Circuit Labs - Electronic Circuits Collection

This circuit arose from the need of the author to provide a 5 V output from the 24 V battery of a solar powered generator. Although solar power is essentially free it is important not to be wasteful especially for small installations; if the battery runs flat at midnight you’ve got a long wait before the sun comes up again. The basic requirement was to make an efficient step-down converter to power low voltage equipment; the final design shown here accepts a wide input voltage from 9 to 60 V with an output current of 500 mA. The efficiency is very good even with a load of 1 mA the design is still better than a standard linear regulator.

The low quiescent current (200 μA) also plays a part in reducing losses. Some of the components specified (particularly the power MOSFET) are not the most economical on the market but they have been deliberately selected with efficiency in mind.

When power is applied to the circuit a reference voltage is produced on one side of R2. D1 connects this to the supply (pin 7) of IC1 to provide power at start-up. Once the circuit begins switching and the output voltage rises to 5 V, D2 becomes forward biased and powers
the IC from the output. Diode D1 becomes reverse biased reducing current through R1. When the circuit is first powered up the voltage on pin 2 of IC1 is below the reference voltage on pin 3, this produces a high level on output pin 6. The low power MOSFET T1 is switched on which in turn switches the power MOSFET T3 via R5 and the speed-up capacitor C4, the output voltage starts to rise.

When the output approaches 5 V the voltage fed back to the inverting input of IC1 becomes positive with respect to the non inverting input (reference) and switches the output of IC1 low. T1 and T3 now switch off and C3 transfers this negative going edge to the base of T2 which conducts and effectively shorts out the gate capacitance of T3 thereby improving its switch off time.

The switching frequency is not governed by a fixed clock signal but instead by the load current; with no load attached the circuit oscillates at about 40 Hz while at 500 mA it runs at approximately 5 kHz. The variable clock rate dictates that the output inductor L1 needs to have the relatively high value of 100 mH. The coil can be wound on ferrite core material with a high AL value to allow the smallest number of turns and produce the lowest possible resistance. Ready-made coils of this value often have a resistance greater than 1 Ω and these would only be suitable for an output load current of less than 100 mA.

The voltage divider ratio formed by R4 and R3 sets the output voltage and these values can be changed if a different output voltage is required. The output voltage must be a minimum of 1 V below the input voltage and the output has a minimum value of 4 V because of the supply to IC1.

A maximum efficiency of around 90 % was achieved with this circuit using an input voltage between 9 and 15 V and supplying a current greater than 5 mA, even with an input voltage of 30 V the circuit efficiency was around 80 %. If the circuit is used with a relatively low input voltage efficiency gains can be made by replacing D4 with a similar device with a lower reverse breakdown voltage rating, these devices tend to have a smaller forward voltage drop which reduces losses in the diode at high currents. At higher input voltage levels the value of resistor R1 can be increased proportionally to reduce the quiescent current even further.

Author: Michel Franke

(Elektor Electronics Magazine – 2006)

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When computer makers switched from RS232 to USB, one of the collateral benefits was TTL compatibility with regard to the supply voltage as well as signal levels. Unfortunately, there was also collateral damage, primarily in the form of an enormous increase in the complexity of the data transmission process. All you need for connection to an RS232 serial port is a level converter, but USB requires an extra IC (such as the Cypress CY7C68000 or a special-purpose microcontroller, such as the PIC18F4550) to make data transmission possible. That sounds like a zero-sum game, which raises the question: if you already have to use an extra IC, why not go directly to Bluetooth? And in fact, there are already several Bluetooth modules available, complete with an integrated antenna and simple serial inputs and outputs. However, many of them have the disadvantage that they are controlled using AT commands, which makes it necessary to use an additional controller.

However, there are also autonomous models available, such as the new LinkMatik module (www.flexipanel.com). If its Mst/Slv pin is held low, it waits until something wants to connect to it. If the pin is high, the module actively searches for devices that are ready to establish a connection. That means you can set up a shortrange radio link without using a computer by simply coupling two modules together. However, generally speaking you will probably want to control an external device from a PC. In that case, you can let your PC search for devices that are ready to establish a connection. When the PC finds a module, it lists its available services. The service known as ‘Serial Port Profile’ (SPP) is the appropriate service of the LinkMatik module. If the security function is enabled, you will have to enter the PIN code of the module (set to ‘0000’ when first delivered). Once a connection has been established, the Status pin of the module goes to a high level to indicate that it is ready for bidirectional data transmission.

Now you can simply use the familiar TxD and RxD lines to transmit serial data. The RTS and CTS lines are also available for controlling the data flow. If you do not need that function, you
can simply connect these two lines together. However, this wireless serial interface does not allow you to use these lines for purposes other than their intended use, as is often done with ‘normal’ serial interfaces. As the data is buffered in the module before and after wireless transmission, RTS and CTS are generated locally. These two signals only control data flow to or from the module, but not over the wireless link.

It is generally not possible to have more than one serial link via Bluetooth per PC. However, Bluetooth-2.0 compatible modules have recently become available, and they allow up to four serial links to be used concurrently along with other functions.

Author: Richard Hoptroff

 (Elektor Electronics Magazine – 7-8/2006)

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