LTC3411 Datasheet, PDF (14/20 Pages) Linear Technology – 1.25A, 4MHz, Synchronous Step-Down DC/DC Converter

LTC3411
APPLICATIO S I FOR ATIO
Thermal Considerations
In a majority of applications, the LTC3411 does not
dissipate much heat due to its high efficiency. However, in
applications where the LTC3411 is running at high ambi-
ent 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 LTC3411 from exceeding the 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 LTC3411 is in
dropout at an input voltage of 3.3V with a load current of
1A. From the Typical Performance Characteristics graph
of Switch Resistance, the R_DS(ON) resistance of the
P-channel switch is 0.11Ω. Therefore, power dissipated
by the part is:
P_D = I² • R_DS(ON) = 110mW
The MS10 package junction-to-ambient thermal resis-
tance, θ_JA, will be in the range of 100°C/W to 120°C/W.
Therefore, the junction temperature of the regulator oper-
ating in a 70°C ambient temperature is approximately:
T_J = 0.11 • 120 + 70 = 83.2°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.
Design Example
As a design example, consider using the LTC3411 in a
portable application with a Li-Ion battery. The battery
provides a V_IN = 2.5V to 4.2V. The load requires a maxi-
mum of 1A in active mode and 10mA in standby mode. The
output voltage is V_OUT = 2.5V. Since the load still needs
power in standby, Burst Mode operation is selected for
good low load efficiency.
First, calculate the timing resistor:
R_T = 9.78 • _{( )} 10¹¹ 1MHz ^−1.08 = 323.8k
Use a standard value of 324k. Next, calculate the inductor
value for about 30% ripple current at maximum V_IN:
L
=
2.5V
1MHz • 510mA
•
_1−
2.5V
4.2V

=
2µH
Choosing the closest inductor from a vendor of 2.2µH,
results in a maximum ripple current of:
∆I_L
=
2.5V
1MHz • 2.2µ
• ^{}1−
2.5V
4.2V

=
460mA
For cost reasons, a ceramic capacitor will be used. C_OUT
selection is then based on load step droop instead of ESR
requirements. For a 5% output droop:
C _OUT
≈
2.5
1A
1MHz • (5%• 2.5V)
=
20µF
The closest standard value is 22µF. Since the output
impedance of a Li-Ion battery is very low, C_IN is typically
10µF. In noisy environments, decoupling SV_IN from PV_IN
with an R6/C8 filter of 1Ω/0.1µF may help, but is typically
not needed.
The output voltage can now be programmed by choosing
the values of R1 and R2. To maintain high efficiency, the
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