LM3405XEVAL National Semiconductor, LM3405XEVAL Datasheet - Page 9

• Manufacturer Part Number: LM3405XEVAL
• Description: BOARD EVALUATION LM3405X
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM3405XEVAL

• Current - Output / Channel: 1A
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 0.2 ~ 13.5 V
• Features: Dimmable
• Voltage - Input: 3 ~ 15V
• Utilized Ic / Part: LM3405
• Kit Contents: Board, Datasheet
• Svhc: No SVHC (15-Dec-2010)
• Kit Features: Cycle-by-Cycle Current Limit,
• Rohs Compliant: No
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

LED PWM DIMMING
The LED brightness can be controlled by applying a periodic
pulse signal to the EN/DIM pin and varying its frequency and/
or duty cycle. This so-called PWM dimming method controls
the average light output by pulsing the LED current between
the set value and zero. A logic high level at the EN/DIM pin
turns on the LED current whereas a logic low level turns off
the LED current. Figure 10 shows a typical LED current wave-
form in PWM dimming mode. As explained in the previous
section, there is approximately a 100µs delay from the EN/
DIM signal going high to fully establishing the LED current as
shown in Figure 11. This 100µs delay sets a maximum fre-
quency limit for the driving signal that can be applied to the
EN/DIM pin for PWM dimming. Figure 12 shows the average
LED current versus duty cycle of PWM dimming signal for
various frequencies. The applicable frequency range to drive
LM3405 for PWM dimming is from 100Hz to 5kHz. The dim-
ming ratio reduces drastically when the applied PWM dim-
ming frequency is greater than 5kHz.
FIGURE 10. PWM Dimming of LEDs using the EN/DIM Pin
FIGURE 9. Startup Response to V
IN
with 200µs rise time
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UNDER-VOLTAGE LOCKOUT
Under-voltage lockout (UVLO) prevents the LM3405 from op-
erating until the input voltage exceeds 2.74V (typical). The
UVLO threshold has approximately 440mV of hysteresis, so
the part will operate until V
teresis prevents the part from turning off during power up if
V
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 com-
pares the FB pin voltage to a threshold voltage that is 60%
higher than the internal reference V
age exceeds this threshold level (typically 328mV), the inter-
nal NMOS power switch is turned off, which allows the
feedback voltage to decrease towards regulation. This
FIGURE 12. Average LED Current versus Duty Cycle of
IN
FIGURE 11. Startup Response to EN/DIM with I
is non-monotonic.
PWM Dimming Signal at EN/DIM Pin
IN
drops below 2.3V (typical). Hys-
REF
. Once the FB pin volt-
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F
= 1A