Texas Instruments LM3429 Evaluation Boards LM3429BSTEVAL/NOPB LM3429BSTEVAL/NOPB Datenbogen
Produktcode
LM3429BSTEVAL/NOPB
SNVS616G – APRIL 2009 – REVISED MAY 2013
For most applications, it is recommended to bypass the V
IN
pin with an 0.1 µF ceramic capacitor placed as close
as possible to the pin. In situations where the bulk input capacitance may be far from the LM3429 device, a 10
Ω
series resistor can be placed between the bulk input capacitance and the bypass capacitor, creating a 150 kHz
filter to eliminate undesired high frequency noise coupling.
filter to eliminate undesired high frequency noise coupling.
N-CHANNEL MosFET (NFET)
The LM3429 requires an external NFET (Q1) as the main power MosFET for the switching regulator. Q1 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. In practice, all switching regulators have some ringing at the
switch node due to the diode parasitic capacitance and the lead inductance. The current rating is recommended
to be at least 10% higher than the average transistor current. The power rating is then verified by calculating the
power loss given the RMS transistor current and the NFET on-resistance (R
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. In practice, all switching regulators have some ringing at the
switch node due to the diode parasitic capacitance and the lead inductance. The current rating is recommended
to be at least 10% higher than the average transistor current. The power rating is then verified by calculating the
power loss given the RMS transistor current and the NFET on-resistance (R
DS-ON
).
In general, the NFET should be chosen to minimize total gate charge (Q
g
) whenever switching frequencies are
high and minimize R
DS-ON
otherwise. This will minimize the dominant power losses in the system. Frequently,
higher current NFETs in larger packages are chosen for better thermal performance.
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 accomplished 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
performing packages minimizing both power losses and temperature rise.
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 accomplished 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
performing packages minimizing both power losses and temperature rise.
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.
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 LM3429 buck regulator contains the input capacitor (C
paths. The main path for discontinuous current in the LM3429 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 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 components. To minimize excessive heating, large copper
pours can be placed adjacent to the short current path of the switch node.
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 RCT, 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.
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 LM3429, 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.
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 © 2009–2013, Texas Instruments Incorporated
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