MAX6948B MAXIM [Maxim Integrated Products], MAX6948B Datasheet - Page 27

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

Converter and Five Constant-Current GPIO Ports
The MAX6948B can use an I/O port to automatically gen-
erate a shutdown/run output. The shutdown/run output is
active-low when the MAX6948B is in run mode, hold-off,
ramp-down, or ramp-up, and goes high automatically
when the device finally enters shutdown after ramp-
down. Programming the port’s output register to value
0x02 puts the output into static constant-current mode
(Table 7). Program the port’s output current to half cur-
rent (Table 9) to minimize operating current. Connect a
220kI pullup resistor 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 220kI pullup resistor can
source.
In shutdown mode, the output goes high impedance
together with any other constant-current outputs. This
output remains low during ramp-up and ramp-down
sequences because the current drawn by the 220kI
pullup resistor is much smaller than the available output
constant current, even at the lowest fade-current step.
The MAX6948B can drive loads needing more than
30mA, like high-current white LEDs, by paralleling out-
puts. For example, consider a white LED that requires
90mA. Drive this LED using the ports P0–P4 connected
in parallel (shorted together). Configure all of the five
ports for full current (2 x 30mA + 3 x 10mA) to meet the
90mA requirement. Control the five ports simultaneously
Driving Load Currents Higher than 30mA
High-Efficiency PWM LED Driver with Boost
with one write access using register 0x0C (Table 1).
Note that because the output ports are current limiting,
they do not need to switch simultaneously to ensure safe
current sharing.
V+ operates with a 2.7V to 5.5V power-supply voltage.
Bypass V+ to GND with a 2.2FF or higher ceramic
capacitor as close as possible to the device. V
ates with a 1.7V to V+ power-supply voltage. Bypass
V
close as possible to the device.
Due to fast switching waveforms and high-current paths,
careful PCB layout is required. Minimize trace lengths
between the IC and the inductor, the diode, the input
capacitor, and the output capacitor. Minimize trace
lengths between the input and output capacitors and
the MAX6948B GND terminal, and place input and
output capacitor grounds as close together as pos-
sible. Use separate power-ground and analog-ground
copper areas, and connect them together at the output-
capacitor ground. Keep traces short, direct, and wide.
Keep noisy traces, such as the LX node trace, away from
sensitive analog circuitry. For improved thermal per-
formance, maximize copper area of the LX and PGND
traces. Refer to the MAX6948B EV kit data sheet for an
example layout.
DD
to GND with a 0.1FF or higher ceramic capacitor as
Power-Supply Considerations
PCB Layout Considerations
DD
oper-
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