User manual

0
V
DHC_READY
V
RAIL(nom)
V
LED
V
RAIL
Time
t
ST-1
t
ST-2
t
ST-3
t
ST-1
open
R
ISR
Startup time
t
ST-2
high
t
ST-3
low
V
IN-UVLO
6.67V
www.ti.com Reducing the System Startup Time
9 Reducing the System Startup Time
The total system startup time is generally dependent on the frequency response of both the primary power
supply and DHC loop of the LM3463. The slower response of the two circuits, the longer time the system
takes to startup. Because the response of the primary is usually not user programmable, the overall
system startup time can be reduced by shortening the time for the V
RAIL
to increase from V
RAIL(nom)
to
V
DHC_READY
, namely the t
ST
. as shown in Figure 8. The t
ST
is adjusted dependent on the value of the C4 and
RISR, which governed by the following equation:
(2)
where
(3)
Figure 8. Adjusting the tST with different value of RISR
The R9 on this evaluation board is opened by default, thus the system startup time is set to the longest.
The startup time of the board can be reduced by putting a 0603 resistor to the position of R9. The value of
the R9 should be no less than 130kΩ.
10 MOSFET Power Dissipation Limit
As the drain voltage of the MOSFETs (Q1, Q2, Q3, Q4, Q5 and Q6) exceeds four times the voltage of the
DRVLIM pin, the output currents are reduced to reduce the power dissipations on the MOSFETs. The
DRVLIM of the LM3463 of this evaluation board is connected to VCC via a 0Ω resistor, R37. Thus the
drain voltage threshold to perform MOSFET power dissipation limit is about 26.4V by default.
11 Analog Dimming Control
The reference voltage for the LED current regulators can be adjusted by changing the voltage at the
IOUTADJ pin of the LM3463. In this evaluation board, the reference voltage for current regulation is set to
200mV by connecting the IOUTADJ pin to VCC via a 0Ω resistor, R10. By default the pull-down resistor to
the IOUTADJ pin, R11 is opened. To adjust the IOUTADJ pin voltage, the R10 and R11 should be
replaced according to the required output current following the equation below:
9
SNVA642A–May 2012–Revised May 2013 AN-2255 LM3463 Evaluation Board
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