TTI PL Series Service Manual page 17

IC7B is a differential amplifier with a voltage gain of 13.3. For 6V output this means the voltage at
IC7A will be 450mV. VR4 trims differential gain and effects voltage regulation. This can be set
more accurately by generating a few hundred millivolts between positive output and positive
sense, see calibration section.
Current Control
IC9A is the error amplifier and compares the voltage on the wiper of VR3 with the voltage
generated across the current sense resistor R62. When this limit is reached IC3A takes control.
Current limit is indicated to the micro-controller by ILIMIT going high. The sense resistor is 50
milli-Ohm, therefore with the current limit set to 7A the voltage on the wiper of VR3 will be
typically 330mV when VR5 is set central.
To limit power dissipation in the series regulator the power supply has foldback current limiting.
When the power supply enters current limit the microcontroller also monitors the output voltage,
and if this falls below 3.5V IC6B is switched from VR2 to a simple 3 bit DAC so that the
microcontroller can take control of the current limit and reduce it as the output voltage falls.
Series Regulator
Q5, Q2 and Q3 form a triple Darlington. When the output current reaches 3 Amps, Q8 starts
conducting.
Over Voltage Protection
When the voltage on the output terminals exceeds 7V, SCR1 is fired crowbaring the output.
Current flowing through D6 is detected by IC9B which drives the trip line high informing the
microcontroller of the trip condition. The microcontroller then turns the series regulator off via Q4.
A to D
IC2 is a 12 bit dual slope converter. Its reference is derived from D9 by R29 and R30 and is
typically 180mV. XL1 is either a 4MHz crystal or ceramic resonator. The buffered oscillator
output is also used by the microcontroller IC1. Analogue multiplexer IC3 selects measurement of
preset voltage or output current.
Microcontroller
The measurement system and display is controlled by a microcontroller IC1.
The 4-digit LED display is driven by IC1 via the segment latch IC4 and the digit latch IC5. Digit
current is provided by IC3 and individual segment current is limited to 25mA by the resistors R10
and R17. The digit multiplex rate is 2ms and is controlled by IC1 which also provides the inter
digit blanking to prevent ghosting segments.
The measurement of output values of voltage and current is performed by the 12 bit analog to
digital converter IC2. The measurement rate is controlled by the 4.0MHz ceramic resonator XL1
connected between pins 22 and 23 and the buffered version of this 4MHz signal at pin 25 is used
as the clock to the microcontroller IC1. The ADC, IC2, is a dual slope converter and provides a
little over 8 readings per second when clocked at 4MHz. The ADC is run in continuous mode and
the status signal on pin 2 is read by the microcontroller every 6ms. When a reading is ready the
microcontroller reads the 12 bit binary value and then converts it to 7 segment BCD and stores it
ready to be sent to the display.
The output is electronically switched. The output from the unit bypasses the output switch (links)
and the OPON signal drives Q4 via inverter Q5. When OPON is low, Q4 is on and removes the
drive to the pass transistors. OPON also drives the multiplexer IC10 which switches to Vp when
OPON is low.
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