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EL5324 Datasheet(PDF) 9 Page - Intersil Corporation |
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EL5324 Datasheet(HTML) 9 Page - Intersil Corporation |
9 / 12 page 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. 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. 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. 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 capacitor and the base of R2 must be kept to a minimum. These constraints should be considered when laying out the PCB. 1V 1V 10µs VS=±2.5V TA=25°C VIN=6VP-P FIGURE 22. OPERATION WITH BEYOND-THE-RAILS INPUT - + R1 R2 VBOOST VCOM 1µF CERAMIC LOW ESR IPCOM INCOM VCOM VDDCOM FIGURE 23. VCOM USED AS A VOLTAGE BUFFER VSSCOM - + R1 R2 VBOOST VCOM 1µF CERAMIC LOW ESR FROM DAC FIGURE 24. VCOM USED AS A BUFFER WITH GAIN 2 π 1µF1MHz × × 6.28S = ESR 6.28 1 > × EL5224, EL5324, EL5424 |
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