MAX6975 Maxim Integrated Products, MAX6975 Datasheet - Page 10

• Manufacturer Part Number: MAX6975
• Description: (MAX6974 / MAX6975) 24-Output PWM LED Drivers
• Manufacturer: Maxim Integrated Products

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
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
(t
frame period and PWM off zones are shown in
for each device.
24-Output PWM LED Drivers
for Message Boards
Table 2. Subframe and PWM Timing
Figure 3. Multiplexed and Nonmultiplexed Output Driver Phasing and Example PWM Values
10
EPWM
MAX6974
MAX6975
PART
______________________________________________________________________________________
) of the PWM period (see
SUBFRAME
(T
16,384
4096
CLKI
)
G0, G2, G4, G6
G1, G3, G5, G7
G0, G2, G4, G6
G1, G3, G5, G7
R0, R2, R4, R6
B0, B2, B4, B6
R1, R3, R5, R7
B1, B3, B5, B7
R0, R2, R4, R6
B0, B2, B4, B6
R1, R3, R5, R7
B1, B3, B5, B7
Individual PWM Control
(T
MUX0
MUX1
t
SPWM
CLKI
16
32
)
MULTIPLEXED
NONMULTIPLEXED
Figure
(T
t
EPWM
t
t
SPWM
SPWM
SPWM
CLKI
16
32
50%
SUBFRAME (n), MUX0
3). The sub-
ON/OFF PHASING
ON/OFF PHASING
OFF/ON PHASING
OFF/ON PHASING
)
SUBFRAME (n)
75%
) and end
Table
(T
t
EMUX
CLKI
CLKI
16
32
75%
)
).
2
t
t
EPWM
SPWM
25%
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
anode power supply.
In multiplexed operation, the subframes are shared
between MUX0 and MUX1 active times, effectively
reducing the number of subframes by 2.
The three levels of intensity control are shown in
for one LED output driver in a MAX6974 in nonmulti-
plexed mode. As an example, the CALDAC is set to
Figure
t
EMUX
t
SPWM
3 to balance the timing of loads on the LED
SUBFRAME (n), MUX1
SUBFRAME (n + 1)
75%
75%
100%
75%
LED-Intensity Control Example
t
t
EPWM
EMUX
Figure
2