LTM4606 datasheet(15/24 Pages) LINER

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LTM8050

8050fc

For more information www.linear.com/LTM8050

APPLICATIONS INFORMATION

Figure 1. Apply an RC Network to RUN/SS to Control the

Soft-Start Behavior of the LTM8050 at Power-Up

Minimum Input Voltage

The LTM8050 is a step-down converter, so a minimum

amount of headroom is required to keep the output in

regulation. In addition, the input voltage required to turn

on is higher than that required to run, and depends upon

whether the RUN/SS is used. As shown in the Typical

Performance Characteristics section, the minimum input

voltage to run a 3.3V output at light load is only about 3.6V,

but, if RUN/SS is pulled up to VIN, it takes 5.5VIN to start.

If the LTM8050 is enabled with the RUN/SS pin after VIN

is applied, the minimum voltage to start at light loads is

lower, about 4.3V. Similar curves detailing this behavior

of the LTM8050 for other outputs are also included in the

Typical Performance Characteristics section.

Soft-Start

The RUN/SS pin can be used to soft-start the LTM8050,

reducing the maximum input current during start-up. The

RUN/SS pin is driven through an external RC network to

create a voltage ramp at this pin. (See Figure 1). By choos-

ing an appropriate RC time constant, the peak start-up

current can be reduced to the current that is required to

regulate the output, with no overshoot. Choose the value

of the resistor so that it can supply at least 20μA when

the RUN/SS pin reaches 2.5V. Output voltage soft-start

waveforms for various values of RSS and CSS are given in

the Typical Performance Characteristics section.

RUN/SS

RUN

100k

CSS

Thisinturnlimitstheamountofenergythatcanbedelivered

to the load under fault. During the start-up time, frequency

foldback is also active to limit the energy delivered to the

potentially large output capacitance of the load.

Synchronization

TheinternaloscillatoroftheLTM8050canbesynchronized

by applying an external 250kHz to 2MHz clock to the SYNC

pin. Do not leave this pin floating. When synchronizing

the LTM8050, select an RTresistorvaluethatcorresponds

to an operating frequency 20% lower than the intended

synchronization frequency (see the Frequency Selection

section).

Inadditiontosynchronization,theSYNCpincontrolsBurst

Mode behavior. If the SYNC pin is driven by an external

clock, or pulled up above 0.7V, the LTM8050 will not

enter Burst Mode operation, but will instead skip pulses

to maintain regulation instead.

Shorted Input Protection

Care needs to be taken in systems where the output will

be held high when the input to the LTM8050 is absent.

This may occur in battery charging applications or in

battery backup systems where a battery or some other

supply is diode ORed with the LTM8050’s output. If the

VIN pin is allowed to float and the SHDN pin is held high

(either by a logic signal or because it is tied to VIN), then

the LTM8050’s internal circuitry will pull its quiescent

current through its internal power switch. This is fine if

your system can tolerate a few milliamps in this state. If

you ground the RUN/SS pin, the input current will drop

to essentially zero. However, if the VIN pin is grounded

while the output is held high, then parasitic diodes inside

the LTM8050 can pull large currents from the output

through the VIN pin. Figure 2 shows a circuit that will run

only when the input voltage is present and that protects

against a shorted or reversed input.

PCB Layout

Most of the headaches associated with PCB layout have

been alleviated or even eliminated by the high level of

integration of the LTM8050. The LTM8050 is neverthe-

less a switching power supply, and care must be taken to

Frequency Foldback

The LTM8050 is equipped with frequency foldback which

actstoreducethethermalandenergystressontheinternal

power elements during a short circuit or output overload

condition.IftheLTM8050detectsthattheoutputhasfallen

out of regulation, the switching frequency is reduced as a

function of how far the output is below the target voltage.

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