9

UNUSED BUFFERS

It is recommended that any unused buffers have their inputs

tied to the ground plane.

DRIVING CAPACITIVE LOADS

The buffers can drive a wide range of capacitive loads. As

load capacitance increases, however, the -3dB bandwidth of

the device will decrease and the peaking increase. The

buffers drive 10pF loads in parallel with 10k

Ω with just 1.5dB

of peaking, and 100pF with 6.4dB of peaking. If less peaking

is desired in these applications, a small series resistor

(usually between 5

Ω and 50Ω) can be placed in series with

the output. However, this will obviously reduce the gain

slightly. Another method of reducing peaking is to add a

snubber circuit at the output. A snubber is a shunt load

consisting of a resistor in series with a capacitor. Values of

150

Ω and 10nF are typical. The advantage of a snubber is

that it does not draw any DC load current or reduce the gain.

back plate of an LCD display. This plate is capacitively

coupled to the pixel drive voltage which alternately cycles

positive and negative at the line rate for the display. Thus the

amplifier must be capable of sourcing and sinking capacitive

pulses of current, which can occasionally be quite large (a

few 100mA for typical applications).

as illustrated in Figure 23. Here, a voltage, corresponding to

the mid-DAC potential, is generated by a resistive divider

and buffered by the amplifier. The amplifier's stability is

designed to be dominated by the load capacitance, thus for

very short duration pulses (< 1µs) the output capacitor

supplies the current. By virtue of its high transconductance

which progressively increases as more current is drawn, it

can maintain regulation within 5mV as currents up to 100mA

are drawn, while consuming only 2mA of quiescent current.

Alternatively, the back plate potential can be generated by a

voltage, with gain if necessary. This is shown in Figure 24. In

this case, the effective transconductance of the feedback is

reduced, thus the amplifier will be more stable, but regulation

will be degraded by the feedback factor.

CHOICE OF OUTPUT CAPACITOR

A 1µF ceramic capacitor with low ESR is recommended for

this amplifier. (For example, GRM42_ 6X7R105K16). This

capacitor determines the stability of the amplifier. Reducing it

will make the amplifier less stable, and should be avoided.

With a 1µF capacitor, the unity gain bandwidth of the

amplifier is close to 1MHz when reasonable currents are

being drawn. (For lower load currents, the gain and hence

bandwidth progressively decreases.) This means the active

trans-conductance is:

This high transconductance indicates why it is important to

have a low ESR capacitor.

If:

then the capacitor will not force the gain to roll off below

unity, and subsequent poles can affect stability. The

recommended capacitor has an ESR of 10m

Ω, but to this

must be added the resistance of the board trace between the

capacitor and the sense connection - therefore this should

be kept short, as illustrated in Figure 21, by the diagonal line

to the capacitor. Also ground resistance between the

These constraints should be considered when laying out the

PCB.

1V

1V

10µs

FIGURE 22. OPERATION WITH BEYOND-THE-RAILS INPUT

-

+

1µF CERAMIC

LOW ESR

IPCOM

INCOM

-

+

1µF CERAMIC

LOW ESR

FROM DAC

2

π 1µF1MHz

×

×

6.28S

=

ESR

6.28

1

>

×

EL5224, EL5324, EL5424