Datasheet

LT3579/LT3579-1
19
35791fa
For more information www.linear.com/LT3579
Table 4. Boost Power Calculations Example with V
IN
= 5V, V
OUT
= 12V, I
OUT
= 1.5A, f
OSC
= 1MHz, V
D
= 0.5V, V
CESAT
= 0.185V
DEFINITION OF VARIABLES EQUATIONS DESIGN EXAMPLE VALUE
DC = Switch Duty Cycle
DC =
V
OUT
– V
IN
+ V
D
V
OUT
+ V
D
– V
CESAT
DC =
12V – 5V + 0.5V
12V + 0.5V –.185V
DC = 60.9%
I
IN
= Average Input Current
η = Power Conversion Efficiency
(typically 90% at high currents)
I
IN
=
V
OUT
• I
OUT
V
IN
• η
I
IN
=
12V • 1.5A
5V • 0.9
I
IN
= 4A
P
SW
= Switch I
2
R Loss
R
SW
= Switch Resistance (typically
45mΩ combined SW1 and SW2)
P
SW
= DC • I
IN
2
•R
SW
P
SW
= 0.609 • (4A)
2
• 45mΩ
P
SW
= 438mW
P
BAC
= Base Drive Loss (AC)
P
BAC
= 13ns• I
IN
• V
OUT
• f
OSC
P
BAC
= 13ns• 4A • 12V • 1MHz
P
BAC
= 624mW
P
BDC
= Base Drive Loss (DC)
P
BDC
=
V
IN
• I
IN
•DC
40
P
BDC
=
5V • 4A • 0.609
40
P
BDC
= 305mW
P
INP
= Input Power Loss
P
INP
= 14mA • V
IN
P
INP
= 14mA • 5V
P
INP
= 70mW
P
TOTAL
= 1.437W
APPLICATIONS INFORMATION
The following example calculates the power dissipation
in the LT3579 for a particular boost application:
(V
IN
= 5V, V
OUT
= 12V, I
OUT
= 1.5A, f
OSC
= 1MHz,
V
D
= 0.5V, V
CESAT
= 0.185V).
To calculate die junction temperature, use the appropriate
thermal resistance number and add in worst-case ambient
temperature:
T
J
= T
A
+ θ
JA
• P
TOTAL
where T
J
=Die Junction Temperature, T
A
=Ambient Tem-
perature, P
TOTAL
is the final result from the calculations
shown in Table 4, and θ
JA
is the thermal resistance from
the silicon junction to the ambient air.