10-Port, Constant-Current LED Driver and

I/O Expander with PWM Intensity Control

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11

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

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.

HIGH-Z

LOW

HIGH-Z

LOW

HIGH-Z

LOW

OUTPUT LOW 254/256 DUTY CONSTANT CURRENT WITH INPUT BUFFER DISABLED (PWM LED DRIVE)

HIGH-Z

LOW

0xFF

HIGH-Z

LOW

HIGH-Z

LOW

OUTPUT LOW 253/256 DUTY CONSTANT CURRENT WITH INPUT BUFFER DISABLED (PWM LED DRIVE)

OUTPUT STATIC-HIGH LOGIC DRIVE WITH INPUT BUFFER ENABLED (GPI)

OUTPUT STATIC-LOW LOGIC DRIVE WITH INPUT BUFFER ENABLED (GPI)

OUTPUT STATIC-LOW CONSTANT CURRENT WITH INPUT BUFFER DISABLED (STATIC LED DRIVE ON)

0xFD

0xFE

0x02

0x00

0x01

OUTPUT STATIC HIGH IMPEDANCE WITH INPUT BUFFER DISABLED (STATIC LED DRIVE OFF)

HIGH-Z

LOW

OUTPUT LOW 3/256 DUTY CONSTANT CURRENT WITH INPUT BUFFER DISABLED (PWM LED DRIVE)

0x03

HIGH-Z

LOW

OUTPUT LOW 252/256 DUTY CONSTANT CURRENT WITH INPUT BUFFER DISABLED (PWM LED DRIVE)

0xFC

REGISTER

VALUE

OUTPUT

7.8125ms NOMINAL PWM PERIOD

HIGH-Z

LOW

OUTPUT LOW 4/256 DUTY CONSTANT CURRENT WITH INPUT BUFFER DISABLED (PWM LED DRIVE)

0x04

Figure 2. Static and PWM Constant-Current Waveforms