SRS Labs CTC100, Руководство пользователя

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Circuit Description 136
CTC100 Programmable Temperature Controller
If 50VSHDN is low, regulator U210 is shut down and the heater voltage is limited to 23.8V.
A 10W 120 ohm internal load resistor, R331, is connected between the +50V supply and
ground. The microcontroller can switch the current through this resistor on or off to keep the
power supply voltage from oscillating at low output voltages and/or currents.
Constant-current heater driver: the card has three independent current-output heater
driver circuits which output 1.333A, 0.465A, and 0.200A. If the user has selected a 2A range, all
three circuits operate in parallel. If a 0.6A range is selected, the 1.333A circuit is disabled. If a
0.2A range is selected, only the 0.2A circuit is enabled. The three circuits are identical except for
the sense resistor. The microcontroller enables each circuit by pulling one of the three lines
2000MA_ONOFF, 200MA_ONOFF, or 20MA_ONOFF low.
A 16-bit DAC, U240, sets the desired output current. The DAC outputs a value between 0 V
(no output current) and 4.0 V (highest possible current for the selected range).
Considering the 2.0A circuit, current from the +50V supply flows through sense resistor R251,
heater.
This high-side configuration is safer than the more common low-side current source, but given
the 55V range, it requires a special high-side-sense IC, U290A. The output of this chip is a voltage
proportional to the voltage across sense resistor R251 multiplied by 20. When the maximum
current is flowing (2A in this case), the output is 4.0 V.
While the current source is enabled, op amp U250A drives FET Q251 such that the output of
U233 is equal to the output of the current control DAC, U240. FET Q233 is needed so that the
gate of Q251 can be driven with a high voltage (up to +50V).
FET Q251 can dissipate up to 10 W of power. If it is not kept sufficiently cool, it may fail in the
Therefore a temperature sensor, U140, measures the temperature of the heatsink.
The sensor outputs a voltage of 1 mV/°F
inputs. The microcontroller requests increasing cooling from the system fan as the heatsink
temperature rises above 35°C. If the heatsink temperature exceeds 60°C, the microcontroller
causes an error message to appear on the CTC
A pair of automatically-resetting fuses (F221, F222) cuts off the output current if it exceeds 2 A.
The current passes through J200. A second sense resistor,
R208, is used to measure the return current. If the return current differs from the output current
by more than 0.25A, the microcontroller requests that an error message be displayed on the
CTC
Voltage and current monitor: a multiplexed 16-bit ADC, U280, monitors the heater
current, the voltage across the heater, and the return current. The ADC has a range of 0 4V. The
heater current is monitored by measuring the voltage across the sense resistor, which is 0.2V when
no current is flowing and 4.0V when the maximum current for the selected range is flowing.
A nalog I/O card
The analog I/O card has four channels that can be used as DAC outputs or ADC inputs.
On-card regulators p5, +15, and -15V analog supply voltages.
A 4-channel DAC, U202, produces four 0 5V outputs, which are converted to ±10V by
U203A-D. Switches U204A-
connectors, changing the affected channels from DAC outputs to ADC inputs.
The outputs of the four switc -
resetting fuse, F301-4, temporarily shuts off the current if it exceeds 200 mA. The normal
resistance of the fuse is about 1.5 ohms. D301 protects the card from electrostatic discharge and
excessive voltages.
U206 multiplexes the four channels into a 24-bit ADC. Since the ADC has a 0 5V range while
the inputs are specified for a ±10V range, the input voltage is divided by 4 and offset by 2.5V.