LM3410_08 Datasheet, PDF (9/32 Pages) National Semiconductor (TI) – PowerWise® 525kHz/1.6MHz, Constant Current Boost and SEPIC LED Driver with Internal Compensation

Design Guide
SETTING THE LED CURRENT
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FIGURE 2. Simplified Boost Topology Schematic
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FIGURE 4. Setting I_LED
The LED current is set using the following equation:
FIGURE 3. Typical Waveforms
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CURRENT LIMIT
The LM3410 uses cycle-by-cycle current limiting to protect
the internal NMOS switch. It is important to note that this cur-
rent limit will not protect the output from excessive current
during an output short circuit. The input supply is connected
to the output by the series connection of an inductor and a
diode. If a short circuit is placed on the output, excessive cur-
rent can damage both the inductor and diode.
where R_SET is connected between the FB pin and GND.
DIM PIN / SHUTDOWN MODE
The average LED current can be controlled using a PWM
signal on the DIM pin. The duty cycle can be varied between
0 & 100% to either increase or decrease LED brightness.
PWM frequencies in the range of 1 Hz to 25 kHz can be used.
For controlling LED currents down to the µA levels, it is best
to use a PWM signal frequency between 200-1 kHz. The
maximum LED current would be achieved using a 100% duty
cycle, i.e. the DIM pin always high.
LED-DRIVE CAPABILITY
When using the LM3410 in the typical application configura-
tion, with LEDs stacked in series between the VOUT and FB
pin, the maximum number of LEDs that can be placed in se-
ries is dependent on the maximum LED forward voltage
(VF_MAX).
(VF_MAX x N_LEDs) + 190 mV < 24V
When inserting a value for maximum VF_MAX the LED forward
voltage variation over the operating temperature range
should be considered.
THERMAL SHUTDOWN
Thermal shutdown limits total power dissipation by turning off
the output switch when the IC junction temperature exceeds
165°C. After thermal shutdown occurs, the output switch
doesn’t turn on until the junction temperature drops to ap-
proximately 150°C.
INDUCTOR SELECTION
The inductor value determines the input ripple current. Lower
inductor values decrease the physical size of the inductor, but
increase the input ripple current. An increase in the inductor
value will decrease the input ripple current.
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