EL5224IRE-T7 Просмотр технического описания (PDF) - Intersil
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EL5224IRE-T7 Datasheet PDF : 12 Pages
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EL5224, EL5324, EL5424
1V
10µs
VS=±2.5V
TA=25°C
1V
VIN=6VP-P
FIGURE 22. OPERATION WITH BEYOND-THE-RAILS INPUT
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 10kΩ 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 5Ω 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
150Ω and 10nF are typical. The advantage of a snubber is
that it does not draw any DC load current or reduce the gain.
The Use of VCOM Amplifier
The VCOM amplifier is designed to control the voltage on the
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).
A simple use of the VCOM amplifier is as a voltage follower,
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. For longer pulses the VCOM amplifier
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.
VBOOST
R1
R2
IPCOM
INCOM
+
-
VDDCOM
VSSCOM
VCOM
VCOM
1µF CERAMIC
LOW ESR
FIGURE 23. VCOM USED AS A VOLTAGE BUFFER
Alternatively, the back plate potential can be generated by a
DAC and the VCOM amplifier used to buffer the DAC
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.
FROM DAC
VBOOST
+
-
R1
R2
VCOM
1µF CERAMIC
LOW ESR
FIGURE 24. VCOM USED AS A BUFFER WITH GAIN
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:
2π × 1µF × 1MHz = 6.28S
This high transconductance indicates why it is important to
have a low ESR capacitor.
If:
ESR × 6.28 > 1
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 10mΩ, 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
capacitor and the base of R2 must be kept to a minimum.
These constraints should be considered when laying out the
PCB.
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