LM7131 datasheet(16/23 Pages) NSC

16 / 23 page

Using the LM7131 (Continued)

Output Short Circuits

The LM7131 has output short circuit protection, however, it is

not designed to withstand continuous short circuits, very fast

high energy transient voltage or current spikes, or shorts to

any voltage beyond the power supply rails. Designs should

reduce the number and energy level of any possible output

shorts, especially when used with ±5V supplies.

A resistor in series with the output, such as the 75

Ω resistor

used to back terminate 75

Ω cables, will reduce the effects of

shorts. For outputs which will send signals off the PC board

additional protection devices, such as diodes to the power

rails, zener-type surge suppressors, and varistors may be

useful.

Thermal Management

Note that the SOT23-5 (Tiny) package has less power dissi-

pation capability (325˚/W) than the S0-8 package (115˚/W).

This may cause overheating with ±5 supplies and heavy

loads at high ambient temps. This is less of a problem when

using +5V single supplies.

Example:

Driving a 150

Ω load to 2.0V at a 40˚C (104 ˚F) ambient tem-

perature. (This is common external maximum temperature

for office environments. Temperatures inside equipment may

be higher.)

No load power-

No load LM7131 supply current - 9.0 mA

Supply voltage is 5.0V

No load LM7131 power - 9.0 mA x 5.0V = 45 mW

Power with load-

Current out is 2.0V/150

Ω = 13.33 mA

Voltage drop in LM7131 is 5.0V (supply) − 2.0V (output) =

3.0V

Power dissipation 13.33 mA x 3.0V = 40 mW

Total Power = 45 mW+40mW = 85 mW = 0.085

Temperature Rise = 0.085 W x 325˚/W = 27.625 degrees

Junction temperature at 40˚ ambient = 40 + 27.625 =

67.6225˚.

This device is within the 0˚ to 70˚ specification limits.

The 325˚/W value is based on still air and the pc board land

pattern shown in this datasheet. Actual power dissipation is

sensitive to PC board connections and airflow.

SOT23-5 power dissipation may be increased by airflow or

by increasing the metal connected to the pads, especially

the center pin (pin number 2, V−) on the left side of the

SOT23-5. This pin forms the mounting paddle for the die in-

side the SOT23-5, and can be used to conduct heat away

from the die. The land pad for pin 2 can be made larger

and/or connected to power planes in a multilayer board.

Additionally, it should be noted that difficulty in meeting per-

formance specifications for the LM7131 is most common at

cold temperatures. While excessively high junction tempera-

tures will degrade LM7131 performance, testing has con-

firmed that most specifications are met at a junction tem-

perature of 85˚C.

See “Understanding Integrated Circuit Package Power Ca-

pabilities”, Application Note AN-336, which may be found in

the appendix of the Operational Amplifier Databook.

Layout and Power Supply Bypassing

Since the LM7131 is a high speed (over 50 MHz) device,

good high speed circuit layout practices should be followed.

This should include the use of ground planes, adequate

power supply bypassing, removing metal from around the in-

put pins to reduce capacitance, and careful routing of the

output signal lines to keep them away from the input pins.

The power supply pins should be bypassed on both the

negative and positive supply inputs with capacitors placed

close to the pins. Surface mount capacitors should be used

for best performance, and should be placed as close to the

pins as possible. It is generally advisable to use two capaci-

tors at each supply voltage pin. A small surface mount ca-

pacitor with a value of around 0.01 microfarad (10 nF), usu-

ally a ceramic type with good RF performance, should be

placed closest to the pin. A larger capacitor, in usually in the

range of 1.0 µF to 4.7 µF, should also be placed near the pin.

The larger capacitor should be a device with good RF char-

acteristics and low ESR (equivalent series resistance) for

best results. Ceramic and tantalum capacitors generally

work well as the larger capacitor.

For single supply operation, if continuous low impedance

ground planes are available, it may be possible to use by-

pass capacitors between the +5V supply and ground only,

and reduce or eliminate the bypass capacitors on the V− pin.

Capacitive Load Driving

The phase margin of the LM7131 is reduced by driving large

capacitive loads. This can result in ringing and slower set-

tling of pulse signals. This ringing can be reduced by placing

a small value resistor (typically in the range of 22

Ω–100Ω)

between the LM7131 output and the load. This resistor

should be placed as close as practical to the LM7131 output.

When driving cables, a resistor with the same value as the

characteristic impedance of the cable may be used to isolate

the cable capacitance from the output. This resistor will re-

duce reflections on the cable.

Input Current

The LM7131 has typical input bias currents in the 15 µA to

25 µA range. This will not present a problem with the low in-

put impedances frequently used in high frequency and video

circuits. For a typical 75

Ω input termination, 20 µA of input

current will produce a voltage across the termination resistor

of only 1.5 mV. An input impedance of 10 k

Ω, however,

would produce a voltage of 200 mV, which may be large

compared to the signal of interest. Using lower input imped-

ances is recommended to reduce this error source.

Gain of +2

DS012313-20

FIGURE 12.

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