Using negative voltage charge pumps and analog switches to build DirectDrive video filter amplifiers in audio/video applications

introduction

The MAX9503/MAX9505 DirectDrive video filter amplifiers integrate an analog switch (MAX9505) and a negative voltage charge pump. These devices operate from a single 2.7V to 3.6V supply and can be connected directly to the output of a video DAC, where they place the black level of the video signal at ground potential. These devices eliminate the need for additional negative supplies and large output DC-blocking capacitors, thereby reducing system cost and saving board area. As can be seen from the following discussion, negative voltage charge pumps and analog switches can improve audio/video system performance.

Using a negative voltage charge pump to adjust the sync header level

When the MAX9503/MAX9505 input sync tip voltage ranges from 0 to 50mV, the output black level is between -0.1V and 0.1V. In some applications, the synchronization head voltage of the DAC output will exceed 50mV. In this case, the black-level voltage at the MAX9503/5 output may exceed 100mV. If the output black level voltage needs to be equal to the ground potential, the circuit shown in Figure 1 can be used to adjust the input voltage to adjust the output black level. Assuming a 150Ω DAC load, connect a 13kΩ resistor between the input and the CPV _SS (-3.3V) of the negative charge pump. The resistor divider network shifts the input voltage of the filter amplifier down by about 40mV _P-P . Using this resistor divider network, the noise ripple at the input of the filter amplifier is less than 0.5mV and can be ignored.

Figure 1. Changing the output black level by adjusting the input voltage of the MAX9503/MAX9505