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10-Port, Constant-Current LED Driver and
I/O Expander with PWM Intensity Control
PWM Intensity Control and Phasing
The MAX6946/MAX6947 use an internal 32kHz oscilla-
tor to generate PWM timing for LED intensity control.
The MAX6946 also features an OSC input to allow for
an external clock for generating PWM timing for LED
intensity control. Select the PWM clock source for the
MAX6946 with configuration register bit D7 (Table 2).
The MAX6947 powers up configured to use the internal
32kHz oscillator by default. The MAX6946 powers up
configured to use the external clock source by default.
A PWM period comprises 256 cycles of the nominal
32kHz PWM clock (Figure 2). Individually set the ports’
PWM duty cycle between 3/256 and 254/256. See
Table 4 for port register settings.
Configure PWM timing by setting the stagger bit in the
configuration register (Table 2), either with output stag-
gering or without. Clearing the stagger bit causes all
outputs using PWM to switch at the same time using
OUTPUT
REGISTER
the timing shown in Figure 2. All outputs, therefore,
draw load current at the exactly same time for the same
PWM setting. This means that if, for example, all out-
puts are set to 0x80 (128/256 duty cycle), the current
draw would be zero (all loads off) for half the time, and
full (all loads on) for the other half.
Setting the stagger bit causes the PWM timing of the 10
port outputs to stagger by 32 counts of the 256-count
PWM period (i.e., 1/8th), distributing the port output
switching points across the PWM period (Figure 3).
Staggering reduces the di/dt output-switching transient
on the supply and also reduces the peak/mean current
requirement.
Set or clear the stagger bit during shutdown. Changing
the stagger bit during normal operation can cause a
transient flicker in any PWM-controlled LEDs because
of the fundamental PWM timing changes.
VALUE
0x00
0x01
0x02
0x03
0x04
0xFC
7.8125ms NOMINAL PWM PERIOD
OUTPUT STATIC-LOW LOGIC DRIVE WITH INPUT BUFFER ENABLED (GPI)
OUTPUT STATIC-HIGH LOGIC DRIVE WITH INPUT BUFFER ENABLED (GPI)
OUTPUT STATIC-LOW CONSTANT CURRENT WITH INPUT BUFFER DISABLED (STATIC LED DRIVE ON)
OUTPUT LOW 3/256 DUTY CONSTANT CURRENT WITH INPUT BUFFER DISABLED (PWM LED DRIVE)
OUTPUT LOW 4/256 DUTY CONSTANT CURRENT WITH INPUT BUFFER DISABLED (PWM LED DRIVE)
OUTPUT LOW 252/256 DUTY CONSTANT CURRENT WITH INPUT BUFFER DISABLED (PWM LED DRIVE)
HIGH-Z
LOW
HIGH-Z
LOW
HIGH-Z
LOW
HIGH-Z
LOW
HIGH-Z
LOW
HIGH-Z
LOW
0xFD
OUTPUT LOW 253/256 DUTY CONSTANT CURRENT WITH INPUT BUFFER DISABLED (PWM LED DRIVE)
HIGH-Z
LOW
0xFE
0xFF
OUTPUT LOW 254/256 DUTY CONSTANT CURRENT WITH INPUT BUFFER DISABLED (PWM LED DRIVE)
OUTPUT STATIC HIGH IMPEDANCE WITH INPUT BUFFER DISABLED (STATIC LED DRIVE OFF)
HIGH-Z
LOW
HIGH-Z
LOW
Figure 2. Static and PWM Constant-Current Waveforms
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