Datasheet

LTC3546
22
3546fc
For more information www.linear.com/3546
applicaTions inForMaTion
tion to exceed the maximum junction temperature of the
part. In a majority of applications, the LTC3546 does not
dissipate much heat due to its high efficiency. However, in
applications where the LTC3546 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 switches
in both regulators will be turned off and the SW nodes will
become high impedance.
To avoid the LTC3546 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 LTC3546 is
in dropout in both regulators at an input voltage of 3.3V
with load currents of 3A (SW1D externally connected to
SW2) and 1A. From the Typical Performance Character
-
istics graph of Switch Resistance, the R
DS(ON)
resistance
of the 3A P-channel switch parallel combination of SW2
and SW1D is 0.06Ω and the R
DS(ON)
of the 1A P-channel
switch is 0.18Ω. The power dissipated by the part is:
P
D
= I1
2
R
DS(ON)1
+ I2
2
R
DS(ON)2
P
D
= 3
2
• 0.064 + 1
2
• 0.19
P
D
= 0.77W
The UFD package junction-to-ambient thermal resistance,
θ
JA
, is about 34°C/W. Therefore, the junction temperature
of the regulator operating in a 85°C ambient temperature
is approximately:
T
J
= 0.77 • 34 + 85
T
J
= 111.2°C
This junction temperature is obtained from an R
DS(ON)
at
25°C. At 125°C the R
DS(ON)
increases by about 30%. This
will put the junction temperature at 122°C. If the supply is
lower, like 2.25V, the R
DS(ON)
is higher still. Special care
needs to be taken if the part is expected to be operating
in dropout so that the maximum junction temperature of
125°C is not exceeded.
Design Example
As a design example, consider using the LTC3546 in a
portable application with a Li-Ion battery. The battery
provides a V
IN
= 2.25V to 4.2V. One output requires 1.8V
at 2.5A in active mode, and 1mA in standby mode. The
other output requires 1.2V at 800mA in active mode,
and 500µA in stand-by mode. Since both loads still need
power in stand-by, Burst Mode operation is selected for
good low load efficiency.
First, determine what frequency should be used. Higher
frequency results in a lower inductor value for a given ΔI
L
(ΔI
L
is estimated as 0.35I
LOAD(MAX)
). Reasonable values
for wire wound surface mount inductors are in the 1µH
and up. Look at the different frequencies with the ΔI
L
=
0.35I
LOAD(MAX)
.
CONVERTER OUTPUT I
LOAD(MAX)
ΔI
L
SW2/SW1D, 1.2V 2.5A 875mA
SW1, 1.8V 800mA 280mA
Using the 1.5MHz frequency setting (FREQ = 143k to GNDA)
we get the following equations for L1 and L2.
L1=
1.2V
1.5MHz • 280mA
• 1−
1.2V
4.2V
⎛
⎝
⎜
⎞
⎠
⎟
= 2µH
L2 =
1.8V
1.5MHz • 875mA
• 1−
1.8V
4.2V
⎛
⎝
⎜
⎞
⎠
⎟
= 0.78µH
Use 1µH and 2.2µH.
C
OUT
selection is typically based on load step rather
than the ripple requirements. The minimum required
capacitance will increase with a decrease in compensa
-
tion loop bandwidth and/or increases in maximum load
step or output voltage tolerance. A good starting point is
about 22µF per ampere of output current for a nominal