Using standard circuits and no auxiliary voltage generators (such as charge pumps or inductive dc-dc converters), it's difficult to build a precision, rail-to-rail ramp generator that operates on a single supply and resets to a well-defined level. Figure 1 implements such a circuit using a bootstrapped series reference and an op amp with rail-to-rail I/O and very low bias current.

The ramp is generated by a constant charging current into capacitor C_RAMP. This is connected between ground and the noninverting input of op-amp IC1, configured as a voltage follower. The current through R_RAMP is the charging current, kept constant by forcing the voltage across R_RAMP to equal the reference voltage from IC1. One side of R_RAMP connects to C_RAMP, and the other joins the reference output. In turn, the reference IC's ground terminal links to the op-amp output, which provides a low-impedance replica of the voltage across C_RAMP.

Thus, the op-amp output follows the C_RAMP voltage and drives the GND pin of the IC2 reference, keeping the voltage across R_RAMP equal to V_REF. A 1-µF capacitor from the op-amp output bootstraps IC1's supply-voltage input. This drives it above the nominal level, yet it's still kept within that device's operating range. This lets the op-amp output reach its own supply-rail voltage.

A MOSFET switch across the ramp capacitor returns the ramp output to 0 V when RAMP_DISABLE goes high, allowing the ramp to develop when RAMP_DISABLE is low. A scope shot of the ramp shows the excursion limits for a supply voltage of 5.00 V . The ramp slope is:

(dV/dt) = V_REF/(C_RAMP × R_RAMP)

For a precise, linear ramp, C_RAMP should be a high-quality capacitor with low leakage current and a low voltage coefficient. The op amp must be capable of rail-to-rail input and output operation. And, the 1.25-V reference IC should have a low quiescent supply current.

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