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Boost Inrush Diode
SNVS574E – JULY 2008 – REVISED MAY 2013
V IN
L1
D1
V O
Q1
Figure 35. Boost Topology with Inrush Diode
CIRCUIT LAYOUT
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 LM3421/23 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 LM3421/23
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, 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 LM3421/23, 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.
Copyright ? 2008–2013, Texas Instruments Incorporated
27
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