LTC3417 Datasheet, PDF (15/20 Pages) Linear Technology – Dual Synchronous 1.4A/800mA 4MHz Step-Down DC/DC Regulator

LTC3417
APPLICATIO S I FOR ATIO
3) I²R losses are calculated from the DC resistances of the
internal switches, R_SW, and the external inductor, R_L. In
continuous mode, the average output current flowing
through inductor L 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)
where R_L is the resistance of the inductor.
4) Other “hidden” losses such as copper trace and internal
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 making sure that C_IN has adequate
charge storage and very low ESR_COUT at the switching
frequency. Other losses including diode conduction losses
during dead-time and inductor core losses generally ac-
count for less than 2% total additional loss.
Thermal Considerations
The LTC3417 requires the package Exposed Pad (PGND2/
GNDD pin) to be well soldered to the PC board. This gives
the DFN and TSSOP packages exceptional thermal prop-
erties, compared to similar packages of this size, making
it difficult in normal operation to exceed the maximum
junction temperature of the part. In a majority of applica-
tions, the LTC3417 does not dissipate much heat due to its
high efficiency. However, in applications where the
LTC3417 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 switches in both regulators
will be turned off and the SW nodes will become high
impedance.
To prevent the LTC3417 from exceeding its maximum
junction 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 tempera-
ture 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 LTC3417 is in
dropout in both regulators at an input voltage of 3.3V with
load currents of 1.4A and 800mA. From the Typical
Performance Characteristics graph of Switch Resistance,
the R_DS(ON) resistance of the 1.4A P-channel switch is
0.09Ω and the R_DS(ON) of the 800mA P-channel switch is
0.163Ω. The power dissipated by the part is:
PD = I₁₂ • R_DS(ON)1 + I₂₂ • R_DS(ON)2
PD = 1.4² • 0.09 + 0.8² • 0.163
PD = 281mW
The DFN package junction-to-ambient thermal resistance,
θ_JA, is about 43°C/W. Therefore, the junction temperature
of the regulator operating in a 70°C ambient temperature
is approximately:
T_J = 0.281 • 43 + 70
T_J = 82.1°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
assume that the actual junction temperature will not
exceed the absolute maximum junction temperature of
125°C.
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