MAX6946 Maxim, MAX6946 Datasheet - Page 20

• Manufacturer Part Number: MAX6946
• Description: The MAX6946/MAX6947 I²C-/SMBus™-compatible, serial-interfaced peripherals provide microprocessors with 10 I/O ports rated to 7V
• Manufacturer: Maxim

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). Optimize phasing when using
fewer than 10 ports as constant-current outputs by allo-
cating the ports with the most appropriate start posi-
tions. If using eight constant-current outputs, choose
P0–P7 because these all have different PWM start posi-
tions. If using four constant-current outputs, choose P0,
P2, P4, P6 or P1, P3, P5, P7 because their 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.
The MAX6946/MAX6947 can use an I/O port to auto-
matically generate a shutdown/run output. The shut-
down/run output is active low when the MAX6946/
MAX6947 are in run mode, hold-off, fade-off, or ramp-
up, and goes high automatically when the devices final-
ly enter shutdown after fade-off. Programming the port’s
output register to value 0x02 puts the output into static
constant-current mode (Table 4). Program the port’s
output current register to half current (Table 6) to mini-
mize operating current. Connect a 220kΩ pullup resis-
tor to this port.
In run mode, the output port goes low, approaching 0V,
as the port’s static constant current saturates trying to
sink a higher current than the 220kΩ pullup resistor can
source.
10-Port, Constant-Current LED Driver and
I/O Expander with PWM Intensity Control
20
______________________________________________________________________________________
Generating a Shutdown/Run Output
Using Stagger with Fewer Ports
In shutdown mode, the output goes high impedance
together with any other constant-current outputs. This
output remains low during ramp-up and fade-down
sequences because the current drawn by the 220kΩ
pullup resistor is much smaller than the available output
constant current, even at the lowest fade current step.
The MAX6946/MAX6947 can drive loads needing more
than 20mA, like high-current white LEDs, by paralleling
outputs. For example, consider a white LED that
requires 70mA. Drive this LED using the ports P0–P3
connected in parallel (shorted together). Configure
three of the ports for full current (20mA) and configure
the last port for half current (10mA) to meet the 70mA
requirement. Control the four ports simultaneously with
one write access using register 0x0B (Table 1). Note
that because the output ports are current limiting, they
do not need to switch simultaneously to ensure safe
current sharing.
The MAX6946/MAX6947 operate with a power-supply
voltage of 2.25V to 3.6V. Bypass the power supply to
GND with a 0.1µF ceramic capacitor as close as possi-
ble to the device.
PROCESS: BiCMOS
Driving Load Currents Higher than 20mA
TOP VIEW
Pin Configurations (continued)
() MAX6947 ONLY
VDD
SCL
SDA
RST
13
14
15
16
Power-Supply Considerations
TQFN (3mm x 3mm)
+
12
1
MAX6946
MAX6947
11
2
Chip Information
10
3
9
4
8
7
6
5
P6
P5
GND
P4