Table 1: Variations in the VF of different LED materials/colors.
Figure 3: The 4-channel White LED Driver
BD81A74EFV-M from ROHM is automotive rated.
(Source: Rohm Semiconductor)
Figure 4: BD81A74EFV-M IC Application example setup with a boost
converter. (Source: ROHM Semiconductor)
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the current. However, engineers must be careful of
electromagnetic interference (EMI) from these circuits
bothering the scores of other electronics packed into
every corner of the vehicle.
Some drivers are buck-boost, which allows the circuit
to handle a high number of series-connected LEDs.
Forward voltage for a blue LED can be as high as 4V at
20ma, and if you run just three in series, you need 12V,
so running from the 12V battery voltage will not work,
even without the fluctuations of the battery voltage.
ROHM BD81A74EFV-M is a great
example of a 4-channel white LED
driver. It handles bus voltages up to
35V and will operate from as low as
4.5V and features a buck-boost or
boost-current driver using an external
inductor. Each of its channels can draw
up to 120mA with ±3 percent accuracy.
PWM frequency is 200kHz to 2.2MHz,
and operating temperature is the full
-40° to 125°C. The controller sets the
current to each channel without regard
for forward voltage, so the channels do
not need to be balanced.
