Datasheet

22
LT3437
3437fc
APPLICATIO S I FOR ATIO
WUU
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THERMAL CALCULATIONS
Power dissipation in the LT3437 chip comes from four
sources: switch DC loss, switch AC loss, boost circuit
current, and input quiescent current. The following formu-
las show how to calculate each of these losses. These
formulas assume continuous mode operation, and should
not be used for calculating efficiency at light load currents.
Switch loss:
P
RI V
V
tIVf
SW
SW OUT OUT
IN
EFF OUT IN
=
()( )
+
()( )()()
2
12/
Boost current loss:
P
VI
V
BOOST
OUT OUT
IN
=
()
()
2
30/
Quiescent current loss:
P
Q
= V
IN
(500µA) + V
OUT
(800µA)
R
SW
= switch resistance (≈1 when hot )
t
EFF
= effective switch current/voltage overlap time
(t
r
+ t
f
+ t
IR
+ t
IF
)
t
r
= (V
IN
/0.6)ns
t
f
= (V
IN
/2)ns
t
IR
= t
IF
= (I
OUT
/0.05)ns
f = switch frequency
Example: with V
IN
= 40V, V
OUT
= 5V and I
OUT
= 250mA:
Pe
W
PW
PW
SW
BOOST
Q
=
()( )()
+
()
()
()()( )
+=
=
()
()
=
=
()
+
()
=
1 0 25 5
40
92 12 0 25 40 200 3
0 008 0 092 0 1
502530
40
0 005
40 0 0005 5 0 0008 0 024
2
2
.
/.
...
./
.
...
Total power dissipation is:
P
TOT
= 0.1 + 0.065 + 0.024 = 0.13W
Thermal resistance for the LT3437 package is influenced
by the presence of internal or backside planes. With a full
plane under the package, thermal resistance will be about
45°C for the FE and DD packages. No plane will increase
resistance to about 150°C/W. To calculate die temperature,
use the proper thermal resistance number for the desired
package and add in worst-case ambient temperature:
T
J
= T
A
+ Q
JA
(P
TOT
)
With the DD package (Q
JA
= 45°C/W) at an ambient
temperature of 70°C:
T
J
= 70 + 45(0.1) = 74.5°C
HIGH TEMPERATURE OPERATION
Extreme care must be taken when designing LT3437
applications to operate at high ambient temperatures. The
LT3437 is designed to work at elevated temperatures but
erratic operation can occur due to external components.
Each passive component should be checked for absolute
value and voltage ratings to ensure loop stability at tem-
perature. Boost and Catch diode leakages, as well as
increased series resistance, will adversely affect efficiency
and low quiescent current operation. Junction tempera-
ture increase in the diodes due to self heating (leakage)
and power dissipation should be measured to ensure their
maximum temperature specifications are not violated.
Input Voltage vs Operating Frequency Considerations
The absolute maximum input supply voltage for the LT3437
is specified at 80V. This is based solely on internal semi-
conductor junction breakdown effects. Due to internal
power dissipation, the actual maximum V
IN
achievable in
a particular application may be less than this.
A detailed theoretical basis for estimating internal power
loss is given in the section Thermal Considerations. Note
that AC switching loss is proportional to both operating
frequency and output current. The majority of AC switch-
ing loss is also proportional to the square of input voltage.
For example, while the combination of V
IN
= 40V, V
OUT
=
5V at 700mA and f
OSC
= 200kHz may be easily achievable,
simultaneously raising V
IN
to 80V and f
OSC
to 700kHz is not
possible. Nevertheless, input voltage transients up to 80V
can usually be accommodated, assuming the resulting