Ref2V5 - GE MAC 5000 Service Manual

+18V
+1.8V
+12V

REF2V5

2-10
Equipment Overview: Theory of Operation
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 (U15), 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.
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.
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 (U15), 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 R135,134
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.
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 (U48), 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.
MAC™ 5000 resting ECG analysis system
2024917-010
Revision B