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3V to 9V DC-DC Converter Never buy another 9V battery Bought a 9V battery lately? They’re horribly expensive and they don’t last very long if you want more than a few milliamps out of them. The solution: build this little DC-DC converter so you can use AA, C or D size cells instead. By PETER SMITH S AY YOU WANT a 9V battery to supply 40mA to a circuit. That’s a pretty modest current but if you use a PP3 style 9V battery it won’t last long at all. In fact, if you’re using a typical “heavy duty” 9V battery, it will last less than 20 minutes before the voltage drops to 7.8V. That may be enough to stop your circuit working. Or maybe you are using an alkaline type. Depending on the brand and price, you might get about two hours life. Not good. By comparison, two AA alkaline cells driving this DC-DC Converter circuit to give 9V at 40mA will last about 7 hours. And rechargeable AA cells can be even better. Table 1 shows the comparisons. This circuit can deliver up to 90mA at 9V (with less life from the cells) or can be set to deliver anywhere between 4.5V and 20V. You might never have to buy another 9V battery ever again. Back in the November 1990 edition of SILICON CHIP, we described a single cell to 9V DC converter suitable for 24  Silicon Chip replacing 9V batteries. That design proved very popular and was subsequently updated in August 1992. Unfortunately, the TL496 power supply IC used in both of these projects is now obsolete. This project is based around the Texas Instruments TL499A, a similar but more versatile variant of the TL496. Most notably, its output voltage is programmable, making it suitable for use in a variety of low-power applications. Main Features • • • • Use it to replace 9V batteries • • Supports DC plugpack input Runs from AA, C or D cells Up to 90mA current at 9V Can be set for 4.5V to 20V output Optional trickle charge for NiCd & NiMH batteries Unlike the original TL496 designs, this new design is specified for use with two cells. This enables the converter to produce more realistic output current levels. For low-power applications, two cells are also more cost effective, as more of their energy is extracted before the terminal voltage falls below the converter’s minimum input voltage. We’ve also included support circuitry for the TL499’s on-board series (linear) regulator, meaning that it can be powered from a plugpack when a mains outlet is available. In addition, a trickle-charge function is provided for use with rechargeable batteries. The PC board is roughly the same size as a 2 x “AA” cell holder, so in some applications it will be possible to build it right in to the equipment that it powers. Alternatively, it could be housed in a small plastic “zippy” box or similar. TL499A basic operation A functional block diagram of the TL499A appears in Fig.1. It contains a switching regulator and series regulator. Let’s look at the switching regulator section first. The switching regulator operates as conventional step-up pulse-width modulated (PWM) DC-DC converter. A variable frequency oscillator drives the base of a power transistor, which acts as a switch between one side of a “boost” inductor and ground. Referring also to the circuit diagram www.siliconchip.com.au Parts List 1 PC board, code 11103041, 59 x 29mm 1 14.8mm toroid (Neosid 17732-22) (Altronics L-5110) 1 700mm-length (approx) 0.63mm enamelled copper wire 1 2 x AA (or C or D) cell holder 2 x 1.5V cells to suit cell holder 1 9V battery snap 1 panel-mount 2.1mm or 2.5mm DC socket (optional) 6 1mm PC board pins (stakes) Hot melt glue or neutral cure silicone sealant Fig.1: the functional block diagram of the TL499A. It’s housed in an 8-pin DIL package and contains both series (linear) and step-up switching regulators. in Fig.2, you can see that one end of the inductor (L1) is connected to battery positive. The other end is connected to pin 6 of the TL499A – the collector of the switching transistor (Q1). When the transistor switches on, the current through L1 ramps up with time, storing energy in the inductor’s magnetic field. When the transistor turns off, the magnetic field collapses, generating an instantaneous voltage which causes the blocking diode to conduct, thereby transferring the inductor’s energy to the output filter capacitor and load via pin 8. The second transistor (Q2) forms part of a cycle-by-cycle current limiting circuit. This circuit turns off the switching transistor (Q1) when the current through it reaches a predetermined level. A 150Ω resistor from pin 4 to ground sets the peak current level to about 500mA. The PWM circuit uses a fixed off time/variable on time scheme to maintain a regulated output voltage under varying line (battery voltage) and load conditions. Under light-load conditions, the switching frequency can be as low as a few kHz. With maximum load and minimum input voltage, it increases to over 20kHz. Now let’s turn our attention to the series regulator section. Again, this section is quite conventional, consisting of an NPN series pass element (Q3), a voltage reference and an error amplifier. DC voltage applied to pin 1 is passed through to the output at pin 8 via transistor Q3. The base of Q3 is driven by an error amplifier, which compares a 1.26V (nominal) reference voltage on its non-inverting input with the voltage at pin 2. Looking at the circuit diagram Semiconductors 1 TL499A Power Supply Controller IC (IC1) 2 1N4004 1A diodes (D1,D2) 1 1N4732A 4.7V 1W Zener diode (ZD1) Capacitors 1 470µF 25V PC electrolytic 1 220µF 25V PC electrolytic 1 100µF 25V PC electrolytic 1 1µF 50V monolithic ceramic 2 100nF 50V MKT polyester Resistors (0.25W 1%) 1 220kΩ 1 150Ω 1 33kΩ 1 10Ω 1 4.7kΩ 1 270Ω 1W 5% 1 220Ω 1W 5% (for testing) Type Service Life Conditions 9V Heavy Duty (Rayovac D1604) 9V Alkaline (Rayovac A1604) ≈ 18 min. 40mA Load, 7.8V Cutoff (Fig.2), you can see that resistors R1, R2 & R3 close the feedback loop, connecting the output voltage back to the error amplifier’s inverting input. The output voltage is determined by the expression: VOUT = VREF (1 + R1||R2/R3) Substituting our listed values gives: VOUT = 1.26 (1 + 33kΩ||220kΩ/4.7kΩ)       = 8.95V In fact, by choosing appropriate values for R1 & R2, the output voltage can be programmed for any value between 4.5V and 20V. A handy list of resistor values for the most common voltage ranges is presented in Table.3. ≈ 2 hours 40mA Load, 7.8V Cutoff Regulator priority Table 1: Battery Life Comparison 2 x AA Alkaline (Energiser E91) ≈ 7 hours 2 x AA NiMH (2000mAh) ≈ 7.7 hours www.siliconchip.com.au 230mA Load, (40mA Output), 1V/Cell Cutoff (9V Output) 230mA Load (40mA Output), 1V/Cell Cutoff (9V Output) A similar voltage feedback scheme is used by the switching regulator control circuits. In this case, however, the error amplifier circuit has been modified so that the output voltage will be about 2-3% lower than from the series March 2004  25 Fig.2: only an external inductor and a few passive components are required to build a complete power supply using the TL499A. D2 & R4 are optional, providing a trickle charge to the battery when a plugpack is connected. 26  Silicon Chip regulator. This gives priority to the series regulator, because its slightly higher output voltage “forces off” the switching regulator. In practice, this means that when the unit is running from batteries and a plugpack is connected, switch-over between the two sources occurs automatically. Power to the output is uninterrupted, ignoring the small increase in voltage (about 180mV for 9V out). When the series regulator is operating, the switching regulator shuts down and battery drain drops to just 15µA (typical). Texas Instruments refers to the voltage difference between the switching and series regulators as the “change voltage”. For more detailed information on the TL499A, you can download the datasheet from www.ti.com Complete circuit Very little external circuitry is required to construct a complete power supply using the TL499A. Looking first at the input side of the circuit (Fig.2), the DC plugpack input is polarity-protected with a series diode (D1) and then filtered with a 100µF capacitor before being applied to the series regulator input (pin 1). At the battery input, a 220µF capacitor compensates for battery lead length, terminal contact resistance and increasing cell impedance during discharge. Additional filtering is provided using a 10Ω resistor and 1µF capacitor before the battery voltage is applied to the switching regulator input (pin 3). This filter removes much of the high frequency switching noise present on the “hot” side of inductor L1. Zener diode ZD1 clamps the voltage on pin 3 to less than the maximum (10V) rating of the IC. It also prevents the trickle charge circuit from powering the output side of the circuit (via L1 and IC1), both unwanted side-effects that would otherwise occur when the circuit is powered from a plugpack without batteries installed. Note: to keep board size to a minimum, polarity protection has not been provided on the battery input. As cell orientation is obvious for most battery holders, you may not be concerned about this omission. However, if your application demands input polarity protection, then the additional circuitry shown in Fig.4 can be inserted prior to the converter’s input terminals. A simple series diode will not suffice in this case, as it would seriously impede circuit performance. Trickle charge circuit If you’re using rechargeable cells, then D2 and R4 can be installed to provide trickle charging whenever a plugpack is connected. A resistor value of 270Ω limits the charge current to about 50mA, dependant on input and battery voltages. This current level is suitable for cells of 1000mAh and higher. For lower cell capacities, you should select a more appropriate value for R4 using the following formula: R4 = (VIN – VD – VBATT) / (Ah x 0.05) Where VIN = plugpack voltage, VD = diode voltage drop, VBATT = fully charged battery voltage, Ah = www.siliconchip.com.au Fig.4: install these components in-line with the battery leads if “fail-safe” polarity protection and/or battery switching is required. The 470µF capacitor may be needed to ensure that the DC-DC converter starts up and regulates properly with the additional series impedance introduced by the switch, fuse and associated wiring. Fig.3: follow this diagram closely when assembling the board. There’s no need to wire up the DC socket if you’ll only be powering the converter from batteries. Note how the 9V battery snap is wired in reverse (red wire to negative terminal, black to positive) to mate with the existing battery snap in the equipment to be powered. battery capacity in amp/hours. For example, if you’re using 650mAh cells with a 12V unregulated plugpack that puts out 16V: R4 = (16 – 0.7 – 3) / (0.65 x 0.05) = 378Ω (use 390Ω) Note that while the trickle charge function will top-up your batteries as well as compensate for self-discharge, it is not intended to recharge flat cells. Do not be tempted to increase the trickle charge current beyond the recommended 0.05C rate. Doing so may shorten the life of your cells, or in the extreme case, cause a fire or explosion! If in doubt, refer to the manufacturer’s data sheets for the maximum recommended trickle charge rate. On the output side of the circuit, the 100nF capacitor across the top two resistors reduces ripple and noise in the feedback signal to pin 2. Finally, 470µF and 100nF capacitors provide the maximum permissible filtering ahead of the output terminals. fied when lightly loaded. Ideally, the input voltage needs to be only about 3V higher than the output to achieve regulation and minimise dissipation. The switching regulator can source up to 100mA of current. Table 4 provides a convenient method of determining the maximum available current for typical input and output voltage combinations when operating from battery power. Although the TL499A includes in-built over-temperature and overcurrent protection, you should not exceed the listed current levels to avoid possible damage to the chip. Excessive loading will also cause high ripple voltage and loss of regulation at the output. Also note that being a step-up (boost) type converter, there is a current path from the battery, through the inductor (L1) and the internal blocking diode to the output, even when the switcher is shut down. The diode is designed for a maximum current of 1A, a level that could easily be exceeded if the output terminals are accidentally shorted together. Voltage and current limits Using the component values shown, the series regulator (plugpack) input can be as high as 17V. This limit is imposed by the maximum continuous power dissipation of the TL499A (0.65W recommended), as well as power dissipation in the trickle charge circuit. If you’ve programmed the output for less than 9V, then use a lower voltage plugpack (less than 12V) to keep IC power dissipation under control. Remember that unregulated plugpacks put out higher voltages than speci- About efficiency & battery life The switching regulator’s efficiency depends on the input and output voltages and the load current. As shown Table 2: Resistor Colour Codes o o o o o o o o No. 1 1 1 1 1 1 1 www.siliconchip.com.au Value 220kΩ 33kΩ 4.7kΩ 150Ω 10Ω 270Ω (5%) 220Ω (5%) 4-Band Code (1%) red red yellow brown orange orange orange brown yellow violet red brown brown green brown brown brown black black brown red violet brown gold red red brown gold 5-Band Code (1%) red red black orange brown orange orange black red brown yellow violet black brown brown brown green black black brown brown black black gold brown not applicable not applicable March 2004  27 Table 3: R1 & R2 Values For Common Output Voltages VOUT R1 R2 4.5V 5V 6V 7.5V 9V 12V 15V 22kΩ 15kΩ 33kΩ 27kΩ 33kΩ 47kΩ 56kΩ 27kΩ 180kΩ 39kΩ 180kΩ 220kΩ 270kΩ 560kΩ Table.3: to program the converter for a different output voltage, just change the values of R1 & R2. Typical voltage ranges together with the necessary resistor values are listed here. in Table 4, the maximum output current with 3V at the input is 90mA. In this configuration, the circuit is about 55% efficient. Therefore, we can say that with a step-up ratio of 3:1, the input power will be about 1.25W at full load. This represents a considerable current demand on the batteries. In the case of alkaline batteries, the voltage decays rapidly to less than 1V/ cell under heavy-load conditions, which means that available output power decreases as well. The most important points to consider are: (1). Alkaline cells are best suited for intermittent and/or light-load use. The high self-discharge rate of rechargeables (especially NiMH types) makes them unsuitable in this application unless trickle-charged. (2). Rechargeable cells are best suited for high current, continuous-use applications. Although the initial terminal voltage is less than for alkaline cells, they have an almost flat voltage discharge curve. The lower (1.2V/cell) terminal voltage means that about 70mA Fig.6: this is the PC board etching pattern. max. output current is possible at 9V, but it will be sustainable over most of the battery life. (3). Carbon cells are not recommended with their positive leads aligned as due to the high peak switching current indicated by the “+” symbol. drawn by the converter. Winding the inductor Assembly Using the overlay diagram in Fig.3 as your guide, begin by installing the wire link (just below IC1) using tinned copper wire. Follow this up with all the resistors and diodes (D1, D2 & ZD1), taking care to align the banded ends of the diodes as shown. Note that the 270Ω 1W resistor should be mounted about 1mm proud of the board to aid heat dissipation. Important: D2 and R4 should only be installed if you’ll be using rechargeable batteries and the plugpack input. Do not install these components if using alkaline batteries. The TL499A (IC1) can go in next. It is important that this chip is soldered directly to the PC board – don’t use an IC socket! This maximises heat transfer and eliminates contact resistance. The notched (pin 1) end must be oriented as shown on the overlay diagram. Install all of the capacitors next, noting that the electrolytics go in The inductor is hand wound on a 14.8mm powered iron toroid, Neosid Part No. 17-732-22. You’ll need about 700mm of 0.63mm enamelled copper wire for the job. In total, 30 turns are required to achieve the 47µH inductance value. The wire must be wound on tightly, with each turn positioned as close as possible to the last. Do not overlap turns. One complete layer should make exactly 30 turns. Be careful not to kink the wire as you thread it through the centre of the toroid, otherwise you won’t be able to fit all 30 turns in the available space. Bend and trim the start and finish ends as necessary to get a neat fit in the PC board holes. Scrape the enamel insulation off the wire ends with a sharp blade and tin with solder prior to soldering to the PC board. With the inductor in place, all that remains is to install an insulated wire link between pin 6 of IC1 and the spare Table 4 Fig.5: this waveform was captured on pin 6 of the TL499A switching regulator IC with a 40mA load (ie, the 220Ω test load). The switching frequency is a little over 9kHz in this case. 28  Silicon Chip Table.4: the maximum switching regulator output current depends on the input and output voltages. This table enables you to predict the maximum current for the chosen output voltage as battery voltage declines. www.siliconchip.com.au hole on one side of the inductor. Make this link from medium-duty hook-up wire and keep it as short as possible. That done, the inductor can be permanently fixed to the PC board using hot-melt glue or neutral cure silicone sealant. Hookup and testing All connections to the board are made with medium-duty hook-up wire. If desired, PC board pins (stakes) can be installed at each connection point rather than soldering the wires directly to the board. Note that the wiring length from the battery holder to the input terminals must not exceed 100mm. Where possible, replace existing light-duty battery www.siliconchip.com.au holder wiring with medium-duty cable and twist the leads tightly together to reduce radiated noise. The converter draws a small quiescent current (a few milliamps) under no-load conditions. Therefore, for light-load or intermittent use, you’ll need to install a switch in series with the battery. Use a switch with a 2A rating or higher. To counter the effects of switch contact resistance (and fuse resistance, if used), you may need to install a capacitor between the switch output and battery negative leads (see Fig.4). In cases where the converter is to be used in place of a 9V battery, a battery clip can be used to make the connection to the existing battery clip in the equipment. As shown on the overlay diagram (Fig.3), you’ll need to wire the clip leads in reverse, so that it mates up with the correct polarity! Before using the converter for the first time, connect a 220Ω 1W resistor across the output terminals and apply battery power. Use your multimeter to measure the voltage across this resistor. If the switching regulator is doing its job, you meter should read close to the desired voltage. If you’ll be using a plugpack as well, then connect it up while monitoring the output voltage. As stated earlier, you should see a small increase in voltage (about 180mV), indicating that the series regulator has taken over and shut SC down the switching regulator. March 2004  29