Datasheet

LTC3614
24
3614fa
APPLICATIONS INFORMATION
Thermal Considerations
In most applications, the LTC3614 does not dissipate much
heat due to its high effi ciency.
However, in applications where the LTC3614 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 LTC3614 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 LTC3614 is in
dropout at an input voltage of 3.3V with a load current of
4A at an ambient temperature of 85°C. From the Typical
Performance Characteristics graph of Switch Resistance,
the R
DS(ON)
resistance of the P-channel switch is 0.038.
Therefore, power dissipated by the part is:
P
D
= (I
OUT
)
2
• R
DS(ON)
= 0.61W
For the QFN package, the θ
JA
is 38°C/W.
Therefore, the junction temperature of the regulator operat-
ing at 85°C ambient temperature is approximately:
T
J
= 0.61W • 38°C/W + 85°C = 108°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
with high ambient temperature and low input voltage.
To maximize the thermal performance of the LTC3614 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 LTC3614 in an
application with the following specifi cations:
V
IN
= 2.25V to 5.5V, V
OUT
= 1.8V, I
OUT(MAX)
= 4A, I
OUT(MIN)
= 200mA, f = 2.6MHz.
Effi ciency 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
11
Hz
2.6MHz
k – 16k = 130kΩ
Next, calculate the inductor value for about 33% ripple
current at maximum V
IN
:
L =
1.8V
2.6MHz • 1.3A
⎛
⎝
⎜
⎞
⎠
⎟
•1–
1.8V
5.5V
⎛
⎝
⎜
⎞
⎠
⎟
= 0.35µH
Using a standard value of 0.33µH inductor results in a
maximum ripple current of:
ΔI
L
=
1.8V
2.6MHz • 0.33µH
⎛
⎝
⎜
⎞
⎠
⎟
•1–
1.8V
5.5V
⎛
⎝
⎜
⎞
⎠
⎟
= 1.41A
C
OUT
will be selected based on the ESR that is required
to satisfy the output voltage ripple requirement and the
bulk capacitance needed for loop stability. For this design,
a 100µF ceramic capacitor is used with a X5R or X7R
dielectric.