As a rule, requirements for sensors, position encoders and also digital devices used in 24 V technology stipulate that that they be supplied with 5 V or, as is increasingly the case, 3.3 V. Another requirement is that the power unit takes up as little space as possible and has low-power dissipation. Also, in decentralized control units, transport systems, operating elements, field bus components or distributed subsystems, digital and analog circuit components usually have to be supplied with voltages of 3.3 or 5 V from higher input voltages. And they must be reliable and have a minimum loss of power. Unfortunately, these input voltages can fluctuate by up to ±50% and be riddled with signal disturbances. Most ICs, however, require supply voltages which are both stable and have as few disturbances as possible. Where feasible, these analog and digital supply voltages should be kept separate so the analog chip sections are not disturbed by spikes from the digital sections.
Due to their relatively high power dissipation at large difference between their input and output voltages, simple series regulators can only be used if the electronics to be connected to them have a very low-power consumption. Switching converters are much more economical but, they have a significantly higher residual ripple. The output voltages generated are of use to digital ICs with moderate filter capacitors; but in most cases, they are not suitable for analog sensor supplies.
A suitable power supply assembly for separate analog and digital supply voltages could include a down converter for the digital supply and a series regulator for the analog circuit components. This solution has the advantage that each form of supply can be designed to individually suit the relevant currents or output voltages. The disadvantages lie in the possibly high number of components and large board space required for a discrete or multiple chip solution and in the high power dissipation of the series regulator. In these cases integrated solutions are a viable alternative.
In an attempt to satisfy the above requirements one possibility would be to have a down converter generate a much lower intermediate voltage. For example, it could range from 5 to 7 V (depending on the required output voltages) from the 24 V input voltage with which the back-end series regulators can then be powered. The residual ripple at the switching converter output would then play a minor role and capacitors could be used which are much smaller than those required for solutions based entirely on switching converters. Separate series regulators for both the digital and analog circuit sections would be ideal. Since the input voltage for the series regulators is now reduced and pre-stabilized, the power dissipation is kept low, even with higher currents, and comparatively small filter capacitors can also be used.
A suitable integrated solution shown in Figure 1 uses few external components. Here the down converters and two back-end series regulators are integrated on one IC, using only a minimum of external components. This also provides two output voltages which are decoupled, allowing the analog processing of sensitive sensor systems with a minimum of disturbance. The input voltage ranges from 8 to 36 V and even at full load capacity the output voltages have a low residual ripple typically about 30 mVss. The series regulators can operate with currents of up to 200 mA (VCC) or 25 mA (VCCA). In order to satisfy various requirements and cope with the necessary output voltages (3.3/5 V) and output currents (200/25 mA) the device comes in four versions, each with a different combination of output voltage at VCC and VCCA (see Figure 1).
Figure 1: Combined voltage supply for digital and analog ICs
An integrated error detection unit logs undervoltage at the regulator outputs and excessive temperature in the IC. With excessive temperature the switching converter is shutdown to reduce the chip temperature. The error output can either directly trigger an LED or act as a reset signal or an interrupt/error messenger for a central control unit.
The converter shutdown current can be suitably lowered for low output currents using the external resistor RVB. This then enables even smaller components to be employed for the converter coil.
A package measuring just 4 mm x 4 mm can meet the minimum space requirements of many applications. It should also have suitable PCB connections with a thermal pad for good heat dissipation on the underside of the package to permit operation at ambient temperatures of up to 85°C, even at maximum load.
Dipl.-Ing. Uwe M. Malzahn
Did his Master in Solid State Electronics at the University of Darmstadt in 1991 and subsequently joined iC-Haus as an IC designer for mixed-signal bipolar and CMOS ASICs. Since 2000 he is Applications Engineering Manager for optical sensors, laser diode drivers and DC/DC converters.