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LTM4619
APPLICATIONS INFORMATION
INTV CC and EXTV CC
The INTV CC 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 INTV CC 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 EXTV CC
pin to bypass the LDO dropout concern, or an external
5V supply can be used to power the EXTV CC 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 EXTV CC would improve design efficiency and reduce
device temperature rise.
Slope Compensation
The module has already been internally compensated for
all output voltages. LTpowerCAD is available for control
loop optimization.
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 INTV CC . 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 switch-
ing frequency. Burst Mode operation and pulse-skipping
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 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 θ JA 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 θ JA for the noted conditions. These equivalent
θ JA 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 galvanic isolation
from V IN to V OUT . 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.
4619fb
14
For more information www.linear.com/LTM4619
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