LTC3618_15 Datasheet, PDF (18/24 Pages) Linear Technology – Dual 4MHz, 3A Synchronous Buck Converter for DDR Termination

LTC3618
Applications Information
2. I²R losses are calculated from the resistances of the
internal switches, R_SW, and external inductor R_L. In
continuous mode the average output current flowing
through inductor L is “chopped” between the main
switch and the synchronous switch. 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. 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 LTC3618 does not dissipate
much heat due to its high efficiency. However, in ap-
plications where the LTC3618 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 LTC3618 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 LTC3618 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 0.086Ω (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 46.9°C/W.
Therefore, the junction temperature of the regulator op-
erating at 70°C ambient temperature is approximately:
T_J = 0.69W • 46.9°C/W + 70°C = 102.4°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 LTC3618,
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 LTC3618 in an
application with the following specifications:
V_IN = 3.3V to 5.5V
V_DDQ = 1.8V
V_TT = 0.9V
I_OUT1(MAX) = 3A
I_OUT2(MAX) = 3A
I_OUT1(MIN) = 200mA
f = 2.25MHz
First, calculate the timing resistor:
R_RT
=
4•
10¹¹ Ω •
2.25MHz
Hz
=
178k
3618fc
18
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