Datasheet

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IN
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OUT
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LM3431
SNVS547G –NOVEMBER 2007–REVISED MAY 2013 www.ti.com
OPERATION DESCRIPTION
The LM3431 combines a boost controller and 3 constant current regulator controllers in one device. To simplify
the description, these two blocks will be described separately as Boost Controller and LED Current Regulator. All
descriptions and component numbers refer to the Figure 14 schematic. The LED bottom cathode nodes (VC1 –
VC4) are referred to simply as the cathode.
BOOST CONTROLLER
The LM3431 is a current-mode, PWM boost controller. Although 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 cycles. 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 supplied by the output capacitors -
C8 and C9 in Figure 14.
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’:
(1)
where D is the duty cycle of the switch.
(2)
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 increase 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 14, 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.
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).
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