THS3115IPWPRG4 datasheet(15/35 Pages) TI1

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THS3112

THS3115

www.ti.com

SLOS385C – SEPTEMBER 2001 – REVISED SEPTEMBER 2010

Power-Down Reference Pin Operation

Printed-Circuit Board Layout Techniques for

Optimal Performance

In addition to the shutdown pin, the THS3115

features a reference pin (REF) which allows the user

Achieving optimum performance with high-frequency

to control the enable or disable power-down voltage

amplifiers such as the THS3115 and THS3112

levels applied to the SHUTDOWN pin. In most

requires careful attention to board layout parasitic and

split-supply

applications,

the

reference

external component types. Recommendations that

connected to ground. In either case, the user needs

optimize performance include:

to be aware of voltage-level thresholds that apply to

•

Minimize parasitic capacitance to any ac ground

the shutdown pin. Table 2 shows examples and

for all of the signal I/O pins. Parasitic capacitance

illustrate the relationship between the reference

on the output and input pins can cause instability.

voltage and the shutdown thresholds. In the table, the

To reduce unwanted capacitance, a window

threshold

levels

are

derived

the

following

around the signal I/O pins should be opened in all

equations:

of the ground and power planes around those

pins. Otherwise, ground and power planes should

SHUTDOWN

≤ REF + 0.8 V for enable

be unbroken elsewhere on the board.

SHUTDOWN

≥ REF + 2V for disable

•

Minimize the distance [0.25 inch, (6,4 mm)] from

Where the usable range at the REF pin is:

the power-supply pins to high-frequency 0.1-µF

and 100-pF decoupling capacitors. At the device

VCC– ≤ VREF ≤ (VCC+ – 4V)

pins, the ground and power plane layout should

The recommended mode of operation is to tie the

not be in close proximity to the signal I/O pins.

REF

midrail,

therefore

setting

the

Avoid

narrow

power

and

ground

traces

enable/disable thresholds to V(midrail) + 0.8 V and

minimize inductance between the pins and the

V(midrail) = 2 V, respectively.

decoupling

capacitors.

The

power-supply

connections should always be decoupled with

Table 2. Shutdown Threshold Voltage Levels

these

capacitors.

Larger

(6.8

µF

more)

tantalum decoupling capacitors, effective at lower

REFERENCE

frequencies, should also be used on the main

SUPPLY

ENABLE

DISABLE

supply pins. These capacitors may be placed

VOLTAGE (V)

LEVEL (V)

somewhat farther from the device and may be

±15, ±5

0.8

2.0

shared among several devices in the same area

±15

2.0

2.8

4.0

of the printed circuit board (PCB).

±15

–2.0

–1.2

•

Careful

selection

and

placement

external

±5

1.0

1.8

3.0

components

preserve

the

high-frequency

±5

–1.0

–0.2

1.0

performance of the THS3115 and THS3112.

+30

15.0

15.8

Resistors should be a very low reactance type.

Surface-mount resistors work best and allow a

+10

5.0

5.8

7.0

tighter overall layout. Again, keep the leads and

PCB trace length as short as possible. Never use

Note that if the REF pin is left unterminated, it floats

wirebound type resistors in a high-frequency

the

positive

rail

and

falls

outside

the

application. Because the output pin and inverting

recommended operating range given above VCC– ≤

input pins are the most sensitive to parasitic

VREF ≤ (VCC+ – 4V). As a result, it no longer serves as

capacitance, always position the feedback and

a reliable reference for the SHUTDOWN pin, and the

series output resistors, if any, as close as possible

enable/disable thresholds given above no longer

to the inverting input pins and output pins. Other

apply.

the

SHUTDOWN

also

left

network components, such as input termination

unterminated, it floats to the positive rail and the

resistors,

should

placed

the

device is disabled. If balanced, split supplies are used

gain-setting resistors. Even with a low parasitic

(±VCC) and the REF and SHUTDOWN pins are

capacitance that shunts the external resistors,

grounded, the device is enabled.

excessively

high

resistor

values

can

create

space

significant

time

constants

that

can

degrade

performance.

Good

axial

metal-film

space

surface-mount resistors have approximately 0.2

space

pF in shunt with the resistor. For resistor values

greater than 2.0 k

Ω, this parasitic capacitance can

add a pole and/or a zero that can affect circuit

operation.

Keep

resistor

values

low

possible,

consistent

with

load

driving

considerations.

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