LM3561 Datasheet, PDF (33/36 Pages) Texas Instruments – LM3561 Synchronous Boost Converter with 600mA High Side LED Driver and I 2 C-Compatible Interface

Layout Recommendations
The high frequency and relatively large switching currents of
the LM3561 make the choice of layout important. The follow-
ing steps should be used as a reference to ensure the device
is stable and maintains proper voltage and current regulation
across its intended operating voltage and current range.
1. Place C_IN on the top layer (same layer as the LM3561)
and as close to the device as possible. The input
capacitor conducts the driver currents during the low-
side MOSFET turn-on and turn-off and can see current
spikes over 500mA in amplitude. Connecting the input
capacitor through short wide traces on both the IN and
GND terminals will reduce the inductive voltage spikes
that occur during switching and which can corrupt the
V_IN line.
2. Place C_OUT on the top layer (same layer as the LM3561)
and as close as possible to the OUT and GND terminal.
The returns for both C_IN and C_OUT should come together
at one point, and as close to the GND pin as possible.
Connecting C_OUT through short wide traces will reduce
the series inductance on the OUT and GND terminals
that can corrupt the V_OUT and GND line and cause
excessive noise in the device and surrounding circuitry.
3. Connect the inductor on the top layer close to the SW pin.
There should be a low-impedance connection from the
inductor to SW due to the large DC inductor current, and
at the same time the area occupied by the SW node
should be small so as to reduce the capacitive coupling
of the fast dV/dt present at SW that can couple into
nearby traces.
4. Avoid routing logic traces near the SW node so as to
avoid any capacitively coupled voltages from SW onto
any high impedance logic lines such as TX1/TORCH/
GPIO1, TX2/GPIO2/INT, HWEN, LEDI/NTC (NTC
mode), SDA, and SCL. A good approach is to insert an
inner layer GND plane underneath the SW node and
between any nearby routed traces. This creates a shield
from the electric field generated at SW.
5. Terminate the Flash LED cathode directly to the GND pin
of the LM3561. If possible, route the LED return with a
dedicated path so as to keep the high amplitude LED
current out of the GND plane. For a Flash LED that is
routed relatively far away from the LM3561, a good
approach is to sandwich the forward and return current
paths over the top of each other on two adjacent layers.
This will help in reducing the inductance of the LED
current paths.
6. The NTC Thermistor is intended to have its return path
connected to the LED's cathode. This allows the
thermistor resistive divider voltage (V_NTC) to trip the
comparators threshold as V_NTC is falling. Additionally, the
thermistor to LED cathode junction can have low thermal
resistivity since both the LED and the thermistor are
electrically connected at GND. The draw back is that the
thermistor's return will see the switching currents from
the LM3561's boost converter. Because of this, it is
necessary to have a filter capacitor at the NTC pin which
terminates close to the GND of the LM3561 and which
can conduct the switched currents to GND.
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