LTC3563
12
3563f
Thermal Considerations
In most applications the LTC3563 does not dissipate much
heat due to its high effi ciency. But in applications where the
LTC3563 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 LTC3563 from exceeding the maximum
junction temperature, the user needs 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
R
= (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.
The junction temperature, T
J
, is given by:
T
J
= T
A
+ T
R
where T
A
is the ambient temperature.
As an example, consider the LTC3563 with an output
voltage of 1.87V, an input voltage of 2.7V, a load current
of 500mA and an ambient temperature of 70°C. From
the typical performance graph of switch resistance, the
R
DS(ON)
of the P-channel switch at 70°C is approximately
0.7Ω and the R
DS(ON)
of the N-channel synchronous
switch is approximately 0.4Ω. The duty cycle in this case
is approximately 70%.
The series resistance looking into the SW pin is:
R
SW
= 0.7Ω (0.7) + 0.4Ω (0.3) = 0.61Ω
Therefore, for the power dissipated by the part is:
P
D
= I
LOAD
2
• R
SW
= 152.5mW
For the DFN package, the θ
JA
is 40°C/W. Thus, the junction
temperature of the regulator is:
T
J
= 70°C + (0.1525)(40) = 76.1°C
APPLICATIO S I FOR ATIO
WUUU
which is below the maximum junction temperature of
125°C.
Note that at higher supply voltages, the junction temperature
is lower due to reduced switch resistance (R
DS(ON)
).
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
OUT
immediately shifts by an amount
equal to ΔI
LOAD
• ESR, where ESR is the effective series
resistance of C
OUT
. ΔI
LOAD
also begins to charge or dis-
charge C
OUT
, generating a feedback error signal used by the
regulator to return V
OUT
to its steady-state value. During
this recovery time, V
OUT
can be monitored for overshoot
or ringing that would indicate a stability problem.
The output voltage settling behavior is related to the stability
of the closed-loop system and will demonstrate the actual
overall supply performance. For a detailed explanation of
optimizing the compensation components, including a re-
view of control loop theory, refer to Application Note 76.
In some applications, a more severe transient can be caused
by switching loads with large (>1µF) bypass capacitors.
The discharged bypass capacitors are effectively put in
parallel with C
OUT
, causing a rapid drop in V
OUT
. No regula-
tor can deliver enough current to prevent this problem, if
the switch connecting the load has low resistance and is
driven quickly. The solution is to limit the turn-on speed of
the load switch driver. A Hot Swap
TM
controller is designed
specifi cally for this purpose and usually incorporates cur-
rent limit, short circuit protection and soft-start.
Design Example
As a design example, assume the LTC3563 is used in
a single lithium-ion battery-powered cellular phone ap-
plication. The V
IN
will be operating from a maximum of
4.2V down to about 2.7V. The load current requirement
is a maximum of 0.5A, but most of the time it will be in
standby mode, requiring only 2mA. Effi ciency at both
low and high load currents is important. Output voltage
is either 1.87V or 1.28V.
