LT6301 datasheet(11/16 Pages) LINER

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LT6301

sn6301 6301f

APPLICATIO S I FOR ATIO

Heat Sinking Using PCB Metal

Designing a thermal management system is often a trial

and error process as it is never certain how effective it is

until it is manufactured and evaluated. As a general rule,

the more copper area of a PCB used for spreading heat

away from the driver package, the more the operating

junction temperature of the driver will be reduced. The

limit to this approach however is the need for very com-

pact circuit layout to allow more ports to be implemented

on any given size PCB.

To best extract heat from the FE28 package, a generous

area of top layer PCB metal should be connected to the four

corner pins (Pins 1, 14, 15 and 28). These pins are fused

to the leadframe where the LT6301 die are attached. The

package also has an exposed metal heat sinking pad on the

bottom side which, when soldered to the PCB top layer

metal, directly conducts heat away from the IC junction.

Soldering the thermal pad to the board produces a thermal

resistance from junction to case,

θJC, of approximately

°C/W.

Important Note: The metal planes used for heat sinking

the LT6301 are electrically connected to the negative

supply potential of the driver, typically –12V. These

planes must be isolated from any other power planes

used in the board design.

Fortunately xDSL circuit boards use multiple layers of

metal for interconnection of components. Areas of metal

beneath the LT6301 connected together through several

small 13 mil vias can be effective in conducting heat away

from the driver package. The use of inner layer metal can

free up top and bottom layer PCB area for external compo-

nent placement.

When PCB cards containing multiple ports are inserted

into a rack in an enclosed cabinet, it is often necessary to

provide airflow through the cabinet and over the cards.

This is also very effective in reducing the junction-to-

ambient thermal resistance of each line driver. To a limit,

this thermal resistance can be reduced approximately

°C/W for every 100lfpm of laminar airflow.

Layout and Passive Components

With a gain bandwidth product of 200MHz the LT6301

requires attention to detail in order to extract maximum

performance. Use a ground plane, short lead lengths and

a combination of RF-quality supply bypass capacitors (i.e.,

0.1

µF). As the primary applications have high drive cur-

rent, use low ESR supply bypass capacitors (1

µF to 10µF).

The four V+ pins (Pins 18, 19, 24, 25) separately provide

power to each amplifier and should be shorted together

with leads as short as possible to the bypass capacitors.

The parallel combination of the feedback resistor and gain

setting resistor on the inverting input can combine with

the input capacitance to form a pole that can cause

frequency peaking. In general, use feedback resistors of

1k or less.

Compensation

The LT6301 is stable in a gain 10 or higher for any supply

and resistive load. It is easily compensated for lower gains

with a single resistor or a resistor plus a capacitor.

Figure 8 shows that for inverting gains, a resistor from the

inverting node to AC ground guarantees stability if the

parallel combination of RC and RG is less than or equal to

RF/9. For lowest distortion and DC output offset, a series

capacitor, CC, can be used to reduce the noise gain at

lower frequencies. The break frequency produced by RC

and CC should be less than 5MHz to minimize peaking.

Figure 9 shows compensation in the noninverting configu-

ration. The RC, CC network acts similarly to the inverting

case. The input impedance is not reduced because the

network is bootstrapped. This network can also be placed

between the inverting input and an AC ground.

Figure 8. Compensation for Inverting Gains

(OPTIONAL)

6301 F08

–RF

< 5MHz

πRCCC

(RC || RG) ≤ RF/9

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