Versatile body controllerIn modern cars, a large network of electronic parts and switches are present. Safety and reliability are high priorities in automotive grade integrated circuits. More and more often, switch relays are being replaced by power FETs. Switching such FETs needs to be done in a controlled and careful way.
Diagnostics and functions like FET temperature, load current and slew rate regulation become more and more important, explaining the need for an integrated solution: the versatile body controller to provide control of multiple external power switches.
In an automotive environment, electrically, there are lots of sources that introduce noise and voltage spikes on the supply circuit of a car or a truck. One can think of noise coming from switching large inductive loads like wind screen wipers, voltage dips introduced by starting the cars' engine, and high frequency noise from the dynamo itself. Voltages up to 100V can occur in normal cars with a 12V power net, or trucks with a 24V power net.
In such a rough environment, several protections and detections [diagnostic features] need to be present for each IC pin or each of the eight externally connected FETs, like:
- ESD protections.
- Over voltage protections.
- Under voltage detections.
- Over temperature protections.
- Over current [short circuit] protections.
- Under current [open circuit] detections.
- Reverse battery polarity protection.
Other features of the IC are:
- Analogue and digital drain current measurements.
- High side and low side [including full bridged pair] configurable.
- Pulse Width Modulation [PWM] switching with programmable frequency and duty cycle.
- Watchdog for invalid or inactive SPI communication.
- Programmable interrupt controller.
All protections are present for the task of protecting the FET and the connected loads, and thus increasing the rate of reliability and safety.
All these protections need to stay accurate under all circumstances. In addition, the switching of power FETs needs to be done in a controlled way, with an accurately determined slew rate, in order to minimise EMC emission, while maintaining the ability of sufficiently fast switching at 512Hz in case of PWM mode.
In order to withstand the electrically rough automotive environment, the controller has been designed in a high voltage process that can cope up with voltages up to 120V. An internal voltage clamp will limit the voltage over the IC. The device is protected against accidental reverse battery polarity, and will in such an event switch on the connected FET to prevent current from flowing through the parasitic source-drain diode of the power FET.
Since the controller can be used to control both low side [switch between load and ground] and high side [switch between battery and load] power switches, an internal charge pump with external charge pump capacitors has been implemented. This charge pump can generate and deliver a voltage higher than the battery voltage, which is needed to steer the gate of the power FET in case of a high side configured switch. In case of a full bridge pair [useful in case of wind screen wiper motors], an underlap protection will prevent both switches from being simultaneously on.
The controller contains eight FET interfaces capable of independently controlling eight power FETs. Each of the eight power FET channels can be programmed to trip [switch off] at a predetermined drain current and/or FET temperature. In addition, an internal ADC can provide information about nominal drain current. Since each of the channels can be configured to run in PWM mode, for certain loads like light bulbs the frequency and duty cycle can be programmed in order to dim the lights. This way, the controller can adapt the brightness [power consumption] of the bulb in such a way, that it becomes independent of the unique battery voltage of each car, increasing the bulbs' life time.
In order to minimise EMC emission, a controlled and programmable slew rate is used to switch the power FETs on and off. In case of a trip condition, a faster slew rate is used to switch off the FET, to protect the FET and its load. This way, both the external FETs and the connected loads can survive repetitive over current situations, or even short circuit events, without the need for replacing fuses.
If severe problems occur, like miscommunication, or even total communication black outs, an internal watchdog will switch the IC to safety mode, enabling some direct inputs of the IC, so that major functions like brake lights or emergency lights will stay functional.