MAX6967 MAXIM [Maxim Integrated Products], MAX6967 Datasheet - Page 25

• Manufacturer Part Number: MAX6967
• Description: 10-Port Constant-Current LED Drivers and I/O Expanders with PWM Intensity Control
• Manufacturer: MAXIM [Maxim Integrated Products]

Figure 13 shows the typical current sunk by a LITEON
LTST-C170TBKT 3.0V blue LED as the LED supply volt-
age is varied from 2.5V to 7V. The LED currents shown
are for ports programmed for 10mA and 20mA constant
current, swept over a 2.5V to 7V LED supply voltage
range. It can be seen that the LED forward voltage falls
with current, allowing the LED current to fall gracefully,
not abruptly, in brownout. In practice, the LED current
drops to 6mA to 7mA at a 3V LED supply voltage,
which is an acceptable performance at end-of-life in
many backlight applications.
The open-drain output architecture allows the ports to
level translate the outputs to higher or lower voltages
than the MAX6966/MAX6967 supply. An external pullup
resistor can be used on any output to convert the high-
impedance logic-high condition to a positive voltage
level. The resistor can be connected to any voltage up
to 7V. When using a pullup on a constant-current out-
put, select the resistor value to sink no more than a few
hundred µA in logic-low condition. This ensures that the
current sink output saturates close to GND. For inter-
facing CMOS inputs, a pullup resistor value of 220kΩ is
a good starting point. Use a lower resistance to
improve noise immunity in applications where power
consumption is less critical, or where a faster rise time
is needed for a given capacitive load.
Figure 13. LED Brownout
10-Port Constant-Current LED Drivers and I/O
______________________________________________________________________________________
3.05
3.00
2.95
2.90
2.85
2.80
2.75
2.70
2.65
2.60
2.55
2.50
Expanders with PWM Intensity Control
Output Level Translation
2.5
3.0
3.5 4.0 4.5
VLED vs. VLED SUPPLY
VLED SUPPLY (V)
5.0 5.5
6.0
6.5
7.0
The stagger option, when selected, applies to all ports
configured as constant-current outputs. The 10 ports’
PWM cycles are separated to eight evenly spaced start
positions (Figure 3). This phasing can be optimized if
fewer than 10 ports are used as constant-current out-
puts by allocating the ports with the most appropriate
start positions. If eight constant-current outputs are
needed, choose P0–P7 because these all have differ-
ent PWM start positions. If four constant-current outputs
are needed, choose P0, P2, P4, P6 or P1, P3, P5, P7
because the PWM start positions are evenly spaced. In
general, choose the ports that spread the PWM start
positions as evenly as possible. This optimally spreads
out the current demand from the ports’ load supply.
An I/O port can be used to automatically generate a
shutdown/run output from the MAX6966/MAX6967. The
shutdown/run output is active low when the
MAX6966/MAX6967 are in run mode, hold-off, fade-off,
or ramp-up, and go high automatically when the
MAX6966/MAX6967 finally enter shutdown after fade-
off. Program the port’s output register to value 0x00,
which puts the output into static constant-current mode
(Table 6). Program the port’s output current register to
half current (Table 7) to minimize operating current. Fit
a 220kΩ pullup resistor to this port.
20
18
16
14
12
10
8
6
4
2
0
2.5
3.0
Generating a Shutdown/Run Output
3.5 4.0 4.5
ILED vs. VLED SUPPLY
VLED SUPPLY (V)
Using Stagger with Fewer Ports
5.0 5.5
6.0
6.5
7.0
25