MIC3223 Datasheet, PDF (13/24 Pages) Micrel Semiconductor – High Power Boost LED Driver with Integrated FET

Micrel, Inc.
MIC3223
Figure 5. PWM Dimming 20% and I_LED 1kHz
In Figure 4 is at 100Hz dimming frequency and Figure 5
is 1kHz dimming frequency. The time it takes for the
LED current to reach it full value is longer with a lower
Dimming frequency. The reason is the output capacitors
slowly discharge between dimming pulses.
Figure 7. 5µs DIM_IN Pulse
Figure 7 shows the minimum DIM_IN pulse at these
operating conditions before the I_LED current starts to drop
due to low V_OUT. The converter is ON (switching) only
during a DIM_IN pulse.
Figure 7 shows that at this DIM_IN pulse width the
converter is ON (switching) long enough to generate the
necessary V_OUT to forward bias the LED string at the
programmed current level. Therefore this condition will
result in the desired I_LED.
Figure 6. PWM Dimming 20% and I_LED 1kHz
Figure 6 shows the output voltage V_OUT discharge
between DIM_IN pulses. The amount of discharge is
dependent on the time between DIM_IN pulses.
Figure 8. 2.5µs DIM_IN Pulse
Figure 8 shows that at this DIM_IN pulse width the
converter in not ON (switching) long enough to generate
the necessary V_OUT to forward bias the LED string at the
programmed current level. As a result the LED current
drops. Therefore, this condition will not result in the
desired I_LED.
January 2010
13
M9999-011510-A