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MX4428 MXP Engineering /Technical Manual Document: LT0273
MXP Technical Description
Page 7-8 24 March 2006 Issue 1.5
Figure 7.2 Analog Loop Typical DC Level and Data Waveform
7.2.5.1 Over-Current Protection
The current drawn by the analog loop passes through current sense resistors R22 - R26.
When the voltage across these resistors exceeds approximately 0.65 volts (corresponding to
a current of just over 400mA), the collector of Q2 begins to conduct. Thermistor RV3 and
resistor R119 compensate for the fact that the VBE threshold voltage of Q2 varies with
temperature. Q2 conducting pulls the gate of Q12 to +40V and switches off Q12. The drain
of Q12 then drops to around 5V or less depending on the load on the analog loop. Q13 is
then turned on by the current through R5. Q13 then holds Q12 off even though the overload
is now gone and Q2 is no longer conducting.
This “latched” over-current situation can only be reset by the CPU. The CPU senses the
absence of 40V through the sense resistors R68 and R67 and the signal OVERLOAD–
going to a logic low. The CPU (periodically) tries to reset the over-current latch by applying a
short (approx 5 ms) pulse to “40V RESET”. This pulse turns on Q4 and Q1, which turns off
Q13. Q12 will then turn back on and stay hard on as long as there is no over-current.
For the duration of this 5 ms pulse, the latching action of the circuit is disabled, and Q12 will
act as a linear current limiter. During this time the dissipation in Q12 can be up to 40V *
400mA i.e. 16 watts. This will be an excessive dissipation for Q12 if it continues indefinitely,
which is why the reset pulse is limited to 5 ms, and indeed why the latching action is required
in the first place.