Switching regulators are inevitable in most modern ECU’s used in automobiles to generate the power supply for different electronic components. Harsh environmental conditions, stringent quality and legal requirements can make the simplest switching regulator to fail, if it is not designed and verified correctly. When a switching regulator system fails, most of the cases we think that something is not correct with the regulator system control device (Integrated Circuit-IC). But a regulator IC can work fine only when the external circuitry and application conditions meet its requirements. Also like any other component, a regulator IC has its own tolerances and robust regulator system design should take such tolerances in to account and validation should catch any marginality in the design.
Key component selections
Regulator IC/Device selection
Switching regulator IC with many topologies and features are available in the market. Depending on the application and cost requirements, devices with certain features and topology can be selected. With voltage mode control devices, pay special attention to compensation (due to its complex compensation scheme). In case of current mode control devices with more than 50% duty cycle, watch out for sub harmonic oscillations (due to insufficient slope compensation). Hysteretic Control devices have switching frequency variation and may need output capacitors with certain minimum ESR value. For regulators with internal compensation, often the range of LC product (output Inductor and Capacitor value) is specified in the datasheet. It is important to follow these recommendations as internally compensated regulators are optimized only for a small range of LC product. Safety related applications require special devices with certain safety features like independent monitoring of supply rails with independent bandgap and watchdog. Some devices have EMI performance related features like slew rate control and spread spectrum modulation.
Pay attention to the device absolute maximum rating on each device pin and have sufficient margin to accommodate the supply variations and any transient overshoot/undershoot voltages. Absolute maximum ratings violations on input pin, SW/Phase pin and load current are very common root causes for IC damage.
The output inductor is one of the most critical components in the switching regulator and it has major impact on the regulator performance. Follow the recommendations and equations provided in the datasheet to calculate the inductor value. If the inductor range is specified in the datasheet, often this value is effective inductance value considering its tolerances, temperature and load current. Inductor series resistance (DCR) has an impact on the regulator efficiency and the inductor with lower DCR value should be selected for better efficiency. When choosing small chip inductors (like metal alloy or thin film), apart from current ratings, consider their voltage ratings too. In terms of Electro Magnetic Interference (EMI) performance, different inductors could show different performance and it is advisable to check the inductors EMI performance before finalizing it. Figure-1 shows how the inductor current will raise quickly when the inductor saturates in a typical buck regulator. Inductor saturation current rating should be more than the device current limit specifications.
Figure 1: Showing the saturated inductor current and non-saturated inductor