LM3431AQMH/NOPB National Semiconductor, LM3431AQMH/NOPB Datasheet - Page 11

• Manufacturer Part Number: LM3431AQMH/NOPB
• Description: IC LED DRVR HP CONS CURR 28TSSOP
• Manufacturer: National Semiconductor
• Series: PowerWise®r
• Type: High Power, Constant Currentr

Specifications of LM3431AQMH/NOPB

• Constant Current: Yes
• Topology: PWM, Step-Up (Boost)
• Number Of Outputs: 3
• Internal Driver: No
• Type - Primary: Automotive
• Type - Secondary: High Brightness LED (HBLED)
• Frequency: 651kHz ~ 1.1MHz
• Voltage - Supply: 4.5 V ~ 36 V
• Mounting Type: Surface Mount
• Package / Case: 28-TSSOP Exposed Pad, 28-eTSSOP, 28-HTSSOP
• Operating Temperature: -40°C ~ 125°C
• Current - Output / Channel: 200mA
• Internal Switch(s): Yes
• Efficiency: 88%
• Led Driver Application: Display Backlighting, Automotive Lighting
• No. Of Outputs: 3
• Output Current: 200mA
• Output Voltage: 40V
• Input Voltage: 5V To 37V
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Voltage - Output: -
• Other names: LM3431AQMH

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’:
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.
11
The EN UVLO should be set just below the minimum input
voltage for the application.
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.
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
FIGURE 2. Typical Startup Waveforms
(From power-on, DIM = high)
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