Automatic load sensing saves power in high voltage converter

There are a surprising number of high output voltage applications for LTC’s micropower DC/DC converter family. These applications include electroluminescent panels, specialized sensing tubes and xenon strobes. One of the key features of the micropower converters is low quiescent current. Since the quiescent current is far less than the self-discharge rate of common alkaline cells, the traditional ON/OFF switch can be eliminated in cases where the load is intermittent or where the load is shut down under digital control.

The maximum switch voltage for many micropower devices is 50 V. For higher outputs the circuit shown in Figure 1 is often recommended. It combines a boost regulator and a charge pump tripler to produce an output voltage of up to 150 V. The output is sensed through a divider network, which consumes a constant current of about 12 µA. This doesn’t seem like much, but reflected back to the 3 V battery it amounts to over 3 mA. Together with the LT1107’s 320 µA quiescent current the battery current is 3.5 mA under no load. In standby applications this is unacceptably high, even for two D cells.

Automatic shutdown reduces battery current to 350 µA.
Figure 1. Automatic shutdown reduces battery current to 350 µA.

A circuit consisting of transistors Q1 and Q2 was added to reduce the standby current to an acceptable level. When a load of more than 50 µA is present, Q1 turns on, Q2 turns off and the 9.1M resistor (R4) serves as a feedback path. R2, R3 and R4 regulate the output at 128 V.

If the load current drops below 50 µA, Q1 turns off and Q2 turns on, shorting out R4. With R4 out of the way, R2 and R3 regulate the output to approximately 15 V. The measured input current under this condition is only 350 µA, just slightly higher than the chip’s no-load quiescent current. When the load returns, Q1 senses the excess current and the output automatically rises to its nominal value of 128 V.

This automatic feedback switching scheme improves the battery current by a factor of ten and eliminates the need for a mechanical ON/OFF switch in applications where the load is under digital control.

Materials on the topic

  1. Datasheet Analog Devices LT1107
  2. Datasheet Nexperia MMBTA92
  3. Datasheet MCC MUR120
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