The charge pump has many applications. While we generally think of power supply applications, it is also very useful in this voltage controlled oscillator. This VCO has a number of useful features including single positive power supply operation, positive VCO reference voltage, fast response, high linearity, temperature compensation and open collector output. Seldom are all these features present simultaneously. Though the component count is high compared to commercially available IC devices, cost is low because all components are garden variety. Central is the inexpensive LM339 quad single-supply comparator.
Charge Pump VCO Schematic
Simply put, the input current through resistors R1 & R11 is repetitively subtracted by the charge in C2. The input current reference charges C1 to the 0.3V threshold voltage of U1A. When comparator U1A switches, a little positive feedback through C5 results in a clean transition. When the output of U1A reaches 6V, the output of comparator U1B switches and quickly discharges (pumps or dumps) C2 into C1 via D1. This is the basic charge pump circuit. This reduces the voltage across C1 to about –0.3V (the limit of the common mode input range of U1). When the two comparators reset, C2 recharges to the clamp voltage set by D6.
R11 accomplishes the calibration –this is necessary because there are no precision components used in the circuit.
For good temperature compensation, C2 must be the stable NP0 type. Since the forward drop of D1 and D2 are temperature dependent, D4 & D5 are required for compensation –two diodes are required: one to compensate for the charge path through D2, and the other for the discharge path through D1. Note that temperature stability was not tested –there could be some surprises…
U1D is normally dormant. It prevents latch up in the event that the input node voltage ever exceeds 0.3V and a single discharge of C2 is insufficient to reduce it below the threshold of U1A. Latch-up may be demonstrated by teasing the power supply with full reference voltage.
U1C provides a convenient means of interface –either with or without a pull-up resistor. The output consists of short negative going pulses.
The circuit is scaled so that 10V input = 10kHZ output. Linearity is well within 1% except for the discontinuity at the low end as shown on the graph. R2 adds a little fixed bias to compensate for non-linearity on the low end. Re-scaling is easy, but remember to keep the ratios of C1 to C2 (10:1) the same in order to control the peak negative voltage at the input of U1A.
Graph of transfer function
Charge Pump Oscillator Applications
- A-D converter in a microcontroller that has a free counter available –I have actually done this one before.
- Signal isolator –in this case, the output is connected to an optical coupler.
- Variable rate LED flasher –greatly reduce the frequency or this one.
For the future
Another charge pump VCO with independently adjustable duty cycle
Simple Exar XR4151 VCO application