SRS Labs SR560 - Manual (Page 19)

CIRCUIT DESCRIPTION
15
regulator that provides twice the current of
the LM317 negative battery regulator.
Charging is controlled by changing the set
voltage of the regulators based on battery
charge status. Flip-flop U703 determines
whether the charge regulators will be set to
15.5 volts for a quick charge or 13.8 volts
for a trickle or "maintain" charge by
grounding the bottom of P701 and P702.
C712 and R704 insure that the charger
always powers up in the "quick" charge
mode. P701 and P702 are provided to
adjust the open circuit trickle charge voltage
to 13.8 volts. D701 and D703 are blocking
diodes for the charging circuits while not
charging, and D707 and D708 are clamps to
guard against battery polarity reversal.
Comparators U708 and U709 are LP365
micropower comparators that monitor the
battery voltage. A resistive divider chain
sets the four trip points for each comparator.
D709 provides a stable 2.5 volt reference
against which levels are compared. For
each battery, three level indications are
provided, and are decoded by multiplexer
U704. The "trip" level is 14.5 volts. The trip
outputs control the state of U703 and switch
the battery charge voltage settings. The
"low" level is 11.3 volts and activates the
front panel low battery indicator. R730
provides some level hysteresis for the low
battery indication to prevent oscillation
around the trip point. The "dead" level is
10.7 volts and is used to disconnect the
load from the batteries before they are
damaged by an excessively deep
discharge. Q701 and Q703 are power
MOSFET switches used to disconnect
battery power from the amplifier. Dead level
hysteresis is provided by R724. R731 and
D711 provide un-interrupted battery power
to the system RAM so that stored
instrument settings are retained when the
power is switched off.
POWER REGULATORS
The +5 V and +10 V supplies are produced
with three-terminal regulators U801 and
U802, respectively. The -10 V supply is
constructed of op-amp U803 and Q801, a
N-channel MOSFET as the pass element.
The +10 V supply serves as the reference
for the -10 V supply through divider R807
and R806.
The power output banana jacks on the rear
panel (J801 and J803) are connected to the
pre-regulated voltages after the power
switch and before the regulators. This
output can provide up to 200 mA of power
for use as an external bias source, etc.
Under some conditions, these jacks may be
used to supply the unit with external DC
power.
U506D and U506B generate the TTL level
input to the processor to indicate when the
unit is operating on the AC line.
Capacitors C801 through C821 are logic
supply bypass capacitors distributed
throughout the printed circuit board.
REAR PANEL INTERFACES
Two optically isolated rear panel interfaces
are provided on the SR560. The blanking
input accepts a TTL-level signal and
grounds the amplifier signal path after the
front end for as long as the input is held
high. The response time of the blanking
input is typically "on" 5 µs after the rising
edge and "off" 10 µs after the falling edge.
The RS-232 interface allows calibration and
control of the instrument at 9600 baud.
Data in and out on the connector are tied
together, echoing data back to the sender.
Hardware handshaking lines CTS, DSR,
and CD are tied to DTR. Refer to
Appendix A-1 for information on remote
programming of the SR560.