ltc3612 Linear Technology Corporation, ltc3612 Datasheet - Page 22

• Manufacturer Part Number: ltc3612
• Description: 3a, 4mhz Monolithic Synchronous Step-down Dc/dc Converter
• Manufacturer: Linear Technology Corporation

LTC3612
APPLICATIONS INFORMATION
Thermal Considerations
In most applications, the LTC3612 does not dissipate much
heat due to its high effi ciency.
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:
where P
is the thermal resistance from the junction of the die to
the ambient temperature. The junction temperature, T
is given by:
where T
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
Therefore, power dissipated by the part is:
For the QFN package, the θ
Therefore, the junction temperature of the regulator operat-
ing at 70°C ambient temperature is approximately:
We can safely assume that the actual junction temperature
will not exceed the absolute maximum junction tempera-
ture of 125°C.
22
T
T
P
T
RISE
J
J
D
DS(ON)
= T
= 0.675W • 43°C/W + 70°C = 99°C
= (I
A
D
= (P
A
OUT
is the power dissipated by the regulator and θ
is the ambient temperature.
+ T
resistance of the P-channel switch is 0.075Ω.
D
)
RISE
2
)(θ
• R
JA
DS(ON)
)
= 675mW
JA
is 43°C/W.
JA
J
,
Note that for very low input voltage, the junction tempera-
ture will be higher due to increased switch resistance,
R
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 specifi cations:
V
= 100mA, 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:
Next, calculate the inductor value for about 30% ripple
current at maximum V
Using a standard value of 0.47μH inductor results in a
maximum ripple current of:
C
satisfy the output voltage ripple requirement and the bulk
capacitance needed for loop stability. For this design, a
68μF (or 47μF plus 22μF) ceramic capacitor is used with
a X5R or X7R dielectric.
IN
OUT
DS(ON)
R
L =
ΔI
= 2.25V to 5.5V, V
T
L
will be selected based on the ESR that is required to
=
⎛
⎝ ⎜
=
. It is not recommended to use full load current
2.6MHz • 1A
3.82
⎛
⎝ ⎜
2.6MHz
2.6MHz • 0.47µH
1.8V
11
Hz
1.8V
k – 16k = 130kΩ
⎞
⎠ ⎟
OUT
IN
• 1–
⎛
⎝ ⎜
:
= 1.8V, I
⎞
⎠ ⎟
1.8V
5.5V
• 1–
⎛
⎝ ⎜
OUT(MAX)
⎞
⎠ ⎟
= 0.466µH
1.8V
5.5V
⎞
⎠ ⎟
= 3A, I
= 0.99A
OUT(MIN)
3612f