LTM4619 datasheet(14/24 Pages) LINER

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LTM4619

4619f

APPLICATIONS INFORMATION

INTVCC and EXTVCC

The INTVCC is the internal 5V regulator that powers the

LTM4619 internal circuitry and drives the power MOSFETs.

The input voltage of the LTM4619 must be 6V or above

for the INTVCC to regulate to the proper 5V level due to

the internal LDO dropout from the input voltage. For ap-

plications that need to operate below 6V input, then the

input voltage can be connected directly to the EXTVCC

pin to bypass the LDO dropout concern, or an external

5V supply can be used to power the EXTVCC pin when

the input voltage is at high end of the supply range to

reduce power dissipation in the module. For example the

dropout voltage for 24V input would be 24V – 5V = 19V.

This 19V headroom then multiplied by the power MOSFET

drive current of ~15mA would equal ~0.3W additional

power dissipation. So utilizing an external 5V supply on

the EXTVCC would improve design efﬁciency and reduce

device temperature rise.

Slope Compensation

The module has already been internally compensated

for all output voltages. The Linear Technology μModule

Power Design Tool will be provided for control loop op-

timization.

Burst Mode Operation and Pulse-Skipping Mode

The LTM4619 regulator can be placed into high efﬁciency

power saving modes at light load condition to conserve

power. The Burst Mode operation can be selected by ﬂoat-

ing the MODE/PLLIN pin, and pulse-skipping mode can

be selected by pulling the MODE/PLLIN pin to INTVCC.

Burst Mode operation offers the best efﬁciency at light

load, but output ripple will be higher and lower frequency

ranges are capable which can interfere with some systems.

Pulse-skipping mode efﬁciency is not as good as Burst

Mode operation, but this mode only skips pulses to save

efﬁciency and maintains a lower output ripple and a higher

switching frequency. Burst Mode operation and pulse-skip-

ping mode efﬁciencies can be reviewed in graph supplied

in the Typical Performance Characteristics section.

Fault Conditions: Current Limit and Overcurrent

Foldback

The LTM4619 has a current mode controller, which inher-

ently limits the cycle-by-cycle inductor current not only in

steady-state operation, but also in transient.

To further limit current in the event of an overload condi-

tion, the LTM4619 provides foldback current limiting. If the

output voltage falls by more than 50%, then the maximum

output current is progressively lowered to one-third of its

full current limit value. Foldback current limiting is disabled

during the soft-start and tracking up.

Thermal Considerations and Output Current Derating

In different applications, the LTM4619 operates in a variety

of thermal environments. The maximum output current is

limited by the environmental thermal condition. Sufﬁcient

cooling should be provided to ensure reliable operation.

When the cooling is limited, proper output current derat-

ing is necessary, considering the ambient temperature,

airﬂow, input/output conditions, and the need for increased

reliability.

Two outputs of LTM4619 are paralleled to get high output

current for derating curve tests. The power loss curves in

Figures 7 and 8 can be used in coordination with the load

current derating curves in Figures 9 to 16 for calculating

an approximate

θJA for the module with various cooling

methods. Application Note 103 provides a detailed ex-

planation of the analysis for the thermal models and the

derating curves. Tables 2 and 3 provide a summary of the

equivalent

θJA for the noted conditions. These equivalent

θJA parameters are correlated to the measured values,

and are improved with airﬂow. The junction temperature

is maintained at 125°C or below for the derating curves.

Safety Considerations

The LTM4619 modules do not provide isolation from VIN to

VOUT. There is no internal fuse. If required, a slow blow fuse

with a rating twice the maximum input current needs to be

provided to protect each unit from catastrophic failure.

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