How to achieve a low-cost solution with dual-output voltage converters

Abstract: Regardless of the application, engineers are faced with the need to bias circuits with negative voltages for specific functions. This need is common in single-supply systems where high performance is required to extract or impose an analog signal. Of course, any system powered by an AC source can solve this problem with an additional winding on the AC/DC converter. However, when powered by a single battery, the situation is more complicated. In this case, a simple DC/DC converter can meet this need without increasing the PCB footprint or adding cost.

Basic Circuit Description

--- The dual voltage converter is based on the NCP5006 chip, whose high voltage output capability was originally used to power white LEDs. This converter is used in our laboratory's high-precision ADC system for performance evaluation of dedicated analog chips. In order to achieve the best performance of the operational amplifier, a dual power supply voltage is required. On the other hand, a +10V voltage is also required for proper biasing of the additional current source to produce a linear voltage slope.

Since these voltages are not available on the board (powered by a 3.6V Li-Ion battery), the circuit shown in the schematic of Figure 1 was designed using standard components. --- In general, the converter can boost the input voltage to 20V and can provide up to 600mW of power to the load. The output voltage is stabilized by the feedback provided by the R1/R2 network connected between Vout and ground. The chip design includes 200mV feedback detection, which reduces the overall loss.

-- The +10V output voltage is rectified by diode D2, preferably a Schottky device, but a silicon device will do since we only need to meet low output current requirements, and the energy storage capacitor is 2.2μF (preferably ceramic). The output capacitance depends on the output load current, and can be increased if the load current is higher and less ripple is required. The circuit is designed to provide 5mA current to +Vout. The -10V is generated by a charge pump-like technique, using a 100nF ceramic capacitor to provide energy to the load from the switching voltage. The use of dual diodes in a tiny SOT563 package allows for a simple PCB layout and limited footprint. Of course, an additional 2.2μF energy storage capacitor (preferably ceramic) must be used to properly filter the noise on the negative output. The negative output is able to provide the load

-5mA current, but larger loads can be supported if C2 and C3 are sized accordingly.

--- Although the negative output voltage varies with the load, this is not a major problem because a high-end operational amplifier is used in this application, which helps to improve the power supply rejection ratio (PSRR) and avoids any problems caused by negative voltage variations. On the other hand, the current absorbed by this application is almost constant, forming an output negative voltage that is almost constant.

--- Output voltage ripple mainly comes from the noise generated by fast transients caused by switching: See the waveform in Figure 2. Although this noise is harmful (see ON Semiconductor application note AND8172D), it can be reduced by adding an additional LC filter in series with the output voltage.

Tiny, high-permeability inductors used in conjunction with existing capacitors (see Figure 3) can significantly reduce noise, as shown in Figure 4. Of course, the PCB must be carefully designed to reduce the acquisition noise, and depending on the sensitivity level, it may be necessary to use a multilayer board to minimize noise.

Output voltage regulation

Depending on the application, a stable and accurate output voltage may be a mandatory requirement; however, the simple circuit given above does not meet this requirement because the output voltage depends on the load. ---The positive voltage output is regulated and protected from overload by a surface-mount LM317B linear regulator. Two additional diodes are used to protect the regulated LM317B during power on/off transients.

---Although there are a number of negative voltage linear regulators on the market, choosing a discrete solution can better support smaller load currents. On the other hand, there is no need for overload protection circuitry here, because the negative output current is automatically limited by the DC/DC architecture. In fact, if a short circuit to ground occurs between -10V and ground, it is impossible for continuous current to flow in the PNP transistor (capacitor C3 blocks the DC current). Therefore, there is no continuous forward bias safe operating area issue for this PNP device, which the small signal BC858C device can well implement. The PSPICE simulation results shown in Figure 6 illustrate the performance of the linear regulator with a 10mA load on each output.