LM3424 Datasheet, PDF (61/69 Pages) National Semiconductor (TI) – Constant Current N-Channel Controller with Thermal Foldback for Driving LEDs

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9 Power Supply Recommendations
LM3424, LM3424-Q1
SNVS603C – AUGUST 2009 – REVISED AUGUST 2015
9.1 Input Supply Current Limit
It is important to set the output current limit of your input supply to an appropriate value to avoid delays in your
converter analysis and optimization. If not set high enough, current limit can be tripped during start up or when
your converter output power is increased, causing a foldback or shut-down condition. It is a common oversight
when powering up a converter for the first time.
10 Layout
10.1 Layout Guidelines
The performance of any switching regulator depends as much upon the layout of the PCB as the component
selection. Following a few simple guidelines will maximimize noise rejection and minimize the generation of EMI
within the circuit.
Discontinuous currents are the most likely to generate EMI, therefore care should be taken when routing these
paths. The main path for discontinuous current in the LM3424 buck regulator contains the input capacitor (C_IN),
the recirculating diode (D1), the N-channel MOSFET (Q1), and the sense resistor (R_LIM). In the LM3424 boost
regulator, the discontinuous current flows through the output capacitor (C_O), D1, Q1, and R_LIM. In the buck-boost
regulator both loops are discontinuous and should be carefully layed out. These loops should be kept as small as
possible and the connections between all the components should be short and thick to minimize parasitic
inductance. In particular, the switch node (where L1, D1, and Q1 connect) should be just large enough to
connect the components. To minimize excessive heating, large copper pours can be placed adjacent to the short
current path of the switch node.
The RT, COMP, CSH, IS, TSENSE, TREF, HSP, and HSN pins are all high-impedance inputs which couple
external noise easily, therefore the loops containing these nodes should be minimized whenever possible.
In some applications the LED or LED array can be far away (several inches or more) from the LM3424, or on a
separate PCB connected by a wiring harness. When an output capacitor is used and the LED array is large or
separated from the rest of the regulator, the output capacitor should be placed close to the LEDs to reduce the
effects of parasitic inductance on the AC impedance of the capacitor.
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