Switch-mode power supply within few minutes, with the L5973D switched regulator

Switch-mode power supplies (SMPS) are being more and more used because of their undoubted advantage – much higher efficiency than classic linear stabilisers. In contrast to linear power supplies, SMPS-es have almost the same efficiency at various input/otput voltage ratios, which uses to be 80-95%. That´s why, if we need a much lower output voltage, than input voltage (e.g. 5V from 24V), an SMPS still has a very good efficiency, unlike almost catastrophic 20% at linear stabiliser. For many applications, the best solution is to use a ready-made DC/DC module or to use switch-mode stabilisers which directly replace stabilisers of 78xx series. However, for many applications a solution from discrete components is more suitable. From our offer of switch-mode regulators, we would like to introduce you a perspective circuit L5973D from ST Microelectronics.

L5973D is a switch-mode buck regulator (step down) with the integrated 2.5A output transistor P-FET D-MOS, working at 250kHz frequency, in a HSOP-8 package. Input voltage can be 4-36V and an output voltage can be from 1,235 to 35V. For a construction of power source, only minimum of external components are necessary. Thanks to the integrated output transistor, it is easy to reach an optimal components layout and short power tracks on PCB. That´s why it is much easier to reach a minimum interference (EMI) in comparison to a solution with external switching transistor. L5973D contains a thermal protection, as well as an overcurrent protection, which limits a maximum current in every cycle. Cooling is provided by a thermal pad on the bottom side of the circuit. A big advantage is, thet the circuit is able to work even with the low drop-out (LDO) and the duty cycle 100%, i.e. if the inpout voltage is getting closer to the output voltage (for example a discharging accumulator), L5973D can carry over to the output practically the same voltage as it has on the input (voltage feedforward). This is especially useful at battery applications, when we need to ensure functionality of the device even in critical conditions.

To design a working power source, it is necessary only to choose the value of inductance and output capacitor, and naturally to use appropriate resistors in the feedback divider, to reach the desired output voltage. As a recuperative diode, almost every sufficiently dimensioned schottky diode is sufficient. For inductors, relatively small SMD types are sufficient, with inductance of tens of uH, and their choice is not critical. As an output capacitor, many electrolythic and tantalum capacitors can be used, also a parallel combination of few electrolythic capacitors is suitable, when their lower overall ESR and thus lower output ripple can be reached at the same time. Ceramic capacitors are very suitable at the input of the circuit (or in parallel with an electrolythic one), but if used on the input, their too low ESR would cause the instability of the circuit. In such case, it would be necessary to use a more complex compensation circuit, as described in the datasheet. With ceramic capacitors on the input and a tantalum capacitor at the output, it is possible to create with L5973D a powerful switch-mode source at a minimum space of few cm². L5973D has an internal 3.3V reference, which we can freely use in our own application, further it has an „inhibit“ pin, for shutdown of the regulator and reaching a zero power consumption. Moreover L5973D has an input „sync“, through which, we can synchronize more circuits or to use it for partial wobbling of the working frequency, thus reaching a further decrease of EMI, by spreading a spectrum. To reach an extremely low output ripple of few mVolts, it is possible to equipe the circuit with a secondary LC filter.

Also available is the version L5973AD with somewhat lower switching current - 2A, but with the higher working frequency of 500 kHz.

Datasheets L5973D and L5973AD contain detailed information. Application note AN1518 contains next useful tips, as well as a simple circuit for a soft start.

A big advantage of the chip L5973D is its relatively very affordable price. In case of interest, please contact us at info@soselectronic.com.