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Document: LT0273 MX4428 MXP Engineering / Technical Manual
MXP Technical Description
Issue 1.5 24 March 2006 Page 7-5
(i) +VS This is a switched version of "+V", which is switched OFF
when the loop supply falls below the voltage required for
correct operation of the MXP.
(ii) 40V ISO A regulated, isolated 40V supply used to drive the Analogue
Loop circuitry and addressable devices.
(iii) 24V ISO Derived from 40V ISO to power 24V relays.
(iv) 5V Used to power the CPU and logic circuitry.
(v) 5VB Used to power the RAM and PAL. Backed up by supercap
(C68).
Descriptions of the circuitry required to generate these power supplies follow.
7.2.3.1 +VS Circuitry
The MXP is specified to operate over an MX4428 loop supply range of 17.0V to 30.0V. The
MX4428 loop fault clearing technique relies on the fact that responders that are not powered
up "look like" a high impedance (see Section
7.2.4 for details). The circuitry is therefore
divided up into two sections, a (relatively) high current portion which becomes active only
when adequate voltage is available, and a low current portion that is continuously powered
up and whose sole purpose is to sense the loop voltage and control the enabling/disabling of
the high current circuitry. Loop voltage sensing is performed by comparator U7:C and
associated components which, in turn, operate power switch Q22 to feed +VS. Since the
voltage regulator draws peak currents up to around 5A, a FET is used for Q22, which gives a
low voltage drop for minimal control (gate) current.
D39, R12, R35, R113 ensure that +VS becomes active if the loop supply exceeds 17.0V,
with R28, R110, D38 providing about 4V of hysteresis (i.e. once switched on +VS will stay
switched on until the supply voltage drops below 13V).
7.2.3.2 40V ISO
The 40V ISO and 5V ISO supplies are produced using the switch mode power supply
controller IC U10, FET Q17, and associated components. The circuit configuration is such
that the circuit operates in flyback mode, energy being stored in L1 primary during Q17's ON
period and transferred to the two secondary windings during the OFF period.
The current into the diode of optocoupler OC6 increases rapidly as the 40V ISO voltage
passes through 40V (adjustable by means of VR1). The optocoupler OC6 controls the
feedback to U10 pin 2 which adjusts the duty cycle of the current pulses into L1's primary
and maintains regulation of 40V ISO.
R6 and C63 provide stability, essentially coupling the sawtooth from the internal oscillator to
the comparator – input, thereby making the + input a much lower gain pulse width control
than it would otherwise be. R107, R65, C38 provide over-current protection, while C34
defines the frequency of oscillation (approximately 80KHz). D41 and C77 provide a “soft
start” circuit to reduce the current taken during the startup time.
7.2.3.3 24V ISO
The 40V ISO voltage is regulated by a linear regulator consisting of Q23, D40 and
associated components. This supply is only required to supply a low current (about 30mA
max) to drive relays. The reference diode (D40) used for the 24V supply is also used as the
reference for the 40V supply.