TLD1326EL_15 Datasheet, PDF (18/34 Pages) Infineon Technologies AG – 3 Channel High Side Current Source

7
FB Pin
The following block diagram shows the feedback pin functionality.
OUT1
OUT2
OUT3
Output voltage
feedback
TLD1326EL
FB Pin
IFB(SOC)
FB
Figure 11 Block Diagram FB pin
7.1
DC/DC Control
With the FB pin the LITIX^TM Basic IC realizes the dynamic overhead control. The IC provides a voltage feedback
to an external DC/DC converter. Using the circuit shown in Figure 20 it is possible to adjust the DC/DC output
voltage in a way that the voltage drop over the output stages of the LITIX^TM Basic IC is minimized - dynamic
overhead control. This leads to a significant reduction of the overall driver’s power dissipation and an increased
system efficiency. Figure 21 shows the same concept but, using a higher number of LEDs per LED chain (please
note that the cathode of the LED chain is connected to V_IN).
Note: For correct output current control and dynamic overhead control the parameters as specified in Pos. 7.2.1
and Pos. 7.2.2 need to be considered. FB source currents higher than given in Pos. 7.2.1 lead to a drop of
the FB regulation voltage V_FB(nom).
The resistor R_FB(PD) can be dimensioned by applying equations Equation (8) and Equation (9). The following
parameters are required:
• V_OUT represents the maximum LED loads forward voltage, i.e. number of LEDs multiplied with the maximum
LED forward voltage. Temperature drifts of the LED’s forward voltage needs to be considered!
• V_BO represents the DC/DC output voltage, which is predefined by the feedback resistors (Figure 20: R_FB1,
R_FB2, Figure 21: R_FB1, R_FB2, R_FB3). Please refer to the according DC/DC device data sheet for the dimensioning
of those resistors.
• n_len represents the numbers of LITIX^TM Basics using the longest LED-chains (e.g. if there are 3 devices
connected to one DC/DC converter and two devices using LED chains with 7 LEDs and one device is used
with LED chain lengths of 6 LEDs the according n_len = 2.)
• β represents the DC gain of the external bipolar transistor, which is connected to the DC/DC’s feedback pin.
R _{F B ( P D ,m i n )}
=
min{
-^V----^O----^U----^T----–-----0---.-5-----V-- ⋅ ----1----- ,
4 ⋅ 10^–5A ⁿlen
---------^V-----^O----^U---^T-----–----1---.--1----V------------ ⋅ ^{1---.--7----â‹}----1---0---⁵--^Ω--- }
V_BO – V_OUT – 1.1 V n_len
(8)
R _{F B ( P D ,m a x )}
=
---------^V----^O----^U----^T----–-----1---.--1----V-----------
V_{-----B----O-----–----}V_{-----O---U----T--}
R_FB1
⋅
β^{------1+-----1-}
(9)
Data Sheet
18
Rev. 1.1, 2015-03-24