GE MAC 5500, Service Manual

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Revision E MAC

5500 resting ECG analysis system 2-11

2020299-020
Equipment Overview: Theory of Operation
+18V
The Main Regulator’s 5V switching output also supports generation of a non-
regulated 18V rail, which is used to provide power for the acquisition module. By
providing the acquisition module with 11.5V linearly regulated power from the
+18V rail of the main regulator rather than the main 12V regulator (U17),
acquisition is not affected by excessive current draw from the printer motor or
external loads on the COM ports (esp. KISS pump). The acquisition module's power
requirements are modest, so efficiency is not a pressing concern and the lower
efficiency of this approach is acceptable.
+1.8V
The Atmel CPU and FPGA (Xilinx Spartan 2) operates their internal core logic at
1.8V, while their I/O ring runs at the system standard 3.3V. The 1.8 Volt regulator, a
low dropout linear regulator, drops +3V-M to 1.8V for use as a core supply.
+12V
The paper motor drive circuit, LCD backlight and external COM ports all require
12V. The Main Regulator’s 18V output cannot provide sufficient current for all of
the systems 12V loads, so a secondary 12V regulator is required. The Main 12V
Regulator (U17), a switching buck regulator, provides the higher currents needed by
these loads. A P-channel MOSFET (Q1) switch precedes the regulator to provide
on/off control. Gate capacitor C12 slows the turn on/off time of the MOSFET switch
to eliminate switching transients. The voltage divider created by R133,134 prevents
the full supply rail from being impressed across Q1’s gate when on. This protection
is necessary, as the maximum Vgs of the MOSFET is less than the peak supply
voltage.
REF2V5
The high power rails are neither precise nor quiet enough to be used as the reference
for analog input/output or internal measurement circuits. The Analog Reference
Regulator (U50), a 2.5V shunt regulator provides a quiet and stable reference
voltage for such purposes. VREF is derived from +5V-EMI rather than +3V-EMI to
minimize the change in reference current with changes in input rail voltage. The
difference between 5V and 2.5V is three times greater than the difference between
3.3V and 2.5V. If the absolute ripple on both supplies is the same, the modulation of
reference current will be 3 times less if power is derived from +5V.