The circuit shown (see figure) converts bidirectional currents to frequency. Its useful current range is from 10 mA to 1 A. Also, the output frequency is proportional to the input current.
The Maxim MAX471 (IC1) is a high-side current-sensing device. Its output current is proportional to the load current with a 1:2000 ratio. If the load current is 1 A, the output current is 0.5 mA. Note that the load current can be in either direction. The output current charges capacitor C1 to form a linear ramp-up voltage. The Maxim MAX952 (IC2) is a multifunction chip, containing a 1.2-V, 2% reference voltage generator, a comparator, and an operational amplifier.
The voltage on C1 is compared against the 1.2-V reference. Once the voltage on C1 exceeds the reference, the comparator outputs a logic high to charge up capacitor C2. The opamp in IC2 serves as another comparator, and its negative input is connected to the 1.2-V reference. Once the voltage on C2 is above 1.2 V, the op amp’s output becomes high. This high voltage will turn on Q1 to discharge capacitor C1.
The time constant of C2 and R2 guarantees that C1 will be totally discharged. The higher the load current, the faster the charge-up slope on C1, resulting in a higher frequency from the comparator’s output.
The table indicates the test results. The load voltage range is from 3 V to 36 V, and is measured at 5 V, 10 V, and 15 V.