LP8555 Datasheet, PDF (36/64 Pages) Texas Instruments – LP8555 High-Efficiency LED Backlight Driver for Tablet PCs

LP8555
SNVS857 – FEBRUARY 2014
www.ti.com
8.5.7 PGEN
Address: 0x12
D7
PFSET
D6
RESERVED
Bits
Field
7
PFSET
Type
R/W
6
RESERVED
R/O
5:3 THRESHOLD
R/W
2:0
PFREQ
R/W
D5
D4
D3
THRESHOLD
D2
D1
D0
PFREQ
Description
0 = PWM frequency is set with PFREQ register bits
1 = Set PFREQ via the FSET/SDA terminal. This bit should only be set to 1 as an EPROM default,
writing to this bit "on-the-fly" does not have effect. Resistor values and their corresponding frequency
settings are seen in Setting PWM Dimming Frequency section.
0
Adaptive dimming threshold. PWM dimming is used below the threshold and current dimming is used
above the threshold.
000 = 100% current dimming
101 = PWM below 25% (10-bit PWM)
111 = 100% PWM (12-bit PWM)
PWM output frequency (typical)
000 = 4.9 kHz
001 = 9.8 kHz
011 = 19.5 kHz
111 = 39.1 kHz
The output PWM frequency can be configured in two different ways: EPROM or FSET/SDA terminal. The
FSET/SDA terminal is always automatically measured. The PGEN.PFSET bit is used to select between the PWM
frequency value measured from the FSET/SDA terminal and the EPROM value. When the PFSET bit is set to 1
the FSET/SDA terminal value is used; otherwise the EPROM value is used. Regardless of EPROM
programming, the PWM frequency can always be configured from I²C by clearing the PGEN.PFSET bit and
configuring the PGEN.PFREQ field as needed.
Full 12-bit precision is achieved in all adaptive dimming thresholds and PWM output frequencies. When the
PGEN register is read via I²C the PFREQ field will contain the active PWM output frequency value. To read the
EPROM value the PFSET bit must be set to 0. To read the R_FSET resistor value the PFSET bit must be set to 1.
If the FSET/SDA terminal is grounded or floating the PFREQ value will be set to 19.5 kHz.
36
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