LM3405AXMY datasheet(13/32 Pages) TI

13 / 32 page

LM3405A

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SNVS508C – OCTOBER 2007 – REVISED MAY 2013

Figure 21. PWM Dimming of LEDs

Figure 22. Startup Response to EN/DIM

Using the EN/DIM Pin

with IF = 1A

Figure 23. Average LED Current

Duty Cycle of PWM Dimming Signal at EN/DIM Pin

UNDER-VOLTAGE LOCKOUT

Under-voltage lockout (UVLO) prevents the LM3405A from operating until the input voltage exceeds 2.74V

(typical). The UVLO threshold has approximately 440mV of hysteresis, so the part will operate until VIN drops

below 2.3V (typical). Hysteresis prevents the part from turning off during power up if VIN is non-monotonic.

CURRENT LIMIT

The LM3405A uses cycle-by-cycle current limit to protect the internal power switch. During each switching cycle,

a current limit comparator detects if the power switch current exceeds 2.0A (typical), and turns off the switch until

the next switching cycle begins.

OVER-CURRENT PROTECTION

The LM3405A has a built in over-current comparator that compares the FB pin voltage to a threshold voltage that

is 60% higher than the internal reference VREF. Once the FB pin voltage exceeds this threshold level (typically

328mV), the internal NMOS power switch is turned off, which allows the feedback voltage to decrease towards

regulation. This threshold provides an upper limit for the LED current. LED current overshoot is limited to

328mV/R1 by this comparator during transients.

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LM3405AXMY 데이터시트(PDF) 13 Page - Texas Instruments