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SC5014 View Datasheet(PDF) - Semtech Corporation

Part Name
Description
MFG CO.
'SC5014' PDF : 34 Pages View PDF
SC5014
Applications Information (continued)
Phase-Shift PWM Dimming
The SC5014 provides an option for phase-shifted LED PWM
dimming. This option is available in both PWMI indirect
control and I2C control. The phase-shift option is set by the
PH_SHIFT bit in the Device Control Register (register address
0x01). This option delays the turn-on of the LED strings
based on the number of the strings in operation (the
number of the strings in operation is determined during
the start-up). The delay time can be calculated by the
following equation:
1
Tφ phase =
fPWM ,
N
N = number of strings in operation
fPWM = LEDPWM dimming frequency
Phase-shift mode is disabled during the soft-start period.
This allows the output to ramp up to the correct voltage in
a controlled fashion.
Phase-shifting reduces the peak input current, decreases
EMI and improves the dimming linearity. The figures in the
Typical Characteristics Section on page 6 show the
improvement in dimming linearity with phase-shifted
versus non-phase-shifted dimming.
Backlight Fade-in and Fade-out Options
The SC5014 features an option for fade-in and fade-out
brightness control, which allows a smooth transition from
one brightness level to another.
Registers associated with these fading functions are
shown in this section.
. Fade Option (register address 0x09) — sets fade
enable options, fade time, fade type.
2. Fade Rate (register address 0x0A) — sets fade step
size option.
The fade option register allows the user to select fading,
choose between linear or logarithmic fading, and to set
the fading time. The default setting is fading enabled with
logarithmic mode. The fading time is determined by the
LED PWM dimming frequency. The fade setting is shown
in Table 2.
An example for calculating the fading time is shown in
this section. Assuming LED PWM dimming frequency is
10kHz, then 10-bits are assigned for 1024 duty cycle
settings.
Table 2 — Fade Setting
Duty
Cycle
Zone
1
2
3
Duty Cycle
Step
Step
Range Increment Interval
0 to 511
1
2
512 to 767
1
1
768 to 1024
2
1
Total Steps
within the
Range
512
256
256
The time required to go from 10% (102/1024) to 90%
(922/1024) duty cycle can be calculated using the follow-
ing equation:
TPWM = 100 µs (with 10kHz dimming frequency)
Cycle in Zone #1 = (511 - Starting Duty Cycle) x [(Zone #1
Step Interval) / (Zone #1 Step Increment)]
Cycle in Zone #2 = Total Steps in Zone #2 x [(Zone #2 Step
Interval) / (Zone #2 Step Increment)]
Cycle in Zone #3 = (End Duty Cycle - 768) x [(Zone #3 Step
Interval) / (Zone #3 Step Increment)]
In this case, the total cycle will be:
Total cycle = 2 x (511-102) + 1 x 256 + 0.5 x (922 - 768)
= 1151
Total Fading Time = Total Cycle x TPWM
= 1151 x 100 µs = 115.1ms
Time required to go from 10% (102/1024) to 90%
(922/1024) duty cycle can be calculated using the follow-
ing equation:
TPWM = 100 μs (PWMDimmingPeriod)
Total Cycle = 2 × (511102) + 256 + 1 × (922 768) = 1151
2
Total Time =1151× TPWM = 115.1ms
17
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