MAX6967 MAXIM [Maxim Integrated Products], MAX6967 Datasheet - Page 24

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

10-Port Constant-Current LED Drivers and I/O
Expanders with PWM Intensity Control
The I/O ports P0–P9 remain high impedance with up to
8V asserted on them when the MAX6966/MAX6967 are
powered down (V+ = 0V). The MAX6966/MAX6967 can
therefore be used in hot-swap applications.
The MAX6966/MAX6967s’ SPI interface is guaranteed to
operate at 26Mbps on a 2.5V supply, and on a 3.3V sup-
ply typically operate at 35Mbps. This means that trans-
mission-line issues should be considered when the
interface connections are longer than 100mm, particular-
ly with higher supply voltages. Avoid running long adja-
cent tracks for SCLK, DIN, and CS without interleaving
GND traces; otherwise, the signals may cross-couple,
giving false clock or chip-select transitions. Ringing may
manifest itself as communication issues, often intermit-
tent, typically due to double clocking due to ringing at
the SCLK input. Fit a 1kΩ to 10kΩ parallel termination
resistor to either GND or V+ at the DIN, SCLK, and CS
inputs to damp ringing for moderately long interface
runs. Use line-impedance matching terminations when
making connections between boards.
The MAX6966 powers up with DOUT/OSC configured
as DOUT output by default. The MAX6967 powers up
with DOUT/OSC configured as OSC input by default.
Both parts allow the DOUT/OSC pin function to be
changed through the configuration register (Table 4). If
any port is used as a logic input, then configure
DOUT/OSC as DOUT to allow the MAX6966/MAX6967
to be read.
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Differences Between the MAX6966 and
Applications Information
SPI Routing Considerations
Hot Insertion
MAX6967
In most applications, the software can be written so that
either MAX6966 or MAX6967 can be fitted, and
DOUT/OSC is configured appropriately on power-up. If
DOUT/OSC is used as OSC, fit a series resistor
between the PWM clock source and DOUT/OSC pin. A
resistor value of 2.2kΩ is recommended as a starting
point, but other values may be more suitable depend-
ing on the serial-interface speed and clock-source
drive capability. This limits the loading on the PWM
clock source on power-up when a MAX6966 is fitted,
because DOUT/OSC initializes as an output. If
DOUT/OSC is used as DOUT, remember that a
MAX6967 cannot be read after power-up until
DOUT/OSC has been reconfigured from OSC to DOUT.
The MAX6966/MAX6967 correctly regulate the con-
stant-current outputs, provided there is a minimum volt-
age drop across the port output. This port output
voltage is the difference between the load (typically
LED) supply and the load voltage drop (LED forward
voltage). If the LED supply drops so that the minimum
port output voltage is not maintained, the driver output
stages brownout and the load current falls. The mini-
mum port voltage is approximately 0.5V at 10mA sink
current, and approximately 1V at 20mA sink current.
In battery applications, it may be important to operate
the LEDs directly from a battery supply. For example,
the LED supply voltage could be a single rechargeable
Li+ battery with a maximum terminal voltage of 4.2V on
charge, 3.4V to 3.7V most of the time, and down to 3V
when discharged. In this scenario, the LED supply falls
significantly below the brownout point when the battery
is at end-of-life voltage.
Driving LEDs into Brownout