exter-

nal source (typically the regulated output voltage). This

improves efficiency. Tie BIAS

n to GND if it is not used.

To enhance efficiency, the LTM8051 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 to just a few µA.

The TRSS

n node acts as an auxiliary input to the error

amplifier. The voltage at FB

n servos to the TRSSn voltage

until TRSS

n goes above 0.8V. Soft-start is implemented

by generating a voltage ramp at the TRSS

n pin using an

external capacitor which is charged by an internal 2μA

constant current. Alternatively, driving the TRSS

n pin with

a signal source or resistive network provides a tracking

function. Do not drive the TRSS

n pin with a low imped-

ance voltage source. See the Applications Information

section for more details.

The LTM8051 contains power good comparators which

trip when the FB

n pin is at about 92% to 108% of its regu-

lated value. The PG

n output is an open-drain transistor

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

external resistor to pull the PG

n pin high.

The LTM8051 is equipped with a thermal shutdown that

inhibits power switching at high junction temperatures.

The activation threshold of this function is above the max-

imum 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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