LM3405 datasheet(12/29 Pages) TI1

12 / 29 page

LM3405

SNVS429B – OCTOBER 2006 – REVISED MAY 2013

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Figure 23. Startup Response to EN/DIM with IF = 1A

Figure 24. Average LED Current versus Duty Cycle of PWM Dimming Signal at EN/DIM Pin

UNDER-VOLTAGE LOCKOUT

Under-voltage lockout (UVLO) prevents the LM3405 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 LM3405 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 LM3405 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.

THERMAL SHUTDOWN

Thermal shutdown limits total power dissipation by turning off the internal power switch when the IC junction

temperature exceeds 165°C. After thermal shutdown occurs, the power switch does not turn on until the junction

temperature drops below approximately 150°C.

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LM3405 Datasheet(PDF) 12 Page - Texas Instruments