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Issue #206 September
2007
Smart Power
An Intelligent Power Supply for Embedded Systems
by Alexander Popov & Jordan Popov
Start | Hardware | Software | Calibration | Voltage Presets | Slowly Rising Voltage | Slowly Falling Voltage | Short Time Test | Periodcal Linear Voltage | Periodical Voltage Drops | Future Development | Sources & PDF
HARDWARE
The system is powered by two voltages. The main voltage is 12 V, and it must provide enough current for the target load, the Butterfly (through a 3.3-V linear regulator), and the other components. The second voltage, –5 V, is used as a negative supply voltage to the op-amps.
The main block is the regulated power supply, which was built with a linear regulator. A 10-bit DAC controls the voltage. The output of this block flows through the current-detection block and then to the output connectors of the power supply.
As you can see in Figure 2, the system features a National Semiconductor LM723 voltage regulator. It has a temperature-stabilized, low-noise voltage reference. In addition to short circuit protection, it can provide output voltages down to 0 V.
The LM723 requires special handling for low output voltages. For output voltages greater than 2–3 V, the V– pin can be connected directly to ground. But for voltages down to 0 V (or even further), the V– pin should be connected to a negative voltage of at least –0.4 V. There are several ways to produce this voltage. One method is to convert positive to negative voltage with a switching capacitive inverter. But note that this method can introduce noise. The LM723’s voltage reference is relative to the V- voltage. That is why it is important that V- be stable and noise free. Thus, another method is used (see Figure 3). VREFF = 1.28 V is produced by U2A, R19, R5, and R6 from the LM723’s reference voltage. This is inverted to VM256 = –2.56 V by the op-amp U2B, R1, and R2. This also works as negative feedback to VREF, partially compensating for its temperature coefficient and stabilizing the voltage reference even more.
Microchip Technology’s TC1321 DAC (U3) is connected to the LM723’s IN+ pin to set the output voltage. The TC1321 was chosen for its 10 bits of resolution, 2.7–5.5 V single-supply operation, good integral and differential linearity, and an output voltage offset of less than 8 mV. The DAC is controlled by the CPU inside the Butterfly via an I2C interface.
The reference voltage for the DAC is VREF = 1.28 V. It’s produced from the reference voltage of U4 in the way described above.
The DAC’s output is filtered through a simple low-pass filter built with R7 and C5. Its purpose is to smooth the output voltage and filter out the sampling frequency on the DAC’s output.
Many electronic devices can’t withstand a small reverse voltage in the power supply. That’s why a voltage-offset correction is provided (R20, R9, R10, R18, and U2D) in the voltage-feedback circuit. This eliminates the possibility of a negative output voltage during startup (when the DAC is zeroed). There are two reasons why this offset is not eliminated in the software with a constant added to the number written in the DAC. First, the offset can be positive, and there may be a need for a negative constant. This won’t work because the DAC works only with unsigned values. As for the second reason, note that in software, the correction is discrete and there can be an offset of up to one-half of it left.
The diode D1 provides additional safety to the powered electronic circuits by not letting the output voltage become lower than –0.7 V. On the power supply’s output, there is the usual capacitor (C7) with the unusual value of only 1 µF! You would typically expect something like 1,000 µF here, but had this been used, the output response would have been slowed down and no high-speed control from the CPU would have been possible. A small capacitor is still needed to prevent the circuit from self-oscillating.
The transistor Q1 is used to provide more output current than the LM723 can source. The power supply is linear and it converts voltages, but since the current is the same for input and output, Q1 has to dissipate the excessive power (i.e., P = (VIN – VOUT) × IOUT). That is why it should be mounted on a large heatsink. Its area is calculated for the worst-case scenario (i.e., VOUT = 0 V, IOUT = IMAX, qAMB = qMAX). If the dissipated power is less than 5 W, a 65-to-90-W transistor in a TO220 case can be used. When the power is between 10 and 15 W, a Darlington transistor in a TO247 case rated for more than 100 W should be used. The area needed for the heatsink can be seen in Table 1.
The resistor R14 has two functions. It sets the LM723’s current threshold. The protection is triggered when the voltage across R14 reaches 0.65 V. It is also used to measure the output current. The common-mode output voltage is suppressed by the differential amplifier U2C and only the voltage drop across the resistor R14 is amplified. The voltage is proportional to the output current in the ratio 1 V/1 A. The voltage is amplified to about 5.6 V per amp to fit the range of the Butterfly’s VIN measurement circuit, which has a 6:1 divider in the input. Any offset is eliminated in the software because it is not as critical as the output voltage.
The intelligent part of the device is the Butterfly. It has a good user interface with a five-way joystick and a six-character alphanumeric LCD. Many other peripherals are connected to the microcontroller, such as a piezo speaker, DataFlash, an NTC thermistor, a light sensor, and an RS-232 lever shifter. All of the peripherals can be used to extend the device’s functionality in the future. Currently, the LCD, joystick, JTAG, ADC, USI (for the I2C driving the DAC), and one red LED are used. The LED was not originally on the board, but it was easy to add. Its cathode was soldered to pin 3 on the USI header, which is unused in the I2C communication, and its anode was connected to the positive power of the Butterfly board through a 220-W resistor. The LED illuminates when the current protection is triggered.
The DAC (U3) and the Butterfly module are powered by a linear power supply that uses Holtek Semiconductor’s HT7533-1 voltage regulator. This low-dropout regulator is reliable and has no risk of oscillation or high-voltage output in low-current mode, which is sometimes seen in low-dropout regulators.
The absence of hum and noise is important for the accuracy of this power supply. All the power grounds should be routed with different thick tracks to a single point on the board. The signal grounds should also be connected to this point.
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