LM3556_15 Datasheet, PDF (30/39 Pages) Texas Instruments – Synchronous Boost LED Flash Driver With High-Side Current Source

LM3556
SNVS796D – AUGUST 2011 – REVISED OCTOBER 2015
10 Layout
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10.1 Layout Guidelines
The high switching frequency and large switching currents of the LM3556 make the choice of layout important.
The following steps should be used as a reference to ensure the device is stable and maintains proper LED
current regulation across its intended operating voltage and current range.
1. Place C_IN on the top layer (same layer as the device), as close as possible to the device. The input capacitor
conducts the driver currents during the low-side MOSFET turnon and turnoff and can see current spikes over
1 A in amplitude. Connecting the input capacitor through short, wide traces to both the IN and GND pins
reduces the inductive voltage spikes that occur during switching which can corrupt the V_IN line.
2. Place C_OUT on the top layer (same layer as the device) and as close as possible to the OUT and GND pins.
The returns for both C_IN and C_OUT must come together at one point, as close as possible to the GND pin.
Connecting C_OUT through short, wide traces reduces the series inductance on the OUT and GND pins that
can corrupt the V_OUT and GND lines and cause excessive noise in the device and surrounding circuitry.
3. Connect the inductor on the top layer close to the SW pin. There must 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 must be small to reduce the capacitive coupling of the high dV/dt present at SW that can couple into
nearby traces.
4. Logic traces must not be routed near the SW node to avoid any capacitively coupled voltages from SW onto
any high-impedance logic lines such as TORCH, STROBE, HWEN, TEMP, 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 cathodes directly to the GND pin of the LM3556. If possible, route the LED returns
with a dedicated path to keep the high amplitude LED currents out of the GND plane. For flash LEDs that are
routed relatively far away from the device, sandwich the forward and return current paths over the top of
each other on two layers. This helps reduce the inductance of the LED current paths.
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