LM3429_0908 Datasheet, PDF (15/34 Pages) National Semiconductor (TI) – N-Channel Controller for Constant Current LED Drivers

RE-CIRCULATING DIODE
A re-circulating diode (D1) is required to carry the inductor
current during t_OFF. The most efficient choice for D1 is a
Schottky diode due to low forward voltage drop and near-zero
reverse recovery time. Similar to Q1, D1 is recommended to
have a voltage rating at least 15% higher than the maximum
transistor voltage to ensure safe operation during the ringing
of the switch node and a current rating at least 10% higher
than the average diode current. The power rating is verified
by calculating the power loss through the diode. This is ac-
complished by checking the typical diode forward voltage
from the I-V curve on the product datasheet and multiplying
by the average diode current. In general, higher current
diodes have a lower forward voltage and come in better per-
forming packages minimizing both power losses and temper-
ature rise.
CIRCUIT LAYOUT
The performance of any switching regulator depends as much
upon the layout of the PCB as the component selection. Fol-
lowing 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 LM3429 buck reg-
ulator contains the input capacitor (C_IN), the recirculating
diode (D1), the N-channel MosFET (Q1), and the sense re-
sistor (R_LIM). In the LM3429 boost and buck-boost regulators,
the discontinuous current flows through the output capacitor
(C_O), D1, Q1, and R_LIM. In either case, this loop 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 com-
ponents. To minimize excessive heating, large copper pours
can be placed adjacent to the short current path of the switch
node.
The RCT, COMP, CSH, IS, HSP and HSN pins are all high-
impedance inputs which couple external noise easily, there-
fore 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 LM3429, or on a separate
PCB connected by a wiring harness. When an output capac-
itor 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.
15
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