LTC3616 Datasheet, PDF (23/28 Pages) Linear Technology – 6A, 4MHz Monolithic Synchronous Step-Down DC/DC Converter

LTC3616
APPLICATIONS INFORMATION
Thermal Considerations
In most applications, the LTC3616 does not dissipate much
heat due to its high efficiency.
However, in applications where the LTC3616 is running at
high ambient temperature with low supply voltage and high
duty cycles, such as in dropout, the heat dissipated may
exceed the maximum junction temperature of the part. If
the junction temperature reaches approximately 160°C,
both power switches will be turned off and the SW node
will become high impedance.
To prevent the LTC3616 from exceeding the maximum
junction temperature, some thermal analysis is required.
The temperature rise is given by:
T_RISE = (P_D)(θ_JA)
where P_D is the power dissipated by the regulator and θ_JA
is the thermal resistance from the junction of the die to
the ambient temperature. The junction temperature, T_J,
is given by:
T_J = T_A + T_RISE
where T_A is the ambient temperature.
As an example, consider the case when the LTC3616 is in
dropout at an input voltage of 3.3V with a load current of
6A at an ambient temperature of 70°C. From the Typical
Performance Characteristics graph of Switch Resistance,
the R_DS(ON) resistance of the P-channel switch is 0.035Ω.
Therefore, power dissipated by the part is:
P_D = (I_OUT)² • R_DS(ON) = 1.26W
For the QFN package, the θ_JA is 38°C/W.
Therefore, the junction temperature of the regulator operat-
ing at 70°C ambient temperature is approximately:
T_J = 1.26W • 38°C/W + 70°C = 118°C
We can safely assume that the actual junction temperature
will not exceed the absolute maximum junction tempera-
ture of 125°C.
Note that for very low input voltage, the junction tempera-
ture will be higher due to increased switch resistance,
R_DS(ON). It is not recommended to use full load current
for high ambient temperature and low input voltage.
To maximize the thermal performance of the LTC3616 the
exposed pad should be soldered to a ground plane. See
the PCB Layout Board Checklist.
3616f
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