MAX6947 Maxim Integrated Products, MAX6947 Datasheet - Page 15

• Manufacturer Part Number: MAX6947
• Description: Constant-Current LED Drivers and I/O Expanders
• Manufacturer: Maxim Integrated Products

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Table 9. Ramp-Up Register Format (0x12)
Ramp-up and ramp-down use the PWM clock for tim-
ing. When using the external oscillator make sure the
oscillator runs until the end of the sequence. The inter-
nal oscillator always runs during a fade sequence, even
if none of the ports use PWM.
The ramp-up and ramp-down circuit operates a 3-bit
DAC. The DAC adjusts the internal current reference
used to set the constant-current outputs in a similar
manner to the global-current register (Table 5). The
MAX6946/MAX6947 scale the master current reference
so all output constant-current and PWM settings adjust
at the same ratio with respect to each other. This
means the LEDs always fade at the same rate even if
with different intensity settings.
The maximum port output current set by the global-cur-
rent register (Table 5) also sets the point during ramp-
down that the current starts falling, and the point during
ramp-up that the current stops rising. Figure 7 shows
the ramp waveforms that occur with different global-
current register settings.
The MAX6946/MAX6947 feature a Reset Run option so
that simply taking the RST input high brings the driver
out of shutdown in addition to its normal function of
enabling the MAX6946/MAX6947s’ I
provides an alternative method of bringing the driver
out of shutdown to writing to the configuration register
through the serial interface. The Reset Run timing uses
the PWM clock, either the internal nominal 32kHz oscil-
lator or a user-provided clock fed into the OSC input
(MAX6946 only).
After enabling the Reset Run option, the MAX6946/
MAX6947 use the rising edge on RST, followed by no
I
to 129 periods of the PWM clock to trigger the Reset
2
Instant full current coming out from shutdown
1/16s ramp-up to full current coming out from shutdown
1/8s ramp-up to full current coming out from shutdown
1/4s ramp-up to full current coming out from shutdown
1/2s ramp-up to full current coming out from shutdown
1s ramp-up to full current coming out from shutdown
2s ramp-up to full current coming out from shutdown
4s ramp-up to full current coming out from shutdown
C interface activity to the MAX6946/MAX6947 for 128
10-Port, Constant-Current LED Drivers and
I/O Expanders with PWM Intensity Control
______________________________________________________________________________________
DESCRIPTION
Reset Run Option
2
C interface. This
D7
X
X
X
X
X
X
X
X
Run option. If this timeout period elapses without the
MAX6946/MAX6947 acknowledging an I
the run bit (D0) in the configuration register sets, bring-
ing the driver out of shutdown and activating any pro-
grammed ramp-up. If RST pulses high for less than this
timeout period to trigger a Reset Run, the MAX6946/
MAX6947 ignore the pulse, and the MAX6946/MAX6947
continue to wait for a suitable trigger.
Cancel the Reset Run trigger by transmitting an I
communication to the MAX6946/MAX6947 before the
timeout period elapses. The trigger cancels when the
MAX6946/MAX6947 acknowledge the I
Figure 7. Global Current Modifies Fade Behavior
CURRENT
17.5mA
12.5mA
2.5mA
7.5mA
20mA
15mA
10mA
5mA
0mA
D6
X
X
X
X
X
X
X
X
GLOBAL CURRENT = 0x07
GLOBAL CURRENT = 0x05
GLOBAL CURRENT = 0x03
GLOBAL CURRENT = 0x01
GLOBAL CURRENT = 0x00
GLOBAL CURRENT = 0x06
GLOBAL CURRENT = 0x04
GLOBAL CURRENT = 0x02
CURRENT
FULL
RAMP-UP
D5
X
X
X
X
X
X
X
X
CURRENT
7/8
REGISTER BIT
CURRENT
6/8
D4
X
X
X
X
X
X
X
X
CURRENT
5/8
D3
CURRENT
X
X
X
X
X
X
X
X
4/8
CURRENT
3/8
D2
0
0
0
0
1
1
1
1
CURRENT
RAMP-DOWN
2/8
2
2
C transaction,
C transaction
D1
CURRENT
0
0
1
1
0
0
1
1
1/8
ZERO
CURRENT
D0
0
1
0
1
0
1
0
1
2
15
C