LT6550/LT6551
12
65501fa
Power Dissipation
The LT6550/LT6551, enhanced
θJA MS package, has
Pin 5 (VEE for the LT6550 and GND for the LT6551) fused
to the lead frame. This thermal connection increases the
efficiency of the PC board as a heat sink. The PCB material
can be very effective at transmitting heat between the pad
area attached to Pin 5 and a ground or power plane layer.
Copper board stiffeners and plated through holes can also
be used to spread the heat generated by the device. Table 1
lists the thermal resistance for several different board
sizes and copper areas. All measurements were taken on
3/32” FR-4 board with 2oz copper. This data can be used
as a rough guideline in estimating thermal resistance. The
thermal resistance for each application will be affected by
thermal interactions with other components as well as
board size and shape.
Table 1. Fused 10-Lead MSOP Package
COPPER AREA
TOPSIDE*
BACKSIDE
BOARD AREA
THERMAL RESISTANCE
(mm2)
(JUNTION-TO-AMBIENT)
540
2500
110
°C/W
100
2500
120
°C/W
100
0
2500
130
°C/W
30
0
2500
135
°C/W
0
2500
140
°C/W
*Device is mounted on topside.
As an example, calculate the junction temperature for the
circuit in Figure 2 assuming an 85
°Cambienttemperature.
The device dissipation can be found by measuring the
supply current, calculating the total dissipation and then
subtracting the dissipation in the load.
The dissipation for the amplifiers is:
PD = (106mA)(5V) – 4 (2.5V)2/150 = 363mW
The total package power dissipation is 363mW. When a
2500 sq mm PC board with 540 sq mm of 2oz copper on
top and bottom is used, the thermal resistance is
110
°C/W. The junction temperature (TJ) is:
TJ = (363mW)(110°C/W) + 85°C = 125°C
The maximum junction temperature for the LT6551 is
150
°C so the heat sinking capability of the board is
adequate for the application.
Figure 2. Calculating Junction Temperature
+
–
5V
1.25V
OA
450
450
+
–
OA
450
450
+
–
OA
450
450
+
–
GND
OA
450
450
75
75
75
75
75
75
75
75
LT6551
6551 F02
APPLICATIO S I FOR ATIO
WU
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