LTM8003 datasheet(20/30 Pages) LINER

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LTM8003

8003fc

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APPLICATIONS INFORMATION

An active pull-down circuit is connected to the TR/SS pin

which will discharge the external soft-start capacitor in

the case of fault conditions and restart the ramp when the

faults are cleared. Fault conditions that clear the soft-start

capacitor are the RUN pin transitioning low, VIN voltage

falling too low, or thermal shutdown.

Pre-Biased Output

As discussed in the Output Voltage Tracking and Soft-

Start section, the LTM8003 regulates the output to the

FB voltage determined by the TR/SS pin whenever TR/

SS is less than 0.97V. If the LTM8003 output is higher

than the target output voltage, the LTM8003 will attempt

to regulate the output to the target voltage by returning a

small amount of energy back to the input supply. If there

is nothing loading the input supply, its voltage may rise.

Take care that it does not rise so high that the input voltage

exceeds the absolute maximum rating of the LTM8003.

Frequency Foldback

The LTM8003 is equipped with frequency foldback which

actstoreducethethermalandenergystressontheinternal

power elements during a short circuit or output overload

condition. If the LTM8003 detects that the output has

fallenoutofregulation,theswitchingfrequencyisreduced

as a function of how far the output is below the target

voltage. This in turn limits the amount of energy that can

be delivered 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. When a clock is applied to the SYNC pin, the

SYNC pin is floated or held high, the frequency foldback

is disabled, and the switching frequency will slow down

only during overcurrent conditions.

Synchronization

To select low ripple Burst Mode operation, tie the SYNC

pin below about 0.4V (this can be ground or a logic low

output). To synchronize the LTM8003 oscillator to an

external frequency, connect a square wave (with about

20% to 80% duty cycle) to the SYNC pin. The square

wave amplitude should have valleys that are below 0.4V

and peaks above 1.5V.

The LTM8003 will not enter Burst Mode operation at low

output loads while synchronized to an external clock, but

insteadwillpulseskiptomaintainregulation.TheLTM8003

maybesynchronizedovera200kHzto3MHzrange.TheRT

resistor should be chosen to set the switching frequency

equal to or below the lowest synchronization input. For

example, if the synchronization signal will be 500kHz and

higher, the RT should be selected for 500kHz.

For some applications it is desirable for the LTM8003 to

operate in pulse-skipping mode, offering two major dif-

ferences from Burst Mode operation. The first is that the

clock stays awake at all times and all switching cycles

are aligned to the clock. The second is that full switching

frequency is reached at lower output load than in Burst

Modeoperation.Thesetwodifferencescomeattheexpense

of increased quiescent current. To enable pulse-skipping

mode, the SYNC pin is floated.

The LTM8003 features spread spectrum operation to

further reduce EMI/EMC emissions. To enable spread

spectrum operation, apply between 2.9V and 4.2V to the

SYNC pin. In this mode, triangular frequency modulation

is used to vary the switching frequency between the value

programmed by RT to about 20% higher than that value.

Themodulationfrequencyisabout3kHz.Forexample,when

the LTM8003 is programmed to 2MHz, the frequency will

vary from 2MHz to 2.4MHz at a 3kHz rate. When spread

spectrum operation is selected, Burst Mode operation is

disabled, and the part will run in pulse-skipping mode.

TheLTM8003doesnotoperateinforcedcontinuousmode

regardless of SYNC signal.

Negative Output

The LTM8003 is capable of generating a negative output

voltage by connecting its VOUT to system GND and the

LTM8003 GND to the negative voltage rail. An example

of this is shown in the Typical Applications section. The

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