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Riding a motorcycle with a faulty turn indicator is dangerous for both the driver and the other vehicles on the road. Turn signals tell the other drivers where the driver is going and allows them to react to what the motorbike is doing. For this reason the Module driving the Blinkers, on top of providing proper oscillating frequency, must provide indication that a Load disconnection is occurring; each time one of the bigger wattage lamps is missing, the operative frequency doubles indicating the driver that something ‘wrong’ is happening. For small motorbike vehicles, up to 150cc, the most common bulbs configuration is 2x10W bulbs + 1.7W on each side; it is requested that, every time a 10W bulb is disconnected, the driving frequency is doubled respect to the nominal one.
In this type of Applications there is no mC setting and controlling frequency of lamps: it is all included in the IC that embeds the Power Part to supply the Current to the Loads and the Logic for controlling all the operative modes (normal frequency, double frequency in case of disconnection of one main load, etc).
In such a configuration the Smart Driver (IC) it is connected between the battery positive terminal (VCC pin) and a mechanical switch to the right or left side. As soon as the series switch connects the OUT pin to the bulbs, the device begins to turn on/off with a 50% duty cycle. External low voltage capacitors are needed for supplying the device, for stabilizing the internal voltage regulator output and for setting the oscillating frequency.
By construction, no direct GND connection is possible: with that peculiar topology, many design challenges have been solved for providing a very robust, reliable and accurate solution. The article will specifically focus how the novel ST’s VN5MB02 device provides high accuracy in detecting Load disconnection and guarantees a proper Fast Lane change by means of a ST patented solution.
Indication of Load disconnection is requested to be fully working in all the Battery range (9-16V) and keeping into account that, for production spread of aging effects, Bulb wattages may change (in the range of +/-10.5%).
The VN5MB02 internally keeps as reference the IDF current level; each time the load current goes below to such a threshold the device, at the next ON time, doubles the operating frequency. The IDF range must be tight enough to allow:
-No change of frequency in case of Nominal Load -10.5% due to Bulb tolerance
-Activating ‘double frequency’ in case of one 10W missing including the +10.5% of Bulb tolerance
In the following picture, considering 2x10W+1.7W scenario, three bands (min-max) are shown:
-Green stripe represents nominal load with +/- 10.5% accuracy on lamps power
-Blue stripe represents Idf range of VN5MB02 (min, typ, max)
-Pink stripe represents Load current when 1x10W bulb is disconnected and it include +/- 10.5% tolerance.
For a proper Open Load operation, the IDF range must not overlap any of the other two ranges; this leads to an overall IDF precision, for the VN5MB02, of around +/-8%; this can be considered the new State of the Art for Self-oscillating system like the one used for Motorbikes. Such an accurate precision also allows discriminating a 10W bulb missing in Load configurations like 2x10W+3W and 2x10W+0.3W.
Nominal blinker frequency, when all the lamps are functional, is set in order to have 85 cycles per minute. In normal condition the frequency is around 1.44Hz with 50% duty cycle, and becomes doubles when a 10W lamp is disconnected (around 170 cycles per minute). VN5MB02 embeds a digital control of the frequency to allow, also for those parameters, high accuracy in the oscillating frequency (both at nominal and open load conditions).
When a nominal load is connected to the device Output pin, the device oscillates by charging the frequency capacitor (Cfreq) up to the threshold voltage Vch, and then discharging same capacitor to the threshold voltage Vcl by an internal constant current.
The self-oscillating frequency of the device is determined by the relation
n is the internal digital counter (equal to 1066).
Nominal Mode (all the nominal loads connected)
If the load current is lower than Idf, the device will detect the low load at the end of the ON phase, and will double the oscillating frequency.
Where Kfreq is between 2.15 and 2.30.
Double Frequency Mode (1x10W disconnected)
Special care has to be placed on two parameters:
-first turn-on (known also as ‘Lighting starting time’)
-minimum turn-on duration
Geographical regulations clearly describe those two timings:
-the delay of first pulse, just after the mechanical switch has been closed, generally has to be sufficiently short because it corresponds to the delay at which the first turn on of the lamp occurs. The delay in first turn on of the lamp, indicate in the below image as delON, should be timely because it indicates to the drivers of surrounding vehicles that the vehicle is changing direction. As per the Motorbike lighting regulations, the value of the delay of the first pulse should be less than 100 ms.
-It also requested to control the value of the tON pulses: the value of this length should be sufficiently high to enable the human eye of the driver of an adjacent vehicle to perceive that a blinker has been activated. Length of the first, and of any other pulse, should be greater than or equal to 200 ms.
How many times the Motorbike blinker triggers a ‘first turn-on event’? The answer is pretty simple: the first turn-on is triggered every time the mechanical switch activates one side (left or right) of the driven load configuration. In the below pictures two different Lane changes are shown, where a ‘first turn-on event’ is triggered and where both minimum tON and maximum delay time must be achieved.
ST has patented a Novel solution that guarantees, in the described conditions, proper blinker operation: every time the user activates the blinker, from left to right, right to left and in general in every case after the mechanical switch has been closed, a right timing is provided. When a Lane Change occurs, the series switch disconnection may happen in two cases:
-the device was already in its OFF phase
-the device was already in its ON phase
No matter in which timing condition the device was, before the change of direction, the ST patented solution guarantees correct blinker operation.
The VN5MB02 comes with protection for external, unwanted, short circuit (hard and soft). The internal current shutdown latches the VN5MB02-E when a heavy overload occurs and, if the overload even is removed, the device may be turned on at the following on time.
Shutdown in case of a hard short circuit
The thermal limitation reduces the stress on the device if the junction temperature rises; If the overload condition lasts more than a time tfault, the VN5MB02-E is latched; when the overload condition is not anymore present, device can be turned on at the following on time
Thermal Limitation in case of a Soft short circuit
Note: The VN5MB02 is housed in standard SO-16 Narrow, plastic package, and it is already available for production.