LT1229 Linear Technology, LT1229 Datasheet - Page 8

• Manufacturer Part Number: LT1229
• Description: Dual and Quad 100MHz Current Feedback Amplifiers
• Manufacturer: Linear Technology

LT1229/LT1230
A
Capacitive Loads
The LT1229/LT1230 can drive capacitive loads directly
when the proper value of feedback resistor is used. The
graph Maximum Capacitive Load vs Feedback Resistor
should be used to select the appropriate value. The value
shown is for 5dB peaking when driving a 1k load at a gain
of 2. This is a worst case condition; the amplifier is more
stable at higher gains and driving heavier loads. Alterna-
tively, a small resistor (10 to 20 ) can be put in series
with the output to isolate the capacitive load from the
amplifier output. This has the advantage that the amplifier
bandwidth is only reduced when the capacitive load is
present, and the disadvantage that the gain is a function of
the load resistance.
Power Supplies
The LT1229/LT1230 amplifiers will operate from single or
split supplies from 2V (4V total) to 15V (30V total). It is
not necessary to use equal value split supplies, however,
the offset voltage and inverting input bias current will
change. The offset voltage changes about 350 V per volt
of supply mismatch, the inverting bias current changes
about 2.5 A per volt of supply mismatch.
Power Dissipation
The LT1229/LT1230 amplifiers combine high speed and
large output current drive into very small packages. Be-
cause these amplifiers work over a very wide supply range,
it is possible to exceed the maximum junction temperature
under certain conditions. To ensure that the LT1229 and
LT1230 remain within their absolute maximum ratings,
we must calculate the worst case power dissipation,
define the maximum ambient temperature, select the
appropriate package and then calculate the maximum
junction temperature.
The worst case amplifier power dissipation is the total of
the quiescent current times the total power supply voltage
plus the power in the IC due to the load. The quiescent
supply current of the LT1229/LT1230 has a strong nega-
tive temperature coefficient. The supply current of each
8
PPLICATI
O
U
S
I FOR ATIO
U
W
U
amplifier at 150 C is less than 7mA and typically is only
4.5mA. The power in the IC due to the load is a function of
the output voltage, the supply voltage and load resistance.
The worst case occurs when the output voltage is at half
supply, if it can go that far, or its maximum value if it
cannot reach half supply.
For example, let’s calculate the worst case power dissipa-
tion in a video cable driver operating on 12V supplies that
delivers a maximum of 2V into 150 .
Now if that is the dual LT1229, the total power in the
package is twice that, or 0.602W. We now must calcu-
late how much the die temperature will rise above the
ambient. The total power dissipation times the thermal
resistance of the package gives the amount of tempera-
ture rise. For the above example, if we use the SO8
surface mount package, the thermal resistance is
150 C/W junction to ambient in still air.
The maximum junction temperature allowed in the plastic
package is 150 C. Therefore, the maximum ambient al-
lowed is the maximum junction temperature less the
temperature rise.
Note that this is less than the maximum of 70 C that is
specified in the absolute maximum data listing. If we must
use this package at the maximum ambient we must lower
the supply voltage or reduce the output swing.
As a guideline to help in the selection of the LT1229/
LT1230 the following table describes the maximum sup-
ply voltage that can be used with each part in cable driving
applications.
P
P
Temperature Rise = P
150 C/W = 90.3 C
Maximum Ambient = 150 C – 90.3 C = 59.7 C
d MAX
d MAX
2
2 12
0 168 0 133 0 301
V I
.
S S MAX
V
.
7
d (MAX)
mA
V
S
R
.
12
–
JA
V
V
W per Amp
= 0.602W
O MAX
–
2
V
150
V
O MAX
2
R
V
L