LT3083IDFPBF View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LT3083IDFPBF' PDF : 28 Pages View PDF
LT3083
APPLICATIONS INFORMATION
The power in the drive circuit equals:
PDRIVE = (VCONTROL – VOUT)(ICONTROL)
where ICONTROL is equal to IOUT/60. ICONTROL is a function
of output current. A curve of ICONTROL vs IOUT can be found
in the Typical Performance Characteristics curves.
The total power equals:
PTOTAL = PDRIVE + POUTPUT
The current delivered to the SET pin is negligible and can
be ignored.
VCONTROL(MAX_CONTINUOUS) = 3.630V (3.3V + 10%)
VIN(MAX_CONTINUOUS) = 1.575V (1.5V + 5%)
VOUT = 0.9V, IOUT = 3A, TA = 50°C
Power dissipation under these conditions is equal to:
PDRIVE = (VCONTROL – VOUT)(ICONTROL)
ICONTROL
= IOUT
60
=
3A
60
= 50mA
PDRIVE = (3.630V – 0.9V)(50mA) = 137mW
POUTPUT = (VIN – VOUT)(IOUT)
POUTPUT = (1.575V – 0.9V)(3A) = 2.03W
Total Power Dissipation = 2.16W
Junction Temperature will be equal to:
TJ = TA + PTOTAL • θJA (using tables)
TJ = 50°C + 2.16W • 16°C/W = 84.6°C
In this case, the junction temperature is below the maxi-
mum rating, ensuring reliable operation.
Reducing Power Dissipation
In some applications it may be necessary to reduce the
power dissipation in the LT3083 package without sacrific-
ing output current capability. Two techniques are available.
The first technique, illustrated in Figure 7, employs a
resistor in series with the regulator’s input. The voltage
drop across RS decreases the LT3083’s input-to-output
differential voltage and correspondingly decreases the
LT3083’s power dissipation.
As an example, assume: VIN = VCONTROL = 5V, VOUT = 3.3V
and IOUT(MAX) = 2A. Use the formulas from the Calculating
Junction Temperature section previously discussed.
Without series resistor RS, power dissipation in the LT3083
equals:
PTOTAL
=
(5V
−
3.3V)
•
2A
60
+
(5V
−
3.3V)
•
2A
= 3.46W
If the voltage differential (VDIFF) across the NPN pass
transistor is chosen as 0.5V, then RS equals:
RS
=
5V
−
3.3V
2A
−
0.5V
=
0.6Ω
Power dissipation in the LT3083 now equals:
PTOTAL
=
(5V
−
3.3V)
•
2A
60
+
0.5V
•
2A
=
1.06W
The LT3083’s power dissipation is now only 30% compared
to no series resistor. RS dissipates 2.4W of power. Choose
appropriate wattage resistors or use multiple resistors in
parallel to handle and dissipate the power properly.
C1
VCONTROL
LT3083
VIN
RS
IN
VIN′
+
–
SET
RSET
OUT
3083 F07
VOUT
C2
Figure 7. Reducing Power Dissipation Using a Series Resistor
3083fa
16
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