Datasheet

LT3692A
28
3692afc
For more information www.linear.com/3692A
applicaTions inForMaTion
as heat sources. After the operating conditions have been
determined, the individual power losses are calculated by:
Power
D1,2
= 1−
V
OUT
V
IN






•I
OUT
• V
FD
Power
IND1,2
= R
IND
•I
OUT2
Power
CH1,2
= 0.1•
V
OUT
V
IN
•I
OUT2
+ 2•10
–3
•V
IN
+
I
OUT
• V
OUT
• V
BOOST
40• V
IN
+
V
IN
•I
OUT
•F
SW
•10
−6
•
V
IN
3
+
I
OUT
0.3






where:
F
SW
= SwitchingFrequency in kHz
R
IND
= Inductor Resis tance
V
FD
= Catch Diode Forward Voltage Drop
V
BOOST
= SwitchBoost Voltage
For the LT3692A demo board (see Figure 19) using the
TSSOP package, the estimated junction temperature rise
above ambient temperature is found by:
T
RISETSSOP
≈ 10 • (Power
D1
+ Power
D2
) +
12.3 • (Power
IND1
+ Power
IND2
) + 17.5 •
Power
CH1
+ Power
CH2
( )
The estimated junction temperature rise above ambient
for the LT3692A QFN layout (see Figure 19) is:
T
RISEQFN
≈ 8.5 • (Power
D1
+ Power
D2
) +
13 • (Power
IND1
+ Power
IND2
) + 23•
Power
CH1
+ Power
CH2
( )
For example, the typical application circuits listed in Table 4
are used to calculate the individual power loss contribu-
tions in Table 5. Table 6 shows the estimated power loss
and junction temperature rise above ambient temperature.
Note that the larger TSSOP package demonstrates better
thermal per
formance than the compact QFN package on
the L
T3692A demo circuit boards. For LT3692A applica
-
tions that favor thermal performance, the TSSOP package
is the preferred package option.
Table 4. LT3692A Operating Conditions
V
IN
(V)
F
SW
(kHz)
V
OUT1
(V)
I
OUT1
(A)
V
OUT2
(V)
I
OUT2
(A)
30 300/600 1.2 2 3.3 2
12 1000 3.3 2.5 3.3 2.5
24 500 5 2 3.3 2
12 500 5 3 3.3 3
Table 5. LT3692A Power Loss Contributions
PD1
(W)
PD2
(W)
PL1
(W)
PL2
(W)
PCH1
(W)
PCH2
(W)
0.86 0.80 0.24 0.24 0.38 0.72
0.82 0.82 0.25 0.25 0.62 0.62
0.71 0.78 0.24 0.24 0.54 0.48
0.79 0.98 0.54 0.54 0.81 0.59
Table 6. Estimated System Power Loss and IC Temperature Rise
P
LOSS
(W) T
RISE
TSSOP (°C) T
RISE
QFN (°C)
CALCULATED MEASURED CALCULATED MEASURED CALCULATED MEASURED
3.2 3.2 41.9 44.9 45.8 49.1
3.4 3.5 44.3 48.8 49 54.8
3.0 3.1 38.5 35.3 42.2 42.3
4.2 4.7 55.4 52.0 61.2 63
The power loss and temperature rise equations provided
in the Thermal Considerations section serve as a good
starting point for estimating the junction temperature
rise. However, the LT3692A is a very versatile converter.
The combination of independent input voltages, output
voltages, output currents, switching frequencies, and
package selections for the LT3692A dictate that no power
loss estimation scheme can accommodate every possible
operating condition. As such, it is absolutely necessary to
evaluate a converter’s performance at the bench.
The power dissipation in the other power components such
as boost diodes, input and output capacitors, inductor
core loss, and trace resistances cause additional copper
heating and can further increase what the IC sees as am
-
bient temperature. See the LT1767 data sheet’s Thermal
Considerations section.