Forward Transmission

Gain (S12)

Current Limit

00922323

00922324

Application Hints

POWER SUPPLY DECOUPLING

The method of supply bypassing is not critical for stability of

the LM6121 series buffers. However, their high current out-

put combined with high slew rate can result in significant

voltage transients on the power supply lines if much induc-

tance is present. For example, a slew rate of 900 V/µs into a

50

Ω load produces a di/dt of 18 A/µs. Multiplying this by a

wiring inductance of 50 nH (which corresponds to approxi-

minimize this problem use high quality decoupling very close

to the device. Suggested values are a 0.1 µF ceramic in

parallel with one or two 2.2 µF tantalums. A ground plane is

recommended.

LOAD IMPEDANCE

The LM6121 is stable to any load when driven by a 50

Ω

source. As shown in the Overshoot vs Capacitive Load

graph, worst case is a purely capacitive load of about

1000 pF. Shunting the load capacitance with a resistor will

reduce overshoot.

SOURCE INDUCTANCE

Like any high frequency buffer, the LM6121 can oscillate at

high values of source inductance. The worst case condition

occurs at a purely capacitive load of 50 pF where up to

100 nH of source inductance can be tolerated. With a 50

Ω

load, this goes up to 200 nH. This sensitivity may be reduced

at the expense of a slight reduction in bandwidth by adding a

resistor in series with the buffer input. A 100

Ω resistor will

ensure stability with source inductances up to 400 nH with

any load.

OVERVOLTAGE PROTECTION

The LM6121 may be severely damaged or destroyed if the

Absolute Maximum Rating of 7V between input and output

pins is exceeded.

If the buffer’s input-to-output differential voltage is allowed to

exceed 7V, a base-emitter junction will be in reverse-

breakdown, and will be in series with a forward-biased base-

emitter junction. Referring to the LM6121 simplified sche-

matic, the transistors involved are Q1 and Q3 for positive

inputs, and Q2 and Q4 for negative inputs. If any current is

allowed to flow through these junctions, localized heating of

the reverse-biased junction will occur, potentially causing

damage. The effect of the damage is typically increased

offset voltage, increased bias current, and/or degraded AC

performance. Furthermore, this will defeat the short-circuit

input with a shorted output will destroy the device.

The device is best protected by the insertion of the parallel

combination of a 100 k

Ω resistor (R1) and a small capacitor

(C1) in series with the buffer input, and a 100 k

Ω resistor

(R2) from input to output of the buffer (see Figure 1). This

network normally has no effect on the buffer output. How-

ever, if the buffer’s current limit or shutdown is activated, and

the output has a ground-referred load of significantly less

than 100 k

Ω, a large input-to-output voltage may be present.

R1 and R2 then form a voltage divider, keeping the input-

output differential below the 7V Maximum Rating for input

voltages up to 14V. This protection network should be suffi-

cient to protect the LM6121 from the output of nearly any op

HEATSINK REQUIREMENTS

A heatsink may be required with the LM6321 depending on

the maximum power dissipation and maximum ambient tem-

perature of the application. Under all possible operating

conditions, the junction temperature must be within the

range specified under Absolute Maximum Ratings.

To determine if a heatsink is required, the maximum power

dissipated by the buffer, P(max), must be calculated. The

formula for calculating the maximum allowable power dissi-

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FIGURE 1. LM6121 with Overvoltage Protection

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