LX Series Digital
Chapter 4, Circuit Descriptions
Power Amplifier Assembly
Volume 2, Rev. 1
4-4
to shunt current away from the LED to
turn it Off. U9 below U8 is used to
enable different features within the
software. Actual use is to be determined.
Page 2
In the lower right corner are voltage
regulator circuits. U22 should allow for
0.14 amps of power using its 92 C/W
rating if Ta = 60°C max and Tj = 125°C
max 0.26 amps can be obtained from
U22 if the mounting pad is 0.5 square
inches. The controller will not need this
much current.
U23 and U24 are low drop out +5 VDC,
voltage regulators with a tolerance
greater than or equal to 1%. 100mA of
current is available from each device but
again the controller will not need this
much current.
In the upper left section are circuits with
U12 and U13. U12 is used to generate a
regulated voltage that is about 5 volts
less than the +32 VDC supply,
approxi26.25 VDC. When the
+32 VDC supply is enabled, the circuitry
around U13B is used to provide gate
voltage to Q10 that is 5 volts greater
than the source pin of this FET. The gate
of Q10 can be turned Off by any one of a
few different circuits.
U10A is used to turn Off the gate of Q10
in the event of high current in amplifier
#1. At 0.886 VDC the current to
amplifier #1 should be greater than 5
Amps. U11B is used to turn off the Q10
FET, if high current is detected in
amplifier #2. U11A is used to turn off
the Q10 FET, if high current is detected
in amplifier #3. With 2.257 VDC at Pin 5
of U11B or Pin 3 of U11A, the voltage
output of current sense amplifier U17 or
U18 at high current shut down should be
greater than 15 Amps.
U14B is used to turn Off the gate of Q10
in the event of high power supply
voltage, approxi35.4 VDC.
U14A is used to keep the FET disabled in
the event of low power supply voltage,
approxi25.4 VDC.
Current monitoring sections of the board.
The ICs U16, U17 and U18 along with
associated components set up the current
monitoring sections of the board. R67,
R68 and R69 are 0.01
Ω
/5W 1% through
hole resistor is used for monitoring the
current through several sections of the
amplifier. The voltage developed across
these resistors are amplified for current
monitoring by U16, U17 or U18. The
LT1787HVCS8 precision high side current
sense IC amplifier accepts a maximum
voltage of 60 VDC. The 43.2 k
Ω
resistor
from pin 5 to ground sets the gain of the
amplifier to about 17.28. This value is not
set with much accuracy since the
manufacturer internally matches the
resistors of this part but their actual
resistance value is not closely defined. A
trimming resistor is suggested to give a
temperature stability of –200 ppm/C, but
instead the microcontroller will determine
the exact gain of the circuit and use a
correction factor for measurements.
Circuit loading components are located in
the lower portion of each current
monitoring circuit. These components
allow for short duration high current
loading of the supply. By measuring the
current through the sense resistor with and
without the additional four 30.1
Ω
1%
resistors. For very short duration pulses, a
1206 resistor can handle up to 60 watts.
The processor requires 226 uSec per
conversion. A supply voltage of +32 VDC
will pass 1.06 amps + 1% through the load
resistors.
A6 is a temperature sensor thermistor
that is used to monitor the temperature of
the module's heat sink. It connects to J6
pins 1 & 2 on the board and is wired to
the comparator IC U10B. If the
temperature increases above 75°C the
output will go Low that is used as a
temperature fault output, which generates
a Fault alert at U15A and disables
Amplifier #1.
