Rainbow Electronics MAX6948B - Manual (Page 16)

High-Efficiency PWM LED Driver with Boost
Converter and Five Constant-Current GPIO Ports
16
MAX6948B
PWM Intensity Control and Phasing
The MAX6948B uses an internal 31.25kHz oscillator to
generate PWM timing for LED intensity control. A PWM
period comprises 256 cycles of the nominal 31.25kHz
PWM clock (Figure 2). Each port can have an individual
PWM duty cycle between 3/256 and 254/256. See Table
7 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. Set PWM stagger = 0 to cause 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 exact same time for the same PWM
setting. This means that if, for example, all outputs 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.
Set PWM stagger = 1 to stagger the PWM timing of the
five port outputs and the integrated boost-converter out-
put, distributing the port output switching points across
the PWM period (Figure 3). Staggering reduces the di/dt
output-switching transient on the supply and reduces the
peak/mean current requirement.
Change the PWM stagger-setting bit during shutdown.
Changing the stagger bit during normal operation can
cause a transient flicker in any PWM-controlled LED
because of the fundamental PWM timing changes.
Figure 2. Static and PWM Constant-Current Waveforms
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