BUF20800-Q1 Datasheet, PDF (16/32 Pages) Texas Instruments – 18-CHANNEL GAMMA VOLTAGE GENERATOR WITH TWO PROGRAMMABLE VCOM CHANNELS

BUF20800-Q1
SBOS571 – AUGUST 2011
www.ti.com
BUF20800-Q1
V_{COM OUT1}
V_{COM OUT2}
V_COM
OUT1
OUT2
OUT17
Gamma
References
OUT18
SDA
SCL
Control Interface
Figure 12. BUF20800-Q1 Used for Programmable V_COM
REFH AND REFL INPUT RANGE
Best performance and output swing range of the BUF20800-Q1 are achieved by applying REFH and REFL
voltages that are slightly below the power-supply voltages. Most specifications have been tested at REFH = V_s −
200mV and REFL = GND + 200mV. The REFH internal buffer is designed to swing very closely to V_s and the
REFL internal buffer to GND. However, there is a finite limit on how close they can swing before saturating. To
avoid saturation of the internal REFH and REFL buffers, the REFH voltage should not be greater than V_s
−100mV and REFL voltage should not be lower than GND + 100mV. Figure 13 shows the swing capability of the
REFH and REFL buffers.
The other consideration when trying to maximize the output swing capability of the gamma buffers is the
limitation in the swing range of output buffers (OUT1−18, V_COM1, and V_COM2), which depends on the load current.
A typical load in the LCD application is 5−10mA. For example, if OUT1 is sourcing 10mA, the swing is typically
limited to about V_s − 200mV. The same applies to OUT18, which typically limits at GND + 200mV when sinking
10mA. An increase in output swing can only be achieved for much lighter loads. For example, a 3mA load
typically allows the swing to be increased to approximately V_s − 100mV and GND + 100mV.
Connecting REFH directly to V_s and REFL directly to GND does not damage the BUF20800-Q1. As discussed
above however, the output stages of the REFH and REFL buffers will saturate. This condition is not desirable
and can result in a small error in the measured output voltages of OUT1−18, V_COM1, and V_COM2. As described
above, this method of connecting REFH and REL does not help to maximize the output swing capability.
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