Datasheet

LT3430/LT3430-1
19
34301fa
THERMAL CALCULATIONS
Power dissipation in the LT3430/LT3430-1 chip comes
from four sources: switch DC loss, switch AC loss, boost
circuit current, and input quiescent current. The follow-
ing formulas show how to calculate each of these losses.
These formulas assume continuous mode operation, so
they should not be used for calculating effi ciency at light
load currents.
Switch loss:
P
RI V
V
tIVf
SW
SW OUT OUT
IN
EFF OUT IN
=
()( )
+
()()()
2
12(/ )
(Note: Switching losses are less for the LT3430-1 oper-
ating at only 100kHz)
Boost current loss:
P
VI
V
BOOST
OUT OUT
IN
=
()
2
36/
Quiescent current loss:
PV V
Q IN OUT
=
()
+
()
0 0015 0 003..
R
SW
= Switch resistance (≈0.15) hot
t
EFF
= Effective switch current/voltage overlap time
= (t
r
+ t
f
+ t
Ir
+ t
If
)
t
r
= (V
IN
/1.2)ns
t
f
= (V
IN
/1.1)ns
t
Ir
= t
If
= (I
OUT
/0.2)ns
f = Switch frequency
Example: with V
IN
= 40V, V
OUT
= 5V and I
OUT
= 2A:
P
W
PW
PW
SW
BOOST
Q
=
( )()()
+
()
()
()( )
()
=+=
=
()
()
=
=+=
−
015 2 5
40
90 10 1 2 2 40 200 10
008 072 08
5236
40
004
40 0 0015 5 0 003 0 08
2
93
2
.
•/ •
...
/
.
(. ) (. ) .
Total power dissipation in the IC is given by:
P
TOT
= P
SW
+ P
BOOST
+ P
Q
= 0.8W + 0.04W + 0.08W = 0.92W
Thermal resistance for the LT3430/LT3430-1 package is in-
fl uenced by the presence of internal or backside planes.
TSSOP (Exposed Pad) Package: With a full plane under
the TSSOP package, thermal resistance will be about
45°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
+ (θ
JA
• P
TOT
)
When estimating ambient, remember the nearby catch
diode and inductor will also be dissipating power:
P
VV V I
V
DIODE
F IN OUT LOAD
IN
=
( )( – )( )
V
F
= Forward voltage of diode (assume 0.52V at 2A)
PW
DIODE
==
(. )( – )()
.
052 40 5 2
40
091
P
INDUCTOR
= (I
LOAD
)
2
(R
IND
)
R
IND
= Inductor DC resistance (assume 0.1Ω)
P
INDUCTOR
(2)
2
(0.1) = 0.4W
Only a portion of the temperature rise in the external
inductor and diode is coupled to the junction of the
LT3430. Based on empirical measurements, the thermal
effect on the LT3430 junction temperature due to power
dissipation in the external inductor and catch diode can
be calculated as:
∆T
J
(LT3430) ≈ (P
DIODE
+ P
INDUCTOR
)(5°C/W)
Using the example calculations for LT3430 dissipation, the
LT3430 die temperature will be estimated as:
T
J
= T
A
+ (θ
JA
• P
TOT
) + [5 • (P
DIODE
+ P
INDUCTOR
)]
With the TSSOP package (θ
JA
= 45°C/W), at an ambient
temperature of 50°C:
T
J
= 50 + (45 • 0.92) + (5 • 1.31) = 98°C
Die temperature can peak for certain combinations of V
IN
,
V
OUT
and load current. While higher V
IN
gives greater
switch AC losses, quiescent and catch diode losses, a
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