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ISL59837 Datasheet(PDF) 12 Page - Renesas Technology Corp |
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ISL59837 Datasheet(HTML) 12 Page - Renesas Technology Corp |
12 / 14 page ISL59837 FN6335 Rev 1.00 Page 12 of 14 March 5, 2007 THE CHARGE PUMP The ISL59837 charge pump provides a bottom rail up to 1.6V below ground while operating on a 0V to 3.3V power supply. The charge pump is internally regulated to one-half the potential of the positive supply. This internal multi-phase charge pump is driven by a 110MHz differential ring oscillator driving a series of inverters and charge storage circuitry. Each series inverter charges and places parallel adjoining charge circuitry slightly out of phase with the immediately preceding block. This generates a negative rail of about -1.6V with a low amplitude ripple voltage from the charge pump action. Some of this ripple is coupled into the output signals at a very low amplitude, as seen in Figure 21. The ripple on the outputs is typically well below the noise floor of the signal. There are two ways to further reduce the output supply noise: • Add a 120 bead in series between VCC and DVCC. This reduces the coupling between the charge pump and the analog amplifier supplies. • Add a 20pF capacitor between the back load 75 resistor and ground (see “ISL59837 + DC-Restore Solution” on page 10). This will attenuate frequencies above 100MHz. The system operates at sufficiently high frequencies that any related charge pump noise is far beyond standard video bandwidth requirements. Still, appropriate bypassing discipline must be observed, and all pins related to either the power supply or the charge pump must be properly bypassed. See “Power Supply Bypassing and Printed Circuit Board Layout” on page 13. THE VREF PIN Applying a voltage to the VREF pin simply places that voltage on what would usually be the ground side of the gain resistor of the amplifier, resulting in a DC-level shift of the output signal. Applying 100mV to the VREF pin would apply a 100mV DC level shift to the outgoing signal. The charge pump provides sufficient bottom room to accommodate the shifted signal. The ISL59837 buffers the VREF voltage before applying it to the triple amplifiers, isolating the input from the amplifiers and allowing it to be driven by moderate-impedance voltage sources. THE VEE PIN The VEE pin is the output pin for the charge pump. A voltmeter applied to this pin will display the output of the charge pump. This pin does not affect the functionality of the part. One may use this pin as an additional voltage source. Keep in mind that the output of this pin is generated by the internal charge pump and a fully regulated supply that must be properly bypassed. We recommend a 0.1µF ceramic capacitor placed as close to the pin and connected to the ground plane of the board. INPUT, OUTPUT AND SUPPLY VOLTAGE RANGE The ISL59837 is designed to operate with a single supply voltage range of from 0V to 3.3V. The need for a split supply has been eliminated with the incorporation of a charge pump capable of generating a bottom rail as much as 1.6V below ground, for a 4.9V range on a single 3.3V supply. This performance is ideal for NTSC video with its negative-going sync pulses. VIDEO PERFORMANCE For good video performance, an amplifier is required to maintain the same output impedance and the same frequency and phase response as DC levels are changed at the output. This is especially difficult when driving a standard video load of 150 because of the change in output current with changing DC levels. Special circuitry has been incorporated into the ISL59837 for the reduction of output impedance variation with the current output. This results in outstanding differential gain and differential phase specifications of 0.06% and 0.1°, while driving 150 at a gain of +2. Driving higher impedance loads would result in similar or better differential gain and differential phase performance. NTSC The ISL59837 (generating a negative rail internally) is ideally suited for NTSC video with its accompanying negative-going sync signals, which is easily handled by the ISL59837 without the need for an additional supply as the ISL59837 generates a negative rail with an internal charge pump referenced at negative 1/2 the positive supply. YPbPr YPbPr signals originating from a DVD player requiring three channels of very tightly-controlled amplifier gain accuracy present no difficulty for the ISL59837. Specifically, this standard encodes sync on the Y-Channel and it is a negative-going signal, which is easily handled by the ISL59837 without the need for an additional supply as the ISL59837 generates a negative rail placed at negative 1/2 the positive supply. Additionally, the Pb and Pr are bipolar analog signals and the video signals are negative-going, and again, easily handled by the ISL59837. DRIVING CAPACITIVE LOADS AND CABLES The ISL59837 (internally-compensated to drive 75 cables) will drive 10pF loads in parallel with 1k with less than 5dB of peaking. If less peaking is required, a small series resistor, usually between 5 to 50, can be placed in series with the output. This will reduce peaking at the expense of a slight closed loop gain reduction. When used as a cable driver, double termination is always recommended for reflection-free performance. For those applications, a back-termination series resistor at the amplifier's output will isolate the amplifier from the cable and allow extensive capacitive drive. However, other applications may have high capacitive loads without a back-termination resistor. Again, a small series resistor at the output can help to reduce peaking. The ISL59837 is a triple amplifier designed to drive three channels; simply deal with each channel separately as described in this section. |
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