Datasheet

LTC3561A
16
3561af
APPLICATIONS INFORMATION
Thermal Considerations
In a majority of applications, the LTC3561A does not
dissipate much heat due to its high effi ciency. However,
in applications where the LTC3561A 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 150°C,
both power switches will be turned off and the SW node
will become high impedance.
To avoid the LTC3561A from exceeding the maximum junc-
tion temperature, the user will need to do some thermal
analysis. The goal of the thermal analysis is 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
RISE
+ T
AMBIENT
As an example, consider the case when the LTC3561A
is in dropout at an input voltage of 3.3V with a load cur-
rent of 1A. From the Typical Performance Characteristics
graph of Switch Resistance, the R
DS(ON)
resistance of the
P-channel switch is 0.17Ω. Therefore, power dissipated
by the part is:
P
D
= I
2
• R
DS(ON)
= 170mW
The DD8 package junction-to-ambient thermal resistance,
θ
JA
, will be in the range of about 43°C/W. Therefore, the
junction temperature of the regulator operating in a 70°C
ambient temperature is approximately:
T
J
= 0.17 • 43 + 70 = 77.31°C
Remembering that the above junction temperature is
obtained from an R
DS(ON)
at 25°C, we might recalculate
the junction temperature based on a higher R
DS(ON)
since
it increases with temperature. However, we can safely as-
sume that the actual junction temperature will not exceed
the absolute maximum junction temperature of 125°C.
Design Example
As a design example, consider using the LTC3561A in
a portable application with a Li-Ion battery. The battery
provides a V
IN
= 2.5V to 4.2V. The load requirement is a
maximum of 1A, but most of the time it will be in standby
mode, requiring only 10mA. The output voltage is V
OUT
= 1.8V. Since the load still needs power in standby, Burst
Mode operation is selected for good low load effi ciency.
First, calculate the timing resistor for 1MHz operation:
R
T
= 5 • 10
7
(10
3
)
–1.6508
= 557.9k
Use a standard value of 549k. Next, calculate the inductor
value for about 40% ripple current at maximum V
IN
:
L =
1.8V
1MHz • 400mA
•1−
1.8V
4.2V
⎛
⎝
⎜
⎞
⎠
⎟
= 2.57μH
Choosing the closest inductor from a vendor of 2.2μH,
results in a maximum ripple current of:
ΔI
L
=
1.8V
1MHz • 2.2μH
•1−
1.8V
4.2V
⎛
⎝
⎜
⎞
⎠
⎟
= 468mA
For cost reasons, a ceramic capacitor will be used. C
OUT
selection is then based on load step droop instead of ESR
requirements. For a 5% output droop:
C
OUT
≈ 2.5
1A
1MHz •(5% • 1.8V)
≅ 27μF