In latest generations of car exterior mirrors, the number of embedded functions has dramatically increased: the side repeater, or directional/flasher; the door’s outside lights; the defroster; and the folding in and adjustment of the mirror, to name a few. On top of those features, the electro-chromatic (EC) control has recently been adopted, which is needed at night in case of headlight glares coming from cars behind the driver. EC mirrors automatically darken, allowing the driver to avoid being dazzled by the reflected light.
EC glass is a capacitive-resistive equivalent load, wherein its control requires a sophisticated strategy. The voltage range of the EC characteristic, between transparency and maximum dipping, lies between zero and 1.2V, with the current profile around 150mA to 250mA at its highest point at the beginning of the dip. With a classic linear controller, starting from 12V battery voltage, too much high power can be dissipated (around 12-1.2Vx0.15A=1.62W); a switching controller is not recommended since it can lead to electro-magnetic control (EMC) issues. For those reasons, the first EC control circuitries, embedded in automotive devices, were based on a shunt controller lying in parallel to the EC glass (Figure 1).
With this topology, a 6-bit DA converter, controlled by an external microprocessor, sets different (and accurate) voltage references for the shunt controller, having as a power output the N-channel MOSFET T2. The maximum power dissipation of the latter is 1.2V*0.15A= 0.18W. The resistor R1 determines the peak current that the EC glass sinks to during the dipping phase. At maximum transparency, that is zero volt control voltage, which means that no current flows into the saturated shunt controller, and T1 switches the current completely off under these operating conditions. The power needed to control the EC mirror, dissipated by the MOSFET T2 and the shunt resistor, comes mainly from the external component. The shunt resistor must be rated high wattage (its power dissipation can be around 2W in case of high battery voltage), and it must be a precision resistor (low tolerance) in order to ensure accurate current control of the EC material. Obviously, this component has a big impact on cost and space of the mirror electronics. Additionally, the new generation of EC glass requires a “fast discharge” in order to implement a prompt brightening of the mirror. Another topology must be used to optimize the control and to make it possible for mid-end cars to adopt EC control.
Historically, several actuator drivers for car door loads have been offered. These devices are characterized by a scalable actuator driving concept that is also package and software compatible, to satisfy the multiplicity of door electronics variants. Those drivers support all regular door zone loads such as door lock motors, mirror adjustment, mirror folder, heater (or defroster) and several lighting functions from incandescent lamps to LEDs. A single device has been introduced that produces the major signals required to drive vehicle door-mounted systems as well as to provide an improved method for controlling electro-chromatic rear-view mirrors. For instance, the L99DZ70XP combined driver IC from STMicroelectronics eliminates discrete drivers that would normally be mounted in multiple locations inside the door, allowing compact, integrated door-zone controllers that are easier to fit and deliver greater long-term reliability. The L99DZ70XP includes control signals for a small external MOSFET driving the EC element (Figure 2). This saves space and cost, and also allows closed-loop current control for enhanced reliability.
The voltage of the electro-chromic element, connected at pin ECV, is controlled to a target value (0...1.2V), which is set by six internal bits. An on-chip differential amplifier and an external MOS source follower, with its gate connected to pin ECDR and which drives the EC mirror voltage at pin ECV, form the control loop. The drain of the external MOS transistor is supplied by OUT10. A diode from pin ECV (anode) to pin ECDR (cathode) has been placed on the chip to protect the external MOS source follower. The external capacitor, at least of 5nF, is added to pin ECDR for improving loop-stability.
The target voltage is binary coded with a full scale range of 1.5V. If a certain bit of the internal control register is set to “1”, the maximum controller output voltage is clamped to 1.2V without changing the resolution of relevant bits. When setting the target voltage to 0V and programming the ECVLS driver to on-state, the voltage at pin ECV is pulled to ground by a 1.6Ω low-side switch (fast discharge). Also available is a “too high” and a “too low” control of the EC element: this facilitates the detection of any unexpected voltage behavior of the component. The status of the voltage control loop and diagnostics are reported via SPI.
This new topology, compared to the first adopted for EC control, has only a single element (outside of the door zone device) that dissipates all the power needed for controlling the glass. This is not an expensive component because, as it always works in its linear zone, its Rds(ON) is not a crucial parameter. That small MOSFET has to be chosen with a package option suitable for handling the dissipated power. ST recommends the STD18NF03L, a 50mΩ MOSFET having BVdss of 30V. It comes in the well known DPAK package, a popular package for the automotive environment.
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Caption
Figure 1: The first EC control circuitries, embedded in automotive devices, were based on a shunt controller lying in parallel to the EC glass.
Figure 2: The L99DZ70XP includes control signals for a small external MOSFET driving the EC element.
