MAX6948B MAXIM [Maxim Integrated Products], MAX6948B Datasheet - Page 18

• Manufacturer Part Number: MAX6948B
• Description: High-Efficiency PWM LED Driver with Boost Converter and Five Constant-Current GPIO Ports
• Manufacturer: MAXIM [Maxim Integrated Products]

High-Efficiency PWM LED Driver with Boost
Converter and Five Constant-Current GPIO Ports
In shutdown mode, all ports configured as constant-
current outputs (output register set to a value between
0x02 and 0xFE) switch off and become high impedance.
Shutdown does not affect ports configured as logic
inputs or outputs (output registers set to value 0x00
or 0x01) (Table 7). This means that any ports used for
GPIOs are still operational in shutdown mode.
Put the MAX6948B into shutdown mode by setting the
run bit (D0) = 0 in the configuration register (0x10) (Table
2). Exit shutdown by setting the run bit high through the
serial interface or by using the reset-run option (see the
Reset-Run Option section). Configure and control the
MAX6948B normally through the serial interface in shut-
down mode. All registers are accessible in shutdown
mode. Entering and/or exiting shutdown mode does not
change any register values.
Changing a port from static logic-low (0x00) or static
logic-high (0x01) to a constant-current value (0x02 to
0xFE) in shutdown mode turns that output off (logic-high
or high impedance) like any other constant-current out-
puts in shutdown. The new constant-current output starts
just like any other constant-current outputs when exiting
shutdown.
Changing a port from a constant-current value (0x02 to
0xFE) to static logic-low (0x00) or static logic-high (0x01)
in shutdown causes that output to set to the value as a
GPIO output. The new GPIO output is unaffected just like
any other GPIO output when exiting shutdown.
The MAX6948B provides controls that allow the output
currents to ramp down into shutdown (ramp-down) and
ramp up again out of shutdown (ramp-down) (Figures
4, 5). Ramp-down comprises a programmable hold-off
delay that maintains the outputs at full current for a time
before the programmed ramp-down time. After the hold-
off delay, the output currents ramp down.
18
Ramp-Up and Ramp-Down Controls
Shutdown Mode
The ramp-down register sets the hold-off and ramp-down
times and allows disabling of hold-off and ramp-down
(zero delay), if desired (Table 11). The ramp-up register
sets the ramp-up time and allows disabling of ramp-up
(zero delay), if desired (Table 12). The configuration
register contains three status bits that identify the condi-
tion of the MAX6948B, hold-off, ramp-down, or ramp-up
(Table 2). The configuration register also enables or dis-
ables ramp-up. One write command to the configuration
register puts the device into shutdown (using hold-off
and ramp-down settings in the ramp-down register) and
one read command to the configuration register deter-
mines whether the reset run is enabled for restart, and
whether the MAX6948B is currently in ramp-up or ramp-
down mode. Reset run needs to be used with ramp-up
for it to work properly.
Ramp-up and ramp-down use the PWM clock for tim-
ing. The internal oscillator always runs during a fade
sequence, even if none of the ports uses PWM.
The ramp-up and ramp-down circuit operates a 3-bit
DAC. The DAC adjusts the internal current reference
used to set the constant-current outputs in a similar
manner to the global-current register (Table 8). The
MAX6948B scales the master-current reference to have
all output constant-current and PWM settings adjust at
the same ratio with respect to each other. This means the
LEDs always fade at the same rate even if with different
intensity settings. The boost circuit does not use the 3-bit
DAC. During ramp-down, the boost circuit remains at its
programmed output until it shuts off completely at the
end of the ramp-down period. The boost circuit turns on
completely at the beginning of the ramp-up sequence.
The maximum port output current set by the global-cur-
rent register (Table 8) also sets the point during ramp-
down that the current starts falling, and the point during
ramp-up that the current stops rising. Figure 7 shows the
ramp waveforms that occur with different global-current
register settings.