SENSITRON
SEMICONDUCTOR
TECHNICAL DATA
DATASHEET 4310, Rev B
• 221 West Industry Court
Deer Park, NY 11729-4681
Phone (631) 586 7600 Fax (631) 242 9798
•
• World Wide Web - http://www.sensitron.com • E-Mail Address - sales@sensitron.com •
SPDXXD28 SERIES
Figure 4 - Electrical Block Diagram
Description
Figure 4 shows the block diagram of the SPDXXD28 SSPC Series. It uses a SN74LVC3G14 device for digital
I/O. This TTL compatible device has a Schmitt-Trigger input to minimize the effects of noise on the input. Its
outputs can each drive more than 10 standard TTL loads. It’s also compatible with CMOS inputs and outputs.
The SN74LVC3G14 is isolated from the remainder of the module circuitry by three optocouplers.
The block labeled “Control & Protection Circuitry” gets power from the DC-DC converter and is referenced to the
output of the SSPC. This block contains an amplifier to gain up the voltage developed across the sense
resistor. It also contains a microcontroller with on-board timers, A/D converter, clock generator and independent
watchdog timer. The microcontroller implements a precision I
2t protection curve as well as an Instant Trip
function to protect the wiring and to protect itself. It performs all of the functions of multiple analog comparators
and discrete logic in one high-reliability component.
The code programmed in the microcontroller acquires the output of the internal A/D converter, squares the
result and applies it to a simulated RC circuit. It checks the output of the simulated circuit to determine whether
or not to trip (turn off the power Mosfets). Because the microcontroller simulates an analog RC circuit, the
SSPC has ‘thermal memory’. That is, it trips faster if there had been current flowing prior to the overload than if
there hadn’t been current flowing. This behavior imitates thermal circuit breakers and better protects the
application’s wiring since the wiring cannot take as much an overload if current had been flowing prior to the
overload.
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