LT3430 Linear Technology, LT3430 Datasheet - Page 19

• Manufacturer Part Number: LT3430
• Description: High Voltage/ 3A/ 200kHz Step-Down Switching Regulator
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
THERMAL CALCULATIONS
Power dissipation in the LT3430 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, so they
should not be used for calculating efficiency at light load
currents.
Switch loss:
Boost current loss:
Quiescent current loss:
Example: with V
Total power dissipation in the IC is given by:
P
P
P
SW
Q
BOOST
R
t
t
t
t
f = Switch frequency
P
P
P
P
EFF
r
f
Ir
SW
Q
TOT
BOOST
SW
= (V
= (V
= t
40 0 0015
= Effective switch current/voltage overlap time
= (t
0 08 1 2 1 28
= Switch resistance ( 0.15) hot
If
( .
= P
= 1.28W + 0.04W + 0.08W = 1.4W
V
0 15 2
.
IN
IN
IN
.
= (I
R
r
/1.1)ns
/1.2)ns
5
SW
SW OUT
+ t
0 0015
40
OUT
V
2
.
f
OUT
40
+ P
.
I
+ t
2 36
)
IN
2
/0.05)ns
/
V
Ir
BOOST
2
= 40V, V
5
5 0 003
U
IN
V
+ t
( .
I
2
IN
OUT
.
If
V
V
)
150 10
0 04
OUT
OUT
W
+ P
/
U
.
36
)
OUT
•
Q
W
0 003
0 08
.
= 5V and I
.
t
9
EFF
W
W
1 2 2 40 200 10
( / )
/
1 2
I
OUT
OUT
= 2A:
U
V
IN
•
f
3
Thermal resistance for the LT3430 package is influenced
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:
When estimating ambient, remember the nearby catch
diode and inductor will also be dissipating power:
Notice that the catch diode’s forward voltage contributes
a significant loss in the overall system efficiency. A larger,
lower V
Typical thermal resistance of the board is 5 C/W. Taking
the catch diode and inductor power dissipation into ac-
count and using the example calculations for LT3430
dissipation, the LT3430 die temperature will be estimated
as:
With the TSSOP package (
temperature of 50 C:
Die temperature can peak for certain combinations of V
V
switch AC losses, quiescent and catch diode losses, a
lower V
losses. In general, the maximum and minimum V
should be checked with maximum typical load current for
OUT
T
V
P
L
P
T
T
P
P
J
DCR
J
J
DIODE
DIODE
F
INDUCTOR
INDUCTOR
= T
= T
= 50 + (45 • 1.4) + (5 • 1.11) = 119 C
= Forward voltage of diode (assume 0.52V at 2A)
and load current. While higher V
F
IN
= Inductor DC resistance (assume 0.1 )
A
A
diode can improve efficiency by several percent.
may generate greater losses due to switch DC
+ (
+ (
( )(
( . )(
V V
0 52 40 5 2
= (I
(2)(0.1) = 0.2W
JA
F
JA
• P
LOAD
• P
IN
40
TOT
TOT
–
)(L
– )( )
V
V
)
) + [10 • (P
IN
OUT
DCR
JA
)(
)
I
LOAD
= 45 C/W), at an ambient
0 91
.
DIODE
)
W
IN
+ P
gives greater
INDUCTOR
LT3430
sn3430 3430is
IN
19
levels
)]
IN
,