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PDF LM3431 Data sheet ( Hoja de datos )
| Número de pieza | LM3431 | |
| Descripción | 3-Channel Constant Current LED Driver | |
| Fabricantes | National Semiconductor | |
| Logotipo |  |
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LM3431/LM3431A/LM3431Q/
LM3431AQ
March 10, 2009
www.DataSheet4U.com
3-Channel Constant Current LED Driver with Integrated
Boost Controller
General Description
The LM3431 is a 3-channel linear current controller combined
with a boost switching controller ideal for driving LED back-
light panels in space critical applications. The LM3431 drives
3 external NPN transistors or MOSFETs to deliver high ac-
curacy constant current to 3 LED strings. Output current is
adjustable to drive strings in excess of 200 mA. The LM3431
can be expanded to drive as many as 6 LED strings.
The boost controller drives an external NFET switch for step-
up regulation from input voltages between 5V and 36V. The
LM3431 features LED cathode feedback to minimize regula-
tor headroom and optimize efficiency.
A DIM input pin controls LED brightness from analog or digital
control signals. Dimming frequencies up to 25 kHz are pos-
sible with a contrast ratio of 100:1. Contrast ratios greater than
1000:1 are possible at lower dimming frequencies.
The LM3431 eliminates audible noise problems by maintain-
ing constant output voltage regulation during LED dimming.
Additional features include LED short and open protection,
fault delay/error flag, cycle by cycle current limit, and thermal
shutdown for both the IC and LED array. The enhanced
LM3431A features reduced offset voltge for higher accuracy
LED current.
Features
■ LM3431Q/LM3431AQ are Automotive Grade products
that are AEC-Q100 grade 1 qualified (-40°C to 125°C
operating junction temperature)
■ 3-channel programmable LED current
■ High accuracy linear current regulation
■ Analog and digital PWM dimming control
■ Up to 25kHz dimming frequency
■ >100:1 contrast ratio
■ Integrated Boost controller
■ 5V-36V input voltage range
■ Adjustable switching frequency up to 1MHz
■ LED short and open protection
■ Selectable fault shutdown or automatic restart
■ Programmable fault delay
■ Programmable cycle by cycle current limit
■ Output Over Voltage Protection
■ No Audible Noise
■ Enable pin
■ LED Over-Temperature shutdown input
■ Thermal Shutdown
■ TSSOP-28 exposed pad package
Applications
■ Automotive Infotainment Displays
■ Small to Medium Format Displays
Typical Application Circuit
© 2009 National Semiconductor Corporation 300411
30041101
www.national.com
1 page  
Symbol
bw
VLEDOFF
VDIM
TDIM
DIMDLY_R
DIMDLY_F
THMODE/F
IMODE/F
VMODE_L
VMODE_H
Protection
VSC_SHORT
VSC_OPEN
IDLY_SC
IDLY_SK
VDLY
VDLY_reset
TDLY_BLK
VTHM
ITHM
IILIM
VAFB_max
VAFB_UVP
TSD
Boost controller
VCFB
ICFB
CFBTC
ΔVCFB / ΔVIN
ISS/SH
VSS_END
VRT
FSW
Ton_min
DMAX
ILIMgm
Vslope
ICOMP_SC
ICOMP_SK
EAgm
RLG
Parameter
SNSx amp bandwidth
LEDOFF voltage
DIM threshold
hysteresis
Minimum internal DIM pulse width
(Note 8)
DIM to NDRV delay time
DIM to NDRV delay time
MODE/F threshold
MODE/F source/sink current
MODE/F minimum voltage
MODE/F peak voltage
SC high threshold
SC open clamp voltage
DLY source current
DLY sink current
DLY threshold voltage
DLY reset threshold voltage
DLY blank time
THM threshold
THM hysteresis current
ILIM max source current
AFB overvoltage threshold
AFB undervoltage threshold
Thermal shutdown threshold
CFB voltage
CFB source current
CFB temperature coefficient
CFB Line regulation
SS/SH source current
SS/SH voltage
RT voltage
Switching Frequency
Minimum Switching Frequency
Maximum Switching Frequency
Minimum on time
Maximum duty cycle
ILIM amplifier transconductance
Slope compensation
COMP source current
COMP sink current
Error amplifier transconductance
Gate Drive On Resistance
ILG Driver Output Current
Conditions
At unity gain
DIM low
MODE/F > 4V
DIM rising
DIM falling
For Digital Dimming control
Analog dimming mode
Analog dimming mode
Min Typ Max Units
ww2w.DataSheetM4UH.cz om
5V
1.9 2.3 V
0.8 V
0.4 µs
100 ns
90 ns
3.8 V
40 uA
0.37 V
2.5 V
LED short circuit fault, SC rising
LED open circuit fault, SC rising
DLY = 1.0V
DLY = 1.0V
DLY rising
DLY falling
DIM rising
THM = 1V
COMP = 2.0V
AFB falling
5.7 6 6.2 V
3.16 3.50 3.87 V
39 57 73 µA
1.8 µA
2.40 2.8 3.16 V
350 mV
1.6 µs
1.19 1.23 1.27 V
9.6 µA
31 40 46 µA
1.87 2.0 2.22 V
0.73 0.85 0.98 V
160 °C
DIM high
DIM high
5.5V < VIN < 36V
At EN going high
At end of soft-start cycle
RRT = 34.8 kΩ
RRT = 34.8 kΩ
RRT = 130 kΩ
RRT = 22.6 kΩ
COMP to ILIM gain
Peak voltage per cycle
VCOMP = 1.2V, AFB = 0.5V
VCOMP =1.2V, AFB = 1.5V
CFB to COMP gain, DIM high
Source Current = 200 mA, VIN = 5.5V
Sink Current = 200 mA
Source, LG = 2.5V, VIN = 5.5V
Sink, LG = 2.5V
1.60 1.71 1.82 V
35 50 65 µA
-2.6 mV/°C
0.001
%/V
13 19 24 µA
1.80 1.85 1.90 V
1.22 V
651 700 749 kHz
180 200 220
900 1000 1100
170 230 ns
80 85
%
85 umho
75 mV
155 µA
150 µA
230 umho
6.4 Ω
2.2 Ω
0.35 A
0.70 A
5 www.national.com
5 Page 
Operation Description
The LM3431 combines a boost controller and 3 constant cur-
rent regulator controllers in one device. To simplify the de-
scription, these two blocks will be described separately as
Boost Controller and LED Current Regulator. All descriptions
and component numbers refer to the Figure 1 schematic. The
LED bottom cathode nodes (VC1 – VC4) are referred to sim-
ply as the cathode.
Boost Controller
The LM3431 is a current-mode, PWM boost controller. Al-
though the LM3431 may be operated in either continuous or
discontinuous conduction mode, the following guidelines are
designed for continuous conduction operation. This mode of
operation gives lower output ripple and better LED current
regulation.
In continuous conduction mode (when the inductor current
never reaches zero), the boost regulator operates in two cy-
cles. In the first cycle of operation, the NFET is turned on and
current ramps up and is storing energy in the inductor. During
this cycle, diode D1 is reverse biased and load current is sup-
plied by the output capacitors - C8 and C9 in Figure 1.
In the second cycle, the NFET is off and the diode is forward
biased. Inductor current is transferred to the load and output
capacitor. The ratio of these two cycles determines the output
voltage and is expressed as D or D’:
The EN UVLO should be set just below the minimum input
voltage for the application.
www.DataSheet4U.com
The internal UVLO is monitored at the VCC pin. When VCC
is below the threshold of 4.4V, the LM3431 is in standby mode
(See VCC section).
SOFT-START
The SS/SH pin is a dual function softstart and sample/hold
pin. The SH function is described in sections below. When the
EN pin is pulled above the programmable UVLO threshold
and VCC rises above the internal UVLO threshold, the SS/SH
pin begins sourcing current. This charges the SS cap (C6)
and the SS pin voltage in turn controls the output voltage
ramp-up, sensed via the AFB pin. The softstart capacitor is
calculated as shown below, where 20 µA is the typical soft-
start source current:
The LED current regulators are held off until softstart is com-
pleted. During softstart, current limit is active and the CFB pin
is monitored for a cathode short fault (See LED protection
section). When the SS/SH voltage reaches 1.85V, the current
regulators are activated, LED current begins flowing, and out-
put voltage control is transferred to the CFB pin. Typical
startup is shown below in Figure 2.
where D is the duty cycle of the switch.
Maximum duty cycle is limited to 85% typically.
As input voltage approaches the nominal output voltage, duty
cycle and switch on time are reduced. When the on time
reaches minimum, pulse skipping will occur. This increases
the output ripple voltage and can cause regulator saturation
and poor LED current regulation. If input voltage equals or
exceeds the set output voltage, switching will stop and the
output voltage will become unregulated. This will force an in-
crease in the LED cathode voltage and NPN regulator power
dissipation. Although this condition can be tolerated, it is not
recommended.
Therefore, input voltage should be restricted to keep on time
above the minimum (see Switching Frequency section) and
at least 1V below the set output voltage.
ENABLE and UVLO
The EN pin is a dual function pin combining both enable and
programmable undervoltage lockout (UVLO). The shutdown
threshold is 0.75V. When EN is pulled below this threshold,
the LM3431 will shutdown and IQ will be reduced to 15 µA
typically. The typical EN pin UVLO threshold is 1.23V. When
the EN voltage is above this threshold, the LM3431 will begin
softstart. Below the UVLO threshold, the LM3431 will remain
in standby mode. A resistor divider, shown as R1 and R2 in
Figure 1, can be used to program the UVLO threshold at the
EN pin. This feature is used to shutdown the IC at an input
voltage higher than the internal VCC UVLO threshold of 4.4V.
30041121
FIGURE 2. Typical Startup Waveforms
(From power-on, DIM = high)
OUTPUT VOLTAGE, OVP, and SH
The LM3431 boost controls the LED cathode voltage in order
to drive the LED strings with sufficient headroom at optimum
efficiency. When the LED strings are on, voltage is regulated
to 1.7V (typical) at the CFB pin, which is one diode Vf above
the LED cathode voltage. Therefore, when the LED strings
are on, the output voltage (LED anodes, shown as VA in Fig-
ure 1) will vary according to the Vf of the LED string, while the
LED cathode voltage will be regulated via the CFB pin.
The AFB pin is used to regulate the output voltage when the
LED strings are off, which is during startup and dimming off
cycles. During LED-off times, the cathode voltage is not reg-
ulated.
AFB should set the initial output voltage to at least 1.0V (CFB
voltage minus one diode drop) above the maximum LED
string forward voltage. This ensures that there is enough
headroom to drive the LED strings at startup and keeps the
11 www.national.com
11 Page | ||
| Páginas | Total 24 Páginas | |
| PDF Descargar | [ Datasheet LM3431.PDF ] | |
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