THS3115IPWPRG4 datasheet(16/35 Pages) TI1

16 / 35 page

DIE

Thermal

Pad

(a) SideView

(b) EndView

THS3112

THS3115

SLOS385C – SEPTEMBER 2001 – REVISED SEPTEMBER 2010

www.ti.com

•

Connections to other wideband devices on the

•

Socketing a high-speed device such as the

board may be made with short direct traces or

THS3115 and THS3112 is not recommended. The

through onboard transmission lines. For short

additional lead length and pin-to-pin capacitance

connections, consider the trace and the input to

introduced by the socket can create an extremely

the next device as a lumped capacitive load.

troublesome parasitic network which can make it

Relatively wide traces [0.05 inch (1,3 mm) to 0.1

almost impossible to achieve a smooth, stable

inch (2,54 mm)] should be used, preferably with

frequency response. Best results are obtained by

ground and power planes opened up around

soldering

the

THS3115/THS3112

amplifiers

them. Estimate the total capacitive load and

directly onto the board.

determine if isolation resistors on the outputs are

necessary. Low parasitic capacitive loads (less

PowerPAD™ Design Considerations

than 4 pF) may not need an RS because the

The THS3115 and THS3112 are available in a

THS3115

and

THS3112

are

nominally

thermally-enhanced PowerPAD family of packages.

compensated to operate with a 2-pF parasitic

These packages are constructed using a downset

load. Higher parasitic capacitive loads without an

leadframe upon which the die is mounted [see

RS are allowed as the signal gain increases (thus

Figure 50(a) and Figure 50(b)]. This arrangement

increasing the unloaded phase margin). If a long

results in the lead frame being exposed as a thermal

trace is required, and the 6-dB signal loss intrinsic

pad

the

underside

the

package

[see

doubly-terminated

transmission

line

Figure 50(c)]. Because this thermal pad has direct

acceptable,

implement

matched-impedance

thermal contact with the die, excellent thermal

transmission line using microstrip or stripline

performance can be achieved by providing a good

techniques (consult an ECL design handbook for

thermal path away from the thermal pad. Note that

microstrip and stripline layout techniques). A 50-

devices such as the THS311x have no electrical

environment is not necessary onboard, and in

connection between the PowerPAD and the die.

fact, a higher impedance environment improves

distortion as shown in the distortion versus load

plots. With a characteristic board trace impedance

based on board material and trace dimensions, a

matching series resistor into the trace from the

output of the THS3115/THS3112 is used as well

as a terminating shunt resistor at the input of the

destination

device.

Remember

also

that

the

terminating impedance is the parallel combination

of the shunt resistor and the input impedance of

Figure 50. Views of Thermally-Enhanced Package

the

destination

device:

this

total

effective

impedance should be set to match the trace

impedance.

the

6-dB

attenuation

The PowerPAD package allows for both assembly

doubly-terminated

transmission

line

and thermal management in one manufacturing

unacceptable,

long

trace

can

operation. During the surface-mount solder operation

series-terminated at the source end only. Treat

(when the leads are being soldered), the thermal pad

the trace as a capacitive load in this case. This

can also be soldered to a copper area underneath the

configuration does not preserve signal integrity as

package. Through the use of thermal paths within this

well as a doubly-terminated line. If the input

copper area, heat can be conducted away from the

impedance of the destination device is low, there

package into either a ground plane or other heat

is some signal attenuation as a result of the

dissipating device.

voltage divider formed by the series output into

The PowerPAD package represents a breakthrough

the terminating impedance.

in combining the small area and ease of assembly of

surface

mount

with

the,

heretofore,

awkward

mechanical methods of heatsinking.

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