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LTM8040
APPLICATION INFORMATION
Capacitor Selection Considerations
The C IN capacitor values in Table 2 are the minimum rec-
ommended values for the associated operating conditions.
Applying capacitor values below those indicated in Table 2
is not recommended, and may result in undesirable
operation. Using larger values is generally acceptable, and
can yield improved performance, if it is necessary. Again,
it is incumbent upon the user to verify proper operation
over the intended system’s line, load and environmental
conditions.
Ceramic capacitors are small, robust and have very low
ESR. However, not all ceramic capacitors are suitable. X5R
and X7R types are stable over temperature and applied
voltage and give dependable service. Other types, includ-
ing Y5V and Z5U have very large temperature and voltage
coef ?cients of capacitance. In an application circuit they
may have only a small fraction of their nominal capacitance
resulting in much higher voltage ripple than expected.
A ?nal precaution regarding ceramic capacitors concerns
the maximum input voltage rating of the LTM8040. A
ceramic input capacitor combined with trace or cable
inductance forms a high Q (under damped) tank circuit. If
the LTM8040 circuit is plugged into a live supply, the input
voltage can ring to twice its nominal value, possibly ex-
ceeding the device’s rating. This situation is easily avoided
by introducing a small series damping resistance into the
circuit. This is most often taken care of by the presence
of an electrolytic bulk capacitor in the board.
High Temperature Considerations
The internal operating temperature of the LTM8040 must
be lower than the 125°C rating, so care should be taken
in the layout of the circuit to ensure good heat sinking of
the LTM8040. To estimate the junction temperature, ap-
proximate the power dissipation within the LTM8040 by
applying the typical ef ?ciency stated in this data sheet to
the desired output power, or, if you have an actual module,
by taking a power measurement. Then calculate the tem-
perature rise of the LTM8040 junction above the surface
of the printed circuit board by multiplying the module’s
power dissipation by the thermal resistance θ JA . The actual
thermal resistance of the LTM8040 to the printed circuit
board depends on the layout of the circuit board, but the
thermal resistance given in the Pin Con?guration, which
is based upon a 40.3cm 2 4 layer FR4 PC board, can be
used as a guide.
The LTM8040 is equipped with thermal protection that
reduces the output LED current if the internal operating
temperature is too high. This thermal protection is active
above the 125°C temperature rating of the LTM8040, so
continuous operation under this operating condition may
impair reliability.
8040fa
13
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