LT3431 LINER [Linear Technology], LT3431 Datasheet - Page 19

• Manufacturer Part Number: LT3431
• Description: High Voltage, 3A,500kHz Step-Down Switching Regulator
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
Switch loss:
Boost current loss:
Quiescent current loss:
Example: with V
Total power dissipation in the IC is given by:
Thermal resistance for the LT3431 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
45 C/W.
To calculate die temperature, use the proper thermal
resistance number for the desired package and add in
worst-case ambient temperature:
P
P
P
SW
Q
BOOST
R
t
t
t
t
f = Switch frequency
P
T
P
P
P
EFF
r
f
Ir
SW
Q
J
TOT
BOOST
SW
= (V
= (V
= t
12 0 0015
= T
= Effective switch current/voltage overlap time
= (t
0 25 0 61 0 86
( . )( ) ( )
= Switch resistance ( 0.15) hot
If
= P
= 0.86W + 0.12W + 0.03W = 1.01W
( .
V
0 15 2 5
A
IN
IN
.
IN
= (I
R
r
+ (
/1.1)ns
/1.2)ns
( ) ( / )
5 2 36
SW
SW OUT
+ t
0 0015
12
OUT
2
V
.
f
12
JA
OUT
+ P
+ t
I
.
IN
)
2
/0.05)ns
• P
Ir
BOOST
V
2
= 12V, V
U
5 0 003
IN
V
+ t
TOT
( .
I
IN
OUT
2
If
(
.
V
V
101 10
)
)
0 12
OUT
+ P
OUT
/
.
U
36
W
OUT
)
•
Q
W
0 003
0 033
.
= 5V and I
.
9
t
EFF
)( / )( )( )(
W
1 2 2 12 500 10
( / )
W
1 2
JA
I
OUT
) will be about
OUT
= 2A
U
V
IN
•
f
3
)
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 LT3431
dissipation, the LT3431 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
calculation of the LT3431 die temperature. If a more
accurate die temperature is required, a measurement of
the SYNC pin resistance (to GND) can be used. The SYNC
pin resistance can be measured by forcing a voltage no
greater than 0.5V at the pin and monitoring the pin current
over temperature in an oven. This should be done with
minimal device power (low V
(V
ambient (oven) temperature.
OUT
C
V
P
R
P
T
T
P
P
J
J
= 0V)) in order to calibrate SYNC pin resistance with
DIODE
F
DIODE
INDUCTOR
INDUCTOR
IND
= T
= 50 + (45 • 1.01) + (5 • 1.01) = 101 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
diode can improve efficiency by several percent.
may generate greater losses due to switch DC
+ (
( )(
( . )( – )( )
V V
0 52 12 5 2
= (I
(2)
F
JA
2
LOAD
• P
(0.1) = 0.4W
IN
12
TOT
–
)
2
V
V
) + [5 • (P
(R
IN
OUT
IND
JA
)(
)
I
LOAD
= 45 C/W), at an ambient
0 61
DIODE
.
IN
)
W
and no switching
+ P
IN
INDUCTOR
gives greater
LT3431
sn3431 3431fs
IN
19
levels
)]
IN
,