LTC3411_15 Datasheet, PDF (16/24 Pages) Linear Technology – 1.25A, 4MHz, Synchronous Step-Down DC/DC Converter

LTC3411
APPLICATIONS INFORMATION
To avoid the LTC3411 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 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 resistance,
θ_JA, will be in the range of 100°C/W to 120°C/W. Therefore,
the junction temperature of the regulator operating 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 as-
sume 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 por-
table application with a Li-Ion battery. The battery provides
a V_IN = 2.5V to 4.2V. The load requires a maximum 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 with 40% ripple current which is 500mA:
L
=
2.5V
1MHz • 500mA
•
⎛
⎝⎜
1−
2.5V
4.2V
⎞
⎠⎟
=
2μH
Choosing the closest inductor from a vendor of 2.2μH,
results in a maximum ripple current of:
ΔI_L
=
2.5V
1MHz • 2.2μ
• ^{⎛⎝⎜}1−
24..52VV ⎞⎠⎟
=
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
1MHz
1A
• (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.
3411fb
16