LTC3569
17
3569fe
For more information www.linear.com/LTC3569
applicaTions inForMaTion
The second consideration is stray capacitance on the FB
pin traces and the R
T
pin trace to GND. This is taken into
account by cutting the ground plane beneath these traces.
However, wherever the ground plane is cut, add additional
decoupling capacitors across the break to provide a path
for high-frequency ground return currents to flow.
Finally, the third consideration is stray impedance between
the SW node and the inductor when operating with a slave
power stage. It is important to keep the stray inductance of
the slave power device to a minimum, by keeping the trace
from slave SW to the main SW as short as possible. This
requirement is necessary to ensure that the slave power
device’s share of the inductor current does not exceed that
of the master as well as to keep the current density in the
slave device under control. The inductor should be placed
close to the master SW pin to minimize stray impedance
and allow the master to control the inductor current.
Thermal Considerations
In the majority of applications, the LTC3569 does not dis-
sipate much heat due to its high efficiency. However, in
applications where the LTC3569 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, the LTC3569
will be turned off and 2k resistive pull-downs are tied to
all the SW nodes.
To prevent the LTC3569 from exceeding 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 junc-
tion temperature of the part. Temperature rise is:
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
A
.
Where T
A
is the ambient temperature.
As an example, consider the case when the LTC3569 is
in dropout at an input voltage of 2.7V with load currents
of 1000mA, 500mA and 500mA for bucks 1, 2 and 3
respectively, at an ambient temperature of 85°C. From
the Typical Performance Characteristics, the R
DS(ON)
of
buck1 is 0.190Ω, and for buck2 and buck3 it is 0.265Ω.
Therefore, power dissipated by the LTC3569 is:
P
D
= I
1
2
R
DS(ON)1
+ I
2
2
R
DS(ON)2
+ I
3
2
R
DS(ON)3
= 190mV + 66.25mW + 66.25mV
= 322.5mW
At 85°C ambient the junction temperature is:
T
J
= 322.5mW•68°C/W + 85°C = 106.9°C.
This junction temperature is below the absolute maximum
junction temperature of 125°C.
Design Example 1: 2.5V, 1.8V and 1.2V From a
Li-Ion Battery
As a design example, consider using the LTC3569 in a
portable application with a Li-Ion battery source. The bat-
tery provides an SV
IN
from 2.9V to 4.2V. The loads require
2.5V, 1.8V and 1.2V with current requirements of up to
800mA, 400mA and 400mA respectively when active. The
first load, with the 2.5V rail has no standby requirements,
however loads 2 and 3 each require a current of 1mA in
standby. Since two of the loads require low current opera-
tion, Burst Mode operation is selected. With V
IN(MAX)
at
4.2V and V
OUT(MIN)
= 1.2V, the maximum clock frequency
is 3.57MHz based on minimum on-time requirements.
To simplify the board layout, the fixed 2.25MHz internal
frequency is selected.
