LT1229 Просмотр технического описания (PDF) - Linear Technology

Номер в каталоге

Компоненты Описание

производитель

LT1229

Dual and Quad 100MHz Current Feedback Amplifiers

Linear Technology 

LT1229/LT1230

APPLICATI S I FOR ATIO

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 (10Ω to 20Ω) 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 350µV per volt

of supply mismatch, the inverting bias current changes

about 2.5µA 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

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 150Ω.

( ) ( ) ( ) ( ) Pd MAX

= 2VS IS MAX

+





VS

–

VO

MAX





VO MAX

RL

( ) ( ) Pd

MAX

= 2 • 12V • 7mA +

12V – 2V

• 2V

150Ω

= 0.168 + 0.133 = 0.301W per Amp

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.

Temperature Rise = Pd (MAX) RθJA = 0.602W •

150°C/W = 90.3°C

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.

Maximum Ambient = 150°C – 90.3°C = 59.7°C

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.

9

Share Link: