LTC3415 Datasheet, PDF (21/28 Pages) Linear Technology – 7A, PolyPhase Synchronous Step-Down Regulator

LTC3415
APPLICATIONS INFORMATION
3) I²R losses are calculated from the DC resistances of
the internal switches, R_SW, and external inductor, R_L.
In continuous mode, the average output current flows
through inductor L but is “chopped” between the internal
top and bottom switches. Thus, the series resistance
looking into the SW pin is a function of both top and
bottom MOSFET R_DS(ON) and the duty cycle (DC) as
follows:
R_SW = (R_DS(ON) TOP)(DC) + (R_DS(ON)BOT)(1-DC)
The R_DS(ON) for both the top and bottom MOSFETs can be
obtained from the Typical Performance Characteristics
curves. Thus, to obtain I²R losses:
I²R losses = I_OUT2(R_SW + R_L)
4) Other “hidden” losses such as copper trace and in-
ternal battery resistances can account for additional
efficiency degradations in portable systems. It is very
important to include these “system” level losses in
the design of a system. The internal battery and fuse
resistance losses can be minimized by ensuring that
C_IN has adequate charge storage and very low ESR at
the switching frequency. Other losses including diode
conduction losses during dead-time and inductor
core losses generally account for less than 2% total
additional loss.
Thermal Considerations
In the majority of applications, the LTC3415 does not
dissipate much heat due to its high efficiency. However,
in applications where the LTC3415 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 150°C,
both power switches will be turned off and the SW node
will become high impedance.
To avoid the LTC3415 from exceeding the maximum junc-
tion temperature, the user will need to do some thermal
analysis. The goal of the thermal analysis is to determine
whether the power dissipated exceeds the maximum
junction temperature of the part. 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_RISE + T_AMBIENT
As an example, consider the case when the LTC3415 is in
dropout at an input voltage of 3.3V with a load current of
5A. From the Typical Performance Characteristics graph
of Switch Resistance, the R_DS(ON) resistance of the P-
channel switch is 0.03. Therefore, power dissipated by
the part is:
P_D = I² • R_DS(ON) = 750mW
The QFN 5mm × 7mm package junction-to-ambient thermal
resistance, θ_JA, is around 34°C/W. Therefore, the junction
temperature of the regulator operating in a 50°C ambient
temperature is approximately:
T_J = 0.75 • 34 + 50 = 75.5°C
Remembering that the above junction temperature is
obtained from an R_DS(ON) at 25°C, we might recalculate
the junction temperature based on a higher R_DS(ON) since
it increases with temperature. However, we can safely as-
sume that the actual junction temperature will not exceed
the absolute maximum junction temperature of 125°C.
Solder the LTC3415’s bottom exposed pad to ground for
optimal thermal performance.
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