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10-Port, Constant-Current LED Driver and
I/O Expander with PWM Intensity Control
Ports Configured as Inputs
Configure a port as a logic input by writing 0x01 to the
port’s output register (Table 4). Reading an input port
register returns the logic levels from the I/O ports con-
figured as a logic input (Table 7). The input port regis-
ter returns logic 0 in the appropriate bit position for a
port not configured as a logic input.
The input port’s registers are read only. The MAX6946/
MAX6947 ignore a write to input ports register.
RST Input
The active-low RST input operates as a reset that voids
any current I 2 C transaction involving the MAX6946/
MAX6947, forcing the devices into the I 2 C stop condi-
tion. Use the D6 bit in the configuration register to con-
figure RST to reset all the internal registers to the
power-on reset state (Tables 2 and 3). The RST input is
overvoltage tolerant to 6V.
The MAX6946/MAX6947 ignore all I 2 C bus activity while
RST remains low. The MAX6946/MAX6947 use this fea-
ture to minimize supply current in power-critical applica-
tions by effectively disconnecting the MAX6946/
MAX6947 from the bus during idle periods. RST also
operates as a bus multiplexer, allowing multiple
MAX6946/MAX6947s to use the same I 2 C slave address.
Drive only one MAX6946/MAX6947 RST input high at any
time to use RST as a bus multiplexer.
The MAX6946/MAX6947 feature a Reset Run option so
that simply taking the RST input high brings the driver
out of shutdown in addition to its normal function of
enabling the devices’ I 2 C interface.
Standby Mode and Operating Current
Configuring all the ports as logic inputs or outputs (all
output registers set to value 0x00 or 0x01) or LED off
(output register set to value 0xFF) puts the MAX6946/
MAX6947 into standby mode. Put the MAX6946/
MAX6947 into standby mode for lowest supply-current
consumption.
Setting a port as a constant-current output increases
the operating current (output register set to a value
between 0x02 and 0xFE), even if a load is not applied
to the port. The MAX6946/MAX6947 enable an internal
current mirror to provide the accurate constant-current
sink. Enabling the internal current mirror increases the
devices’ supply current. Each output contains a gated
mirror, and each mirror is only enabled when required.
In PWM mode, the current mirror is only turned on for
the output’s on-time. This means that the operating cur-
rent varies as constant-current outputs are turned on
and off through the serial interface, as well as by the
PWM intensity control.
7.8125ms NOMINAL PWM PERIOD
NEXT PWM PERIOD
NEXT PWM PERIOD
0
32
64
96
128
160
192
224
256
OUTPUTS P0, O8 IN-PHASE PWM PERIOD
OUTPUT P1, O9 STAGGERED PWM PERIOD
OUTPUTS P0, O8 IN-PHASE PWM PERIOD
OUTPUT P1, O9 STAGGERED PWM PERIOD
OUTPUTS P0, O8 IN-PHASE PWM PERIOD
OUTPUT P1, O9 STAGGERED PWM PERIOD
OUTPUT P2 STAGGERED PWM PERIOD OUTPUT P2 STAGGERED PWM PERIOD
OUTPUT P3 STAGGERED PWM PERIOD
OUTPUT P4 STAGGERED PWM PERIOD
OUTPUT P5 STAGGERED PWM PERIOD
OUTPUT P3 STAGGERED PWM PERIOD
OUTPUT P4 STAGGERED PWM PERIOD
OUTPUT P5 STAGGERED PWM PERIOD
OUTPUT P6 STAGGERED PWM PERIOD
OUTPUT P7 STAGGERED PWM PERIOD
Figure 3. Staggered PWM Waveform
Table 7. Input Ports Registers
OUTPUT P6 STAGGERED PWM PERIOD
OUTPUT P7 STAGGERED PWM PERIOD
DESCRIPTION
ADDRESS
CODE (HEX)
D7
D6
D5
REGISTER BIT
D4 D3
D2
D1
D0
Input ports
P7–P0
Input ports
P9 and P8
0x0E
0x0F
Port P7
0
Port P6
0
Port P5
0
Port P4
0
Port P3
0
Port P2
0
Port P1
Port P9
Port P0
Port P8
12
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