3.1
Circuit Description
3 – 5
SIM928 can be forced to switch in a fresh battery (see section 1.2.2.2)
prior to connecting to the sensitive user system.
A fully charged, unloaded battery has a terminal voltage several volts
higher than that of a loaded, almost fully discharged battery. During
switch-over both batteries are connected to the isolated user regulator
for approx. 30 seconds to ensure that the output voltage never drops
out. Two discrete, low-drop voltage regulators (with Q207 and Q208
as series elements) equalize the battery voltages to 23.5 V and make
sure that the maximum voltage jump at the output regulator power
supply rail stays well below a few hundred millivolts.
3.1.3
Battery pack
Each SIM928 battery pack contains two sets of three 9 V NiMH batter-
ies each. Each 9 V battery contains seven NiMH cells with a nominal
voltage of 1.2 V. A fully charged battery will produce a voltage of
approx. 21
×
(1.2 V
∼
1.3 V)
=
25.2 V
∼
27.3 V. Under load this voltage
quickly settles to a slowly falling ramp until the battery is nearly
empty. At approx. 1.15 V per cell, the battery quickly starts losing
voltage and the voltage detection circuitry signals the microcontroller
(through U502A) that the battery needs to be switched over.
Each battery pack carries a non-volatile EEPROM with information
about cell type, voltage, capacity, production date, charge cycles
and serial number. This information is used by the microcontroller
to determine the optimum charging time and when to turn on the
”Sevice Battery” indicator.
3.1.4
Isolated voltage regulator stage
The output stage is built around the low power, precision volt-
age reference U601, a 16-bit serial digital-to-analog converter U602,
and a gain 10 output stage containing U604 as the central precision
opamp. Precision is ensured by using a low temperature coe
ffi
cient,
adjustable, precision voltage divider network (R634–R642). O
ff
set
voltage compensation is partially digital by adding a digital o
ff
set
word to the DAC code and partially analog by trimming of U604’s
o
ff
set voltage.
U604 drives a current mirror Q610
/
Q608 used to translate its limited
output voltage swing into a
>
20 V signal which can drive the volt-
age follower made of Q601 and Q602. The biased low current output
stage is built with bipolar transistors and diodes that are specified for
400 V operation. Current and voltage sense transistors QN603
/
Q604
limit the maximum output current of this stage to
<
18 mA and make
sure that overvoltages can not damage the output stage. On detec-
tion of a permanent overvoltage, the microcontroller disconnects the
SIM928
Isolated Voltage Source
