Output pwm led drivers for message boards, Table 2. subframe and pwm timing – Rainbow Electronics MAX6975 User Manual

Page 10

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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

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