TLD5098EL Datasheet, PDF (27/39 Pages) Infineon Technologies AG – DC/DC Boost, Buck-Boost, SEPIC controller

TLD5098EL
Analog Dimming
10
Analog Dimming
This pin is influencing the Feedback Voltage Error Amplifier by generating an internal current accordingly to an
external reference voltage (V_SET). If the analog dimming feature is not needed this pin must be connected to IVCC
or external > 1.6V supply. Different application scenarios are described in Figure 18. This pin can also go outside
of the ECU for instance if a thermistor is connected on a separated LED Module and the Analog Dimming Input is
used to thermally protect the LEDs. For reverse battery protection of this pin an external series resistor should be
placed to limit the current.
10.1
Purpose of Analog Dimming:
1) It is difficult for LED manufacturers to deliver LEDs which have the same Brightness, Colorpoint and Forward
Voltage Class. Due to this relatively wide spread of the crucial LED parameters automotive customers order LEDs
from one or maximum two different colorpoint classes. The LED manufacturer must preselect the LEDs to deliver
the requested colorpoint class. Those preselected LEDs are matched in terms of the colorpoint but a variation of
the brightness remains. To correct the brightness deviation an analog dimming feature is needed. The mean LED
current can be adjusted by applying an external voltage V_SET at the SET pin.
2) If the DC/DC application is separated from the LED loads the ECU manufacturers aim is to develop one
hardware which should be able to handle different load current conditions (e.g. 80mA to 400mA) to cover different
applications. To achieve this average LED current adjustment the analog dimming is a crucial feature.
10.2
Description
Application Example:
Desired LED current = 400mA. For the calculation of the correct Feedback Resistor R_FB the following equation can
be used: This formula is valid if the analog dimming feature is disabled and V_SET > 1.6V.
I _LED
=
V_REF
R_FB
-->
R_FB
=
V_REF
I _LED
-->
R_FB
=
0.3V _{= 750mΩ}
400mA
A decrease of the average LED current can be achieved by controlling the voltage at the SET pin (V_SET) between
0V and 1.6V. The mathematical relation is given in the formula below:
I _LED
= ^{V SET − 0 ,1 V}
5 * R _FB
If V_SET is 100mV the LED current is only determined by the internal offset voltages of the comparators. For this
example I_LED = 0A if V_SET < 100mV. Refer to the concept drawing in Figure 17.
Datasheet
27
Rev. 1.0, 2010-10-13