Output pwm led drivers for message boards, Table 2. subframe and pwm timing – Rainbow Electronics MAX6975 User Manual
Page 10
MAX6974/MAX6975
Individual PWM Control
The MAX6974/MAX6975 further modulate the time that
each subframe is ON by a pulse-width modulation
(PWM) value. Each output current driver in the R, G,
and B ports has a unique 12-bit (MAX6974) or 14-bit
(MAX6975) PWM control value providing fine resolution
adjustment of average current output. Each bit time of
the PWM corresponds to one period of CLKI (T
CLKI
).
The PWM setting determines the amount of time, out of
the total period, that the output is on. The subframes
have PWM off-zones at the start (t
SPWM
) and end
(t
EPWM
) of the PWM period (see
Figure
3). The sub-
frame period and PWM off zones are shown in
Table
2
for each device.
The MAX6974 subdivides each subframe by 4096
(12-bit) PWM steps and has 16 cycle off zones, leaving
an active PWM region of 4064 PWM steps ranging from
16 to 4079. The MAX6975 subdivides each subframe
by 16,384 (14-bit) PWM steps and has 32 cycle off
zones, leaving an active PWM region of 16,320 PWM
steps ranging from 32 to 16,351. The PWM phase for
outputs R0, R2, R4, R6, G0, G2, G4, G6, B0, B2, B4,
and B6 use phasing with the outputs on first and off
second. Inverse phasing is used for outputs R1, R3,
R5, R7, G1, G3, G5, G7, B1, B3, B5, and B7 as shown
in
Figure
3 to balance the timing of loads on the LED
anode power supply.
In multiplexed operation, the subframes are shared
between
MUX0 and MUX1 active times, effectively
reducing the number of subframes by 2.
LED-Intensity Control Example
The three levels of intensity control are shown in
Figure
2
for one LED output driver in a MAX6974 in nonmulti-
plexed mode. As an example, the CALDAC is set to
24-Output PWM LED Drivers
for Message Boards
10
______________________________________________________________________________________
R0, R2, R4, R6
G0, G2, G4, G6
B0, B2, B4, B6
R1, R3, R5, R7
G1, G3, G5, G7
B1, B3, B5, B7
R0, R2, R4, R6
G0, G2, G4, G6
B0, B2, B4, B6
R1, R3, R5, R7
G1, G3, G5, G7
B1, B3, B5, B7
SUBFRAME (n)
SUBFRAME (n + 1)
t
SPWM
t
EMUX
t
EMUX
MUX0
SUBFRAME (n), MUX0
SUBFRAME (n), MUX1
MUX1
t
SPWM
t
SPWM
t
EPWM
MULTIPLEXED
NONMULTIPLEXED
t
SPWM
t
EPWM
ON/OFF PHASING
OFF/ON PHASING
ON/OFF PHASING
OFF/ON PHASING
50%
75%
100%
25%
75%
75%
75%
75%
Figure 3. Multiplexed and Nonmultiplexed Output Driver Phasing and Example PWM Values
PART
SUBFRAME
(T
CLKI
)
t
SPWM
(T
CLKI
)
t
EPWM
(T
CLKI
)
t
EMUX
(T
CLKI
)
MAX6974
4096
16
16
16
MAX6975
16,384
32
32
32
Table 2. Subframe and PWM Timing