LM3431EVAL National Semiconductor, LM3431EVAL Datasheet - Page 4

• Manufacturer Part Number: LM3431EVAL
• Description: BOARD EVAL FOR LM3431 LED DRIVER
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM3431EVAL

• Current - Output / Channel: 140mA
• Outputs And Type: 4, Non-Isolated
• Features: Dimmable
• Voltage - Input: 8 ~ 18V
• Utilized Ic / Part: LM3431
• Kit Contents: Board, Datasheet
• Svhc: No SVHC (15-Dec-2010)
• Kit Features: Designed To Drive 4 Parallel Strings Of 8 Series LEDs At 140mA Per String From An Input 8V To 18V
• Mcu Supported Families: LM3431
• Rohs Compliant: No
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Voltage - Output: -
• Lead Free Status / Rohs Status: Supplier Unconfirmed

www.national.com
R21 - R23
Optional Components
Several of the components shown in the schematic are not
installed and are optional. These components are typically not
required, but may be useful in some applications. C10 is pro-
vided for adding additional output capacitance or for using
more capacitors with smaller case sizes. C11 and C12 can
be used to filter reference voltage noise. Noise at the REF_IN
pin can appear as noise in the LED current. Whether these
capacitors are required depends on the amount of switching
noise, type of NPN used, and how much noise is tolerable in
the LED current. C13 is a second compensation capacitor,
which may be required if a high ESR output capacitor is used.
Refer to the datasheet for details. D10 provides a current path
from VIN to VA during startup. By bypassing the inductor, D10
prevents VA from rising above VIN at the initial power-on. In
some conditions an OVP fault may occur at startup without
D10 installed. R20 is a gate drive resistor for the NFET, Q1.
An R20 value of up to 10Ω can be used to reduce the rise and
fall time of the gate drive to reduce switching noise. To install
R20, first cut the trace between the R20 pads. R21-R23 are
NDRV pull-down resistors. A value of 300Ω to 1k can reduce
LED current noise. These resistors are typically most effective
when driving NFET current regulators. RVCC connects VCC
to VIN. For 5V input applications, a 4.7Ω resistor should be
installed here to pull-up VCC. For these applications, addi-
tional input capacitance at C16 may also be required. R-
restart enables the automatic restart function. Auto-restart
cannot be enabled when four LED channels are used. If 3 or
fewer channels are being driven, install R-restart with 0Ω to
enable the auto- restart function, which restarts the LM3431
after a fault shutdown. R-MODE selects the DIM control mode
as was described previously. For lower current applications,
there is a second Q1 pad which fits SOT-23 size NFETs. An
external NTC thermistor can be connected either between the
THM pad and SGND terminal post or via pins 7 and 8 on the
8-pin header. This thermistor is used to monitor LED temper-
ature. With the installed R17 value, the LM3431 will enter
standby when the thermistor resistance falls to 31kΩ. For ex-
ample, a thermistor with a 25°C resistance of 680k and a beta
constant of 4125 will yield a shutdown temperature of 110°C.
R25 is a hysteresis resistor for the THM function. The default
value is 0Ω which results in a restart hysteresis of 10°C. Use
R25 to increase the THM hysteresis according to the
datasheet description. C14 can be installed as a noise filter
for the external thermistor voltage.
Rrestart
Rmode
RVCC
Ref #
R18
R19
R20
R24
R25
restart select
Function
THM hys
MODE/F
NDRV
AFB
AFB
ch.4
HG
Description
not installed
not installed
not installed
not installed
not installed
33.2k
562k
0Ω
0Ω
4
LED String Options
Tables 2 and 3 give the required component changes for driv-
ing LED arrays other than the default. The values shown
assume a typical LED Vf of 3.2V at 140mA. These tables are
designed for simplicity and ease of modification, not optimum
performance. Therefore, any of the changes listed below will
result in a lower maximum contrast ratio as compared to the
default LED array. For optimum performance, all component
values should be reviewed. R5 in the tables refers to the sum
of R5a and R5b shown on the schematic. For the best per-
formance, R5 can be manually adjusted for the specific input
voltage, dimming frequency, and LED current of each appli-
cation.
2 Strings
3 Strings
4 Strings
# of LEDs
LED current
160mA
130mA
115mA
100mA
TABLE 2. LED Current Settings (nc = no change)
TABLE 3. LED Array Options at 140mA Setting
6
7
8
6
7
8
9
6
7
9
432k
499k
432k
499k
634k
432k
499k
634k
R19
Part Number
nc
nc
78.7k
93.1k
100k
115k
R7
(nc = no change)
-
-
-
-
-
80m
90m
70m
40m
R3
nc
nc
nc
nc
nc
nc
9.09k
13k
13k
13k
R8
24.9k
28.7k
30.1k
24.3k
19.1k
19.1k
16.9k
14.3k
28k
20k
R5
Manufacturer
17.4k
18.7k
-
-
-
-
-
20k
R5
nc
43.2k
26.7k
23.2k
43.2k
30.1k
43.2k
R6
nc
nc
nc
nc