The automotive lighting system requires peak in-rush current of up to 55A. Using relays and fuses in a conventional method to switch and limit the current for power delivery can be unpredictable and unreliable. Combining a high-side pre-FET driver and power FET is an optimal solution to control a vehicle's lighting system.
Resistance of a tungsten filament in an incandescent bulb can vary more than a ratio of 1:10 over temperature. To protect against over heating the element and degradation over time, an electronic switch with programmable short-circuit and over-current conditioning is beneficial. Unlike standard and re-settable fuses, which can interrupt the power to the load once an over-current is detected and may take a variable time to reset, an electronic switch can be programmed to react more predictably.
A fuse-based system can be set only to a specific reset value and cannot accommodate a 10:1 current ratio like the electronic solution. The electronic switch can be programmed to auto retry on a periodic basis in the event of an over-current condition, or to check if the fault is removed. Also monitoring, detecting and reporting the nature of the fault helps to decipher the system easily.
The multichannel pre-FET drivers and external power FET concept provide system flexibility to optimize power dissipation and cost for load control. When compared to a fully-integrated solution, selecting power dissipation and different FET resistances independently helps prevent interaction and system malfunction due to a fault on a single channel.
The pre-FET driver power switch combination allows the system to control FET switching profiles and address any electromagnetic interference (EMI) issues when using external RC components on the gate drive output.
Pre-FET driver and N-channel power FET combination for load control
A pre-FET driver, like the TPIC44H01 from Texas Instruments, is used to control four different loads in a system. This combination works well for controlling a resistive load with temperature coefficient. Typically, the load is connected on the low side and the power FET is configured on the high side to source power to the load. Each channel can be controlled by either a parallel input signal or serial programmed register from the microcontroller. In a parallel configuration, a general purpose I/O or a timer-based output is used to control load current.
Gate drive outputs are typically a constant current source and sink to control FET gate capacitance charge and discharge characteristics. An external resistor in series with the outputs limits the rise and fall times of the FET switching transition. This effect allows slew rate control and helps to reduce fast current changes during the switching edges—which contributes to EMI (which could increase switching losses and power dissipation). These outputs are internally clamped to 17V maximum output voltage to protect the external FET gate from source breakdown. In comparison to an integrated solution, the combination of pre-FET driver and power FET can be configured to protect against dynamic and static faults in the application.
Dynamic fault threshold for in-rush currents
The illumination of a filament lamp requires a dynamic fault threshold to compensate for high in-rush currents and to prevent false triggering of over-current conditions during initial turn on. The use of an RC network with a switch can set up the dynamic fault threshold (see below). Using this method, the short-circuit current can be optimized for different incandescent lamps. The VPEAK voltage setting is when the initial over-current threshold is set, then decays at an RC time constant to a resistor divider value set on VCOMP terminal. This variable over-current threshold waveform is generated every time the gate is turned ON from the OFF state, via a parallel or serial input bit for the appropriate channels.
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The programmed VPEAK(X) value is reflected on VCOMP(X) when the particular channel is OFF. Once a particular channel is enabled, the reference voltage for detection of over-current is dynamic and represented by the internal VPEAK setting and values of external components on the VCOMP(X) terminal.
