LM3410_08 Datasheet, PDF (16/32 Pages) National Semiconductor (TI) – PowerWise® 525kHz/1.6MHz, Constant Current Boost and SEPIC LED Driver with Internal Compensation

Total Power Losses are:
TABLE 2. Power Loss Tabulation
V_IN
V_OUT
I_LED
V_D
f_SW
I_Q
t_RISE
I_Q
R_DSON
L_DCR
D
3.3V
16.7V
50mA
0.45V
1.6MHz
10nS
10nS
3mA
225mΩ
75mΩ
0.82
P_OUT
P_DIODE
P_SWR
P_SWF
P_Q
P_COND
P_IND
825W
23mW
40mW
40mW
10mW
17mW
7mW
η
85%
P_LOSS
137mW
P_INTERNAL = P_COND + P_SW = 107 mW
Calculating and
SOT23-5 = 93°C/W
SOT23-5 = 56°C/W
Typical LLP & eMSOP typical applications will produce
numbers in the range of 50°C/W to 65°C/W, and will vary
between 18°C/W and 28°C/W. These values are for PCB’s
with two and four layer boards with 0.5 oz copper, and four to
six thermal vias to bottom side ground plane under the DAP.
The thermal impedances calculated above are higher due to
the small amount of power being dissipated within the device.
Note: To use these procedures it is important to dissipate an
amount of power within the device that will indicate a true
thermal impedance value. If one uses a very small internal
dissipated value, one can see that the thermal impedance
calculated is abnormally high, and subject to error. Figure 12
shows the nonlinear relationship of internal power dissipation
vs . .
We now know the internal power dissipation, and we are try-
ing to keep the junction temperature at or below 125°C. The
next step is to calculate the value for and/or . This is
actually very simple to accomplish, and necessary if you think
you may be marginal with regards to thermals or determining
what package option is correct.
The LM3410 has a thermal shutdown comparator. When the
silicon reaches a temperature of 165°C, the device shuts
down until the temperature drops to 150°C. Knowing this, one
can calculate the or the of a specific application. Be-
cause the junction to top case thermal impedance is much
lower than the thermal impedance of junction to ambient air,
the error in calculating is lower than for . However,
you will need to attach a small thermocouple onto the top case
of the LM3410 to obtain the value.
Knowing the temperature of the silicon when the device shuts
down allows us to know three of the four variables. Once we
calculate the thermal impedance, we then can work back-
wards with the junction temperature set to 125°C to see what
maximum ambient air temperature keeps the silicon below
the 125°C temperature.
Procedure:
Place your application into a thermal chamber. You will need
to dissipate enough power in the device so you can obtain a
good thermal impedance value.
Raise the ambient air temperature until the device goes into
thermal shutdown. Record the temperatures of the ambient
air and/or the top case temperature of the LM3410. Calculate
the thermal impedances.
Example from previous calculations (SOT23-5 Package):
P_INTERNAL = 107 mW
T_A @ Shutdown = 155°C
T_C @ Shutdown = 159°C
30038551
FIGURE 12. R_θJA vs Internal Dissipation
For 5-pin SOT23 package typical applications, R_θJA numbers
will range from 80°C/W to 110°C/W, and will vary between
50°C/W and 65°C/W. These values are for PCB’s with two &
four layer boards with 0.5 oz copper, with two to four thermal
vias from GND pin to bottom layer.
Here is a good rule of thumb for typical thermal impedances,
and an ambient temperature maximum of 75°C: If your design
requires that you dissipate more than 400mW internal to the
LM3410, or there is 750mW of total power loss in the appli-
cation, it is recommended that you use the 6 pin LLP or the 8
pin eMSOP package with the exposed DAP.
SEPIC Converter
The LM3410 can easily be converted into a SEPIC converter.
A SEPIC converter has the ability to regulate an output volt-
age that is either larger or smaller in magnitude than the input
voltage. Other converters have this ability as well (CUK and
Buck-Boost), but usually create an output voltage that is op-
posite in polarity to the input voltage. This topology is a perfect
fit for Lithium Ion battery applications where the input voltage
for a single cell Li-Ion battery will vary between 2.7V & 4.5V
and the output voltage is somewhere in between. Most of the
www.national.com
16