Texas Instruments AN-2255, Руководство пользователя, Страница: 17 / 26

Analog voltage
at the DIMn pin
2/256
PWM dimming duty
3/256
4/256
0
5/256
5
.7
V
-V
L
SB
5
.7
V
-2
V
L
SB
5
.7
V
5
.7
V
-3
V
L
SB
256/256
5
.7
V
-4
V
L
SB
255/256
254/256
253/256
252/256
0
.8
V
+
3
V
L
SB
0
.8
V
+
2
V
L
SB
0
.8
V
+
4
V
L
SB
0
.8
V
+
V
L
SB
0
.8
V
6/256
V
LSB
=
5.7V - 0.8V
256
(Skipped) 1/256
www.ti.com
Disabling Output Channel(s)
Figure 11. Conversion characteristic of the analog voltage to PWM dimming control circuit
13 Disabling Output Channel(s)
An output channel of this evaluation board can be disabled by not connecting an LED string to the output
terminal. A disabled channel is excluded from the DHC loop and remained in OFF state until a falling edge
at the EN pin or system repower is applied. The channel 0 must be used regardless of the number of
disabled channel.
14 Cascading the LM3463 evaluation board
A number of the LM3463 evaluation boards can be cascaded to expand the number of output channels.
The connection among boards differs depending on the selected mode for dimming control. The
connection diagrams for the serial interface mode, DC interface mode and direct PWM mode are as
illustrated in the Figure 10, Figure 11 and Figure 12 respectively.
When a number of the LM3463 evaluation boards are cascaded, one of the boards must be set as master
unit and the other boards must be set as slave units. The master unit is the board which has the VFB
terminal connected to the primary power supply. The master unit controls the system startup time and
distributes dimming control signals to the slave units, the connections among the boards differs depending
on the mode of dimming control being selected.
By default, the LM3463 evaluation board is set as a master unit in direct PWM dimming mode with 100%
on duty. To set a board to be a slave unit, the resistors R2 and R4 must be removed and the terminal TP3
(VLedFB pin of the LM3463) should be connected to the terminal TP22 (VCC pin of the LM3463) using an
external connection.
In cascade operation, the number of slave units is virtually unlimited. However, in high power applications
the accumulated voltage drop on the power return part could impair the function of the DHC. Generally it
is suggested not to cascade more than four pieces of the LM3463 evaluation board to secure stable
system operation.
17 SNVA642 – May 2012 AN-2255 LM3463 Evaluation Board
Submit Documentation Feedback
Copyright © 2012, Texas Instruments Incorporated