LTC3026
12
3026ff
operaTion
Boost Converter Component Selection
A 10µH chip inductor with a peak saturation current (I
SAT
)
of at least 150mA is recommended for use with the internal
boost converter. The inductor value can range between
4.7µH to 40µH, but values less than 10µH result in higher
switching frequency, increased switching losses, and lower
max output current available at the BST pin. See Table 1
for a list of component suppliers.
Table 1. Inductor Vendor Information
SUPPLIER PART NUMBER WEBSITE
Coilcraft 0603PS-103KB www.coilcraft.com
Murata LQH2MCN100K02 www.murata.com
Taiyo Yuden LB2016T100M www.t-yuden.com
TDK NLC252018T-100K www.TDK.com
It is also recommended that the BST pin be bypassed to
ground with a 4.7µF or greater ceramic capacitor. Larger
values of capacitance will not reduce the size of the BST
ripple much, but will decrease the ripple frequency propor-
tionally. The BST pin should maintain 1µF of capacitance
at all times to ensure correct operation (See the “Output
Capacitance and Transient Response” section about
capacitor selection). High ESR tantalum and electrolytic
capacitors may be used, but a low ESR ceramic must be
used in parallel for correct operation.
Thermal Considerations
The power handling capability of the device will be limited
by the maximum rated junction temperature (125°C).
The majority of the power dissipated in the device will be
the output current multiplied by the input/output voltage
differential: (I
OUT
)(V
IN
– V
OUT
). Note that the BST current
is less than 200µA even under heavy loads, so its power
consumption can be ignored for thermal calculations.
The LTC3026 has internal thermal limiting designed to
protect the device during momentary overload conditions.
For continuous normal conditions, the maximum junction
temperature rating of 125°C must not be exceeded. It is
important to give careful consideration to all sources of
thermal resistance from junction to ambient. Additional
heat sources mounted nearby must also be considered.
For surface mount devices, heat sinking is accomplished
by using the heat-spreading capabilities of the PC board
and its copper traces. Copper board stiffeners and plated
through holes can also be used to spread the heat gener-
ated by power devices.
A junction-to-ambient thermal coefficient of 40°C/W is
achieved by connecting the exposed pad of the MSOP or
DFN package directly to a ground plane of about 2500mm
2
.
Calculating Junction Temperature
Example: Given an output voltage of 1.2V, an input voltage
of 1.8V ±4%, an output current range of 0mA to 1A and
a maximum ambient temperature of 50°C, what will the
maximum junction temperature be?
The power dissipated by the device will be approximately:
I
OUT(MAX)
(V
IN(MAX)
– V
OUT
)
where:
I
OUT(MAX)
= 1A
V
IN(MAX)
= 1.87V
so:
P = 1A(1.87V – 1.2V) = 0.67W
Even under worst-case conditions LTC3026’s BST pin
power dissipation is only about 1mW, thus can be ignored.
The junction to ambient thermal resistance will be on the
order of 40°C/W. The junction temperature rise above
ambient will be approximately equal to:
0.67W(40°C/W) = 26.8°C
The maximum junction temperature will then be equal to
the maximum junction temperature rise above ambient
plus the maximum ambient temperature or:
T
A
= 26.8°C + 50°C = 76.8°C
Short-Circuit/Thermal Protection
The LTC3026 has built-in output short-circuit current
limiting as well as overtemperature protection. During
short-circuit conditions, internal circuitry automatically
limits the output current to approximately 3A. At higher
