MIC3223 Datasheet, PDF (12/24 Pages) Micrel Semiconductor – High Power Boost LED Driver with Integrated FET

Micrel, Inc.
MIC3223
If high dimming ratios are required, a lower Dimming
frequency is required. During each DIM_IN pulse the
inductor current has to ramp up to it steady state value in
order for the programmed LED current to flow. The
smaller the inductance value the faster this time is and a
narrower DIM_IN pulse can be achieved. But smaller
inductance means higher ripple current.
Figure 1. Soft start
LED Dimming
The MIC3223 LED driver can control the brightness of
the LED string via the use of pulse width modulated
(PWM) dimming. An input signal from DC up to 20kHz
can be applied to the DIM_IN pin (see Typical
Application) to pulse the LED string ON and OFF. It is
recommended to use PWM dimming signals above
120Hz to avoid any recognizable flicker by the human
eye. PWM dimming is the preferred way to dim an LED
in order to prevent color/wavelength shifting. Color
wavelength shifting will occur with analog dimming. By
employing PWM Dimming the output current level
remains constant during each DIM_IN pulse. The boost
converter switches only when DIM_IN is high. Between
DIM_IN pulses the output capacitors will slowly
discharge. The higher the DIM_IN frequency the less the
output capacitors will discharge.
PWM Dimming Limits
The minimum pulse width of the DIM_IN is determined
by the DIM_IN frequency and the L and C used in the
boost stage output filter. At low DIM_IN frequencies
lower dimming ratios can be achieved.
Dim_ratio = ^LED_ON_TIME
PERIOD_PWMD
Figure 3. PWM Dimming 20%
Figure 3 shows that switching occurs only during DIM_IN
on pulses. When DIM_IN is low the boost converter
stops switching and the external LED is turned off. The
LED current flows only when DIM_IN is high. Figure 3
shows that the compensation pin (V_COMP) does not
discharge between DIM_IN pulses. Therefore, when the
DIM_IN pulse starts again the converter resumes
operation at the same V_COMP voltage. This eliminates the
need for the comp pin to charge up during each DIM_IN
pulse and allows for high Dimming ratios.
Figure 2. DIM_IN Dimming Ratio
January 2010
Figure 4. PWM Dimming 10% and I_LED 100Hz
12
M9999-011510-A