Datasheet

LT6600-10
12
66001fe
APPLICATIONS INFORMATION
Power Dissipation
The LT6600-10 amplifi ers combine high speed with large-
signal currents in a small package. There is a need to
ensure that the dies’s junction temperature does not exceed
150°C. The LT6600-10 S8 package has Pin 6 fused to the
lead frame to enhance thermal conduction when connect-
ing to a ground plane or a large metal trace. Metal trace
and plated through-holes can be used to spread the heat
generated by the device to the backside of the PC board.
For example, on a 3/32" FR-4 board with 2oz copper, a
total of 660 square millimeters connected to Pin 6 of the
LT6600-10 S8 (330 square millimeters on each side of the
PC board) will result in a thermal resistance, θ
JA
,of about
85°C/W. Without the extra metal trace connected to the
V
–
pin to provide a heat sink, the thermal resistance will
be around 105°C/W. Table 2 can be used as a guide when
considering thermal resistance.
Table 2. LT6600-10 SO-8 Package Thermal Resistance
COPPER AREA
TOPSIDE
(mm
2
)
BACKSIDE
(mm
2
)
BOARD AREA
(mm
2
)
THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT)
1100 1100 2500 65°C/W
330 330 2500 85°C/W
35 35 2500 95°C/W
35 0 2500 100°C/W
0 0 2500 105°C/W
Junction temperature, T
J
, is calculated from the ambient
temperature, T
A
, and power dissipation, P
D
. The power
dissipation is the product of supply voltage, V
S
, and
supply current, I
S
. Therefore, the junction temperature
is given by:
T
J
= T
A
+ (P
D
• θ
JA
) = T
A
+ (V
S
• I
S
• θ
JA
)
where the supply current, I
S
, is a function of signal level, load
impedance, temperature and common mode voltages.
For a given supply voltage, the worst-case power dis-
sipation occurs when the differential input signal is
maximum, the common mode currents are maximum
(see the
Applications Information section regarding com-
mon mode DC currents), the load impedance is small and
the ambient temperature is maximum. To compute the
junction temperature, measure the supply current under
these worst-case conditions, estimate the thermal resis-
tance from Table 2, then apply the equation for T
J
. For
example, using the circuit in Figure 3 with DC differential
input voltage of 250mV, a differential output voltage of 1V,
no load resistance and an ambient temperature of 85°C,
the supply current (current into V
+
) measures 48.9mA.
Assuming a PC board layout with a 35mm
2
copper trace,
the θ
JA
is 100°C/W. The resulting junction temperature is:
T
J
= T
A
+ (P
D
• θ
JA
) = 85 + (5 • 0.0489 • 100) = 109°C
When using higher supply voltages or when driving small
impedances, more copper may be necessary to keep T
J
below 150°C.
Figure 8
FREQUENCY (MHz)
0.1
SPECTRAL DENSITY (nV
RMS
/√Hz)
INTEGRATED NOISE (mV
RMS
)
35
30
25
20
15
10
5
0
140
120
100
80
60
40
20
0
1.0 10 100
6600 F08
SPECTRAL DENSITY
INTEGRATED
NOISE