LTC3612_15 Datasheet, PDF (23/30 Pages) Linear Technology – 3A, 4MHz Monolithic Synchronous Step-Down DC/DC Converter

LTC3612
Applications Information
The R_DS(ON) for both the top and bottom MOSFETs can
be obtained from the Typical Performance Character-
istics curves. To obtain I²R losses, simply add R_SW to
R_L and multiply the result by the square of the average
output current.
Other losses including C_IN and C_OUT ESR dissipative
losses and inductor core losses generally account for
less than 2% of the total loss.
Thermal Considerations
In most applications, the LTC3612 does not dissipate much
heat due to its high efficiency.
However, in applications where the LTC3612 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 LTC3612 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 LTC3612 is in
dropout at an input voltage of 3.3V with a load current of
3A 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 0.075Ω.
Therefore, power dissipated by the part is:
P_D = (I_OUT)² • R_DS(ON) = 675mW
For the QFN package, the θ_JA is 43°C/W.
Therefore, the junction temperature of the regulator op-
erating at 70°C ambient temperature is approximately:
T_J = 0.675W • 43°C/W + 70°C = 99°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 LTC3612 the
Exposed Pad should be soldered to a ground plane. See
the PCB Layout Board Checklist.
Design Example
As a design example, consider using the LTC3612 in an
application with the following specifications:
V_IN = 2.25V to 5.5V, V_OUT = 1.8V, I_OUT(MAX) = 3A, I_OUT(MIN)
= 100mA, f = 2.6MHz.
Efficiency is important at both high and low load current,
so Burst Mode operation will be utilized.
First, calculate the timing resistor:
R_T
=
3.82 •10¹¹Hz
2.6MHz
Ω
–
16kΩ
=
130kΩ
Next, calculate the inductor value for about 30% ripple
current at maximum V_IN:
L
=


1.8V
2.6MHz •1A


•


1–
1.8V
5.5V


=
0.466µH
Using a standard value of 0.47µH inductor results in a
maximum ripple current of:
∆I_L
=


1.8V 
2.6MHz• 0.47µH
•


1–
1.8V
5.5V


=
0.99A
For more information www.linear.com/LTC3612
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