LTC3603 Linear Dimensions Semiconductor, LTC3603 Datasheet - Page 14

• Manufacturer Part Number: LTC3603
• Description: Monolithic Synchronous Step-Down Regulator
• Manufacturer: Linear Dimensions Semiconductor

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APPLICATIONS INFORMATION
Thermal Considerations
In most applications, the LTC3603 does not dissipate much
heat due to its high effi ciency. But, in applications where the
LTC3603 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 prevent the LTC3603 from exceeding the maximum
junction 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:
where P
is the thermal resistance from the junction of the die to
the ambient temperature.
The junction temperature, T
where T
As an example, consider the LTC3603 in dropout at an
input voltage of 8V, a load current of 2.5A and an ambient
temperature of 70°C. From the Typical Performance graph
of Switch Resistance, the R
is approximately 85mΩ. Therefore, power dissipated by
the part is:
For the TSSOP package, the θ
junction temperature of the regulator is:
which is below the maximum junction temperature of
125°C.
LTC3603
14
T
T
P
T
R
J
J
D
= T
= 70°C + (0.53W)(38°C/W) = 90.1°C
= (P
= (I
A
D
A
LOAD
is the power dissipated by the regulator and θ
is the ambient temperature.
D
+ T
) • (θ
R
2
)(R
JA
DS(ON)
)
) = (2.5A)
DS(ON)
J
, is given by:
JA
of the top switch at 70°C
2
is 38°C/W. Thus, the
(85mΩ) = 0.53W
JA
Checking Transient Response
The regulator loop response can be checked by looking
at the load transient response. Switching regulators take
several cycles to respond to a step in load current. When
a load step occurs, V
equal to ΔI
resistance of C
charge C
regulator to return V
this recovery time, V
or ringing that would indicate a stability problem. The I
pin external components and output capacitor shown in the
front page application will provide adequate compensation
for most applications.
Design Example
As a design example, consider using the LTC3603 in
an application with the following specifi cations: V
12V, V
f = 1MHz. Because effi ciency is important at both high and
low load current, Burst Mode operation will be utilized.
First, calculate the timing resistor:
Next, calculate the inductor value for about 40% ripple
current at maximum V
Using a 2.2μH inductor results in a maximum ripple cur-
rent of:
C
to satisfy the output voltage ripple requirement and the
bulk capacitance needed for loop stability. In this applica-
tion, a tantalum capacitor will be used to provide the bulk
OUT
R
L
Δ =
OSC
=
I
L
will be selected based on the ESR that is required
OUT
⎛
⎜
⎝
(
OUT
=
1
⎛
⎜
⎝
MHz
= 3.3V, I
1 15 10
(
LOAD
, generating a feedback error signal used by the
1
3 3
.
MHz
.
1
MHz
)
OUT
V
•(ESR), where ESR is the effective series
•
( )
3 3
1
.
)
A
. ΔI
(
OUT(MAX)
2 2
11
V
OUT
OUT
OUT
⎞
⎟
⎠
.
LOAD
–
IN
•
µH
⎛
⎝ ⎜
10
:
immediately shifts by an amount
to its steady-state value. During
can be monitored for overshoot
1
)
–
k
⎞
⎟
⎠
also begins to charge or dis-
3 3
=
= 2.5A, I
•
12
.
⎛
⎝ ⎜
105
1
V
V
–
⎞
⎠ ⎟
k
3 3
12
=
.
2 39
OUT(MIN)
V
V
. µ µ H
⎞
⎠ ⎟
= = 1 1 . A
= 100mA,
IN
3603f
TH
=