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Design Ideas |
It's easy to effect an interface between power MOSFETs and TTL
circuits. The power devices are thus useful as digitally controlled switching
elements in applications involving solenoid valves, dc-motor drive, and
stepper-motor drive, to name a few. The circuit in
Figure 1 uses a MAX234 RS-232C driver, with
the addition of a few capacitors and diodes, as a level translator. The circuit
allows TTL-level signals to drive four power MOSFETs. The MAX234 is a
line-driver chip that translates TTL-level signals to the RS-232C levels that
serial communications require.
The external capacitors form part of a charge pump that converts the 5V-supply level to ±10V. One charge pump uses C1 to double the 5V to 10V; the second pump uses C2 to convert the 10V to –10V. The TTL-level signals connect to inputs A through D of the MAX234. The output voltage swings to 10V when the input is at logic 0 and to –10V when the input is at logic 1. The diodes isolate the –10V from the MOSFET gates. The MOSFETs have a turn-on threshold voltage of 4V; the 10V gate voltage thus provides full enhancement. (DI #1939)
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