MIC3263 Datasheet, PDF (18/32 Pages) Micrel Semiconductor – Six-Channel WLED Driver for Backlighting Applications with Flicker-Free Dimming

Micrel, Inc.
Figure 9 shows the waveforms during PWM dimming
pulses. The DRC duty cycle is 75% and therefore the
dimming ratio (DR) is 37%. Ch1 is the switch node. Ch2 is
the sum of all six ILED channels. Figure 9 shows the boost
converter is OFF (not switching) between PWM dimming
pulses.
MIC3263
Figure 9. PWM Dimming Pulses
(Ch1 Switch Node; Ch2 is the I_LED Total)
Figure 10. Direct Dimming Control
Direct Dimming
For direct dimming control connect DFS to V_DD and use
the MODE pin for the dimming pulse. This method will
bypass the internal dimming control and allows for
dimming control by the external PWM Dimming pulse (see
Figure 9).
Boost Stage
A current-mode control is easier to compensate than
voltage mode control, thus allowing for a less complex
control loop stability design. An error amplifier amplifies
the difference between the feedback voltage and the
voltage on the CRV capacitor. This amplified error signal is
called the V_CONTROL. A PWM comparator compares the
output of the error amp (V_CONTROL) to the sum of inductor
current and slope compensation currents. When the
current sums reach V_CONTROL, the PWM pulse is terminated
and the boost power switch is turned off. A portion of the
energy stored in the inductor flows into the output
capacitor.
January 2010
18
M9999-012110