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 CRAMP. This is connected between ground and the noninverting input of op-amp IC1, configured as a voltage follower. The current through RRAMP is the charging current, kept constant by forcing the voltage across RRAMP to equal the reference voltage from IC1. One side of RRAMP connects to CRAMP, 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 CRAMP.
Thus, the op-amp output follows the CRAMP voltage and drives the GND pin of the IC2 reference, keeping the voltage across RRAMP equal to VREF. 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) = VREF/(CRAMP × RRAMP)
For a precise, linear ramp, CRAMP 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.
