Datasheet

LT8582
19
8582f
APPLICATIONS INFORMATION
Table 4 calculates the power dissipation of one
channel of the LT8582 for a particular boost
application (V
IN
= 5V, V
OUT
= 12V, I
OUT
= 0.8A, f
OSC
= 1.5MHz,
V
D
= 0.5V, V
CESAT
= 0.270V).
From P
TOTAL
in Table 4, die junction temperature can be
calculated using the appropriate thermal resistance number
and worst-case ambient temperature:
T
J
= T
A
+ θ
JA
• P
TOTAL
where T
J
= die junction temperature, T
A
= ambient tem-
perature and θ
JA
is the thermal resistance from the silicon
junction to the ambient air.
The published θ
JA
value is 34°C/W for the 7mm × 4mm
24-pin DFN package package. In practice, lower θ
JA
values
are realizable if board layout is performed with appropriate
grounding (accounting for heat sinking properties of the
board) and other considerations listed in the Board Layout
Guidelines section. For instance, a θ
JA
value of ~16°C/W
was consistently achieved for DFN packages of the LT8582
(at V
IN
= 5V, V
OUT
= 12V, I
OUT
= 0.8A, f
OSC
= 1.5MHz) when
board layout was optimized as per the suggestions in the
Board Layout Guidelines section.
Junction Temperature Measurement
The duty cycle of CLKOUT2 is linearly proportional to die
junction temperature (T
J
) near the CLKOUT2 pin. To get an
accurate reading, measure the duty cycle of the CLKOUT
signal and use the following equation to approximate the
junction temperature:
T
J
=
DC
CLKOUT
– 34.5%
0.3%
where DC
CLKOUT
is the CLKOUT duty cycle in % and T
J
is
the die junction temperature in °C. Although the absolute
die temperature can deviate from the above equation by
±10°C, the relationship between the CLKOUT duty cycle and
change in die temperature is well defined. A 3% increase
in CLKOUT duty cycle corresponds to ~10°C increase in
die temperature.
Note that the CLKOUT pin is only meant to drive capacitive
loads up to 120pF.
Thermal Lockout
When the die temperature exceeds 165°C (see Operation
Section), a fault condition occurs and the part goes into
thermal lockout. The fault condition ceases when the die
temperature drops to ~160°C (nominal).
Table 4. Calculations Example with V
IN
= 5V, V
OUT
= 12V, I
OUT
= 0.8A, f
OSC
= 1.5MHz, V
D
= 0.5V, V
CESAT
= 0.27V
DEFINITION OF VARIABLES EQUATION 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 – 0.270V
DC = 61.3%
I
IN
= Average Input Current
η = Power Conversion Efficiency
(typically 88% at high currents)
I
IN
=
V
OUT
•I
OUT
V
IN
• η
I
IN
=
12V •0.8A
5V •0.88
I
IN
= 2.18A
P
SW
= Switch I
2
R Loss
R
SW
= Switch Resistance (typically
95m combined SWA and SWB)
P
SW
= DC • I
IN
2
• R
SW
P
SW
= 0.613 • (2.18A)
2
• 95m P
SW
= 277mW
P
BAC
= Base Drive Loss (AC) P
BAC
= 13ns • I
IN
• V
OUT
• f
OSC
P
BAC
= 13ns • 2.18A • 12V • 1.5MHz P
BAC
= 511mW
P
BDC
= Base Drive Loss (DC)
P
BDC
=
V
IN
•I
IN
•DC
β
SW _at_I
IN
P
BDC
=
5V •2.18A •0.613
50
P
BDC
= 134mW
P
INP
= Chip Bias Loss P
INP
= 11mA • V
IN
P
INP
= 11mA • 5V P
INP
= 55mW
P
TOTAL
= 977mW
Note: These power calculations are for one channel of the LT8582. The power consumption of both channels should be taken into account when
calculating die temperature.