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i A PHYSICAL IMPLEMENTATION WITH CUSTOM LOW POWER ...

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7.2.1 Ramp<br />

Generator<br />

The ramp generator has been<br />

implemented based on the description given in patent number US<br />

6,650,153 B2 [23]. The schematic of the ramp generator is shown in Figure 7-13. The ramp<br />

generator design consists of a differential pair powered by the voltage Vpp, generated using a<br />

charge pump. The differential pair acts as an OPAMP with a high gain which forces the inputs<br />

IN+ and IN- to be virtually at the same potential. The IN+ terminal of the OPAMP is connected<br />

to Vref. Vref used in the design is 1.33V (bandgap reference voltage). The IN- terminal of the<br />

OPAMP is connected to the feedback node. Feedback ensures that the voltage at the feedback<br />

node stays close to 1.33V. The presence of a constant current source forces a constant current<br />

through the ramp capacitor C.<br />

The current through the capacitor and the voltage across the capacitor are related as<br />

shown in Equation 7-3. The current<br />

source is constant and so the voltage across the capacitor<br />

ramps with<br />

the slope of I/C as shown in Equation 7-4. The slope can be varied by varying C or I.<br />

I = C<br />

dV<br />

dT<br />

Equation 7-3: Capacitor Current Equation<br />

I dt<br />

V = ∫ =<br />

C<br />

It<br />

C<br />

Equation<br />

7-4: Capacitor Voltage Ramp<br />

129

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