LM3431MHX National Semiconductor, LM3431MHX Datasheet - Page 10

• Manufacturer Part Number: LM3431MHX
• Description: 3-Channel Constant Current LED Driver with Integrated Boost Controller
• Manufacturer: National Semiconductor

www.national.com
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.
FIGURE 1. Typical Application Schematic
10
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.
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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