LTC3615 Datasheet, PDF (25/32 Pages) Linear Technology – Dual 4MHz, 3A Synchronous Step-Down DC/DC Converter

LTC3615
Applications Information
Thermal Considerations
In most applications, the LTC3615 does not dissipate
much heat due to its high efficiency. However, in ap-
plications where the LTC3615 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, all four power
switches will be turned off and the SW node will become
high impedance.
To prevent the LTC3615 from exceeding the maximum
junction temperature, the user will need to do some ther-
mal analysis. 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_A + T_RISE
where T_A is the ambient temperature.
As an example, consider this case: the LTC3615 is in
dropout at an input voltage of 3.3V with a load current for
each channel of 2A at an ambient temperature of 70°C.
Assuming a 20°C rise in junction temperature, to 90°C,
results in an R_DS(ON) of 0.086mΩ (see the graph in the
Typical Performance Characteristics section). Therefore,
the power dissipated by the part is:
P_D = (I₁₂ + I₂₂) • R_DS(ON) = 0.69W
For the QFN package, the θ_JA is 37°C/W.
Therefore, the junction temperature of the regulator operat-
ing at 70°C ambient temperature is approximately:
T_J = 0.69W • 37°C/W + 70°C = 95°C
Note that for very low input voltage, the junction tem-
perature will be higher due to increased switch resistance
R_DS(ON). It is not recommended to use full load current at
high ambient temperature and low input voltage.
To maximize the thermal performance of the LTC3615, the
Exposed Pad should be soldered to a ground plane. See
the PC Board Layout Checklist.
Design Example
As a design example, consider using the LTC3615 in an
application with the following specifications:
V_IN = 3.3V to 5.5V
V_OUT1 = 2.5V
V_OUT2 = 1.2V
I_OUT1(MAX) = 1A
I_OUT2(MAX) = 3A
I_OUT(MIN) = 100mA
f = 2.25MHz
Because efficiency is important at both high and low load
current, Burst Mode operation will be selected by connect-
ing the MODE pin to SGND.
First, calculate the timing resistor:
R _{RT / SYNC}
=
4E11 Ω • Hz
2.25MHz
=
178k
Next, calculate the inductor values for about 1A ripple
current at maximum V_IN:
L1 =


2.5V
2.25MHz

• 1A 
•


1–
2.5V 
5.5V 
=
0.6µH
L2
=


1.2V
2.25MHz

• 1A 
•


1–
1.2V 
5.5V 
=
0.42µH
Using a standard value of 0.56µH and 0.47µH inductors
results in maximum ripple currents of:
∆I_L1 =


2.5V

2.25MHz • 0.56µH
•


1–
2.5V 
5.5V 
=
1.08A
∆I_L2 =


1.2V

2.25MHz • 0.47µH
•


1–
1.2V
5.5V


=
0.89A
3615f
25