MIC3263 Datasheet, PDF (17/32 Pages) Micrel Semiconductor – Six-Channel WLED Driver for Backlighting Applications with Flicker-Free Dimming

Micrel, Inc.
VDD
This is the output of the internal LDO regulator. Connect a
10μF ceramic capacitor to this pin.
PWM Dimming
The duty cycle of the PWM pulse applied to the DRC input
is converted to 16 log levels. This logarithmic dimming is a
unique feature of the MIC3263 which better matches the
sensitivity of the human eye compared to linear dimming.
The DRC duty-cycle to DR duty-cycle conversion is shown
in Table 2.
MIC3263
DRC Duty PWM Dimming Ratio (DR)
N
Cycle
DR = 10^{(N − 1) / 7}
%
%
0
0
0
1
6.25
1.0
2
12.5
1.4
3
18.75
1.9
4
25
2.7
5
31.25
3.7
6
37.5
5.2
7
43.75
7.2
8
50
10
9
56.25
14
10
62.5
19
11
68.75
27
12
75
37
13
81.25
52
14
87.5
72
15
93.75
100
Table 2. Dimming Ratio
To avoid skipping between dimming levels, the MIC3263
uses Flicker-Free Dimming control. This technique uses a
digital filter and hysteresis on the DRC pulse to provide a
clean DR output. The digital filter has a 0.1μF capacitor on
the CINT pin to average the duty cycles of the PWM
pulses. The averaged duty cycle has to be 4.16% higher
than the nominal value before moving to the next dimming
level as shown in Figure 7. Likewise, to move the previous
dimming level the duty cycle has to be −4.16% lower than
the nominal. To prevent flicker the duty-cycle hysteresis is
set a 2%.
Figure 7. Duty-Cycle Thresholds and Hysteresis
PWM Dimming Limits
The minimum pulse width of the PWM Dim is determined
by the PWM Dimming frequency and the L and C used in
the boost stages output filter. At low-PWM Dimming
frequencies, higher dimming ratios can be achieved:
Dim Ratio = ^TPWMD
TLEDON
Figure 8. PWM Dimming Ratio
Consider that the human eye will perceive light flicker at a
PWM dimming frequency below 100Hz. At 100Hz the time
between pulses is 10μs. If the PWM dimming minimum
pulse width is 5μs, then:
10ms
Dim Ratio =
= 2000/1
5μs
If high dimming ratios are required, a lower dimming
frequency is required. During each DR pulse, the inductor
current has to ramp up to it steady state value to generate
the necessary boost output voltage in order for the full
programmed LED current to flow in the LED channels. The
smaller the inductance value the faster this time is and a
narrower PWM dimming pulse can be achieved. But
smaller inductance means higher ripple current.
January 2010
17
M9999-012110