lt3692a Linear Technology Corporation, lt3692a Datasheet - Page 27

• Manufacturer Part Number: lt3692a
• Description: Monolithic Dual Tracking 3.5a Step-down Switching Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
as heat sources. After the operating conditions have been
determined, the individual power losses are calculated by:
Where:
For the LT3692A demo board (see Figure 19) using the
TSSOP package, the estimated junction temperature rise
above ambient temperature is found by:
The estimated junction temperature rise above ambient
for the LT3692A QFN layout (see Figure 19) is:
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
Power
Power
Power
F
R
•V
T
12.3 • (Power
(
T
13 • (Power
(
V
V
V
SW
Power
Power
RISETSSOP
FD
BOOST
RISEQFN
IND
IN
IN
• I
= Catch Diode Forward Voltage Drop
= Switching Frequency in KHz
+
= Inductor Resis tance
OUT
D1,2
IND1,2
CH1,2
I
CH1
CH1
OUT
= Switch Boost Voltage
≈ 8.5 • (Power
• F
= 1−
+ Power
+ Power
≈ 10 • (Power
= 0.1•
• V
IND1
= R
SW
⎛
⎝ ⎜
IND1
40 • V
OUT
IND
+ Power
• 10
V
+ Power
V
OUT
• V
V
CH2
• I
IN
CH2
IN
V
−6
OUT
IN
OUT2
BOOST
⎞
⎠ ⎟
)
•
)
D1
IND2
• I
⎛
⎝ ⎜
• I
D1
V
+ Power
OUT
IND2
OUT2
3
IN
+ Power
) + 23 •
+
+
• V
) + 17.5 •
+ 2 • 10
I
OUT
0.3
FD
D2
D2
⎞
⎠ ⎟
) +
) +
–3
thermal performance than the compact QFN package on
the LT3692A demo circuit boards. For LT3692A applica-
tions that favor thermal performance, the TSSOP package
is the preferred package option.
Table 4: LT3692A Operating Conditions
Table 5: LT3692A Power Loss Contributions
Table 6: Estimated System Power Loss and IC Temperature Rise
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.
Calculated
PD1
0.86
0.82
0.71
0.79
(W)
V
(V)
3.2
3.4
3.0
4.2
30
12
24
12
IN
P
LOSS
(W)
Measured
300/600
(KHz)
1000
PD2
0.80
0.82
0.78
0.98
F
500
500
(W)
3.2
3.5
3.1
4.7
SW
Calculated
V
T
0.24
0.25
0.24
0.54
41.9
44.3
38.5
55.4
PL1
(W)
OUT1
(V)
1.2
3.3
RISE
5
5
TSSOP (°C)
Measured
I
0.24
0.25
0.24
0.54
44.9
48.8
35.3
52.0
OUT1
PL2
(W)
(A)
2.5
2
2
3
Calculated
LT3692A
V
PCH1
0.38
0.62
0.54
0.81
45.8
42.2
61.2
(W)
OUT2
(V)
3.3
3.3
3.3
3.3
T
49
RISE
QFN (°C)
Measured
27
PCH2
I
0.72
0.62
0.48
0.59
49.1
54.8
42.3
OUT2
(W)
(A)
2.5
63
2
2
3
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