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LT2940 28V Power to Frequency Converter

28V Power to Frequency Converter

In some environments isolation is necessary for safety or noise reasons. The circuit above shows a power-to-frequency converter using the LT2940. The PMON output alternately charges and discharges a film capacitor, C5. When the voltage on C5 charges to the upper threshold on hysteretic comparator LTC1440, its output flips the phase of the voltage sense inputs V+ and V– using a pair of MOSFETs (Q2 and Q3). The LT2940’s comparator serves as a phase splitter to develop complementary signals with which to drive Q2 and Q3. When the phasing of the voltage sense inputs is reversed, PMON discharges C5 to the LTC1440’s lower threshold, whereupon the action is repeated. The frequency is proportional to the current at PMON and, ultimately, the power consumed by the load. An opto-isolator conveniently communicates the frequency across an isolation barrier.