LM3424 Datasheet, PDF (20/50 Pages) National Semiconductor (TI) – Constant Current N-Channel Controller with Thermal Foldback for Driving LEDs

MAIN MosFET / DIMMING MosFET
The LM3424 requires an external NFET (Q1) as the main
power MosFET for the switching regulator. Q1 is recommend-
ed 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 regu-
lators 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).
When PWM dimming, the LM3424 requires another MosFET
(Q2) placed in series (or parallel for a buck regulator) with the
LED load. This MosFET should have a voltage rating equal
to the output voltage (V_O) and a current rating at least 10%
higher than the nominal LED current (I_LED) . The power rating
is simply V_O multiplied by I_LED, assuming 100% dimming duty
cycle (continuous operation) will occur.
In general, the NFETs should be chosen to minimize total gate
charge (Q_g) when f_SW is 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 cho-
sen 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 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 LM3424 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 LM3424 boost regulator, the discontinu-
ous current flows through the output capacitor (C_O), D1, Q1,
and R_LIM. In the buck-boost regulator both loops are discon-
tinuous 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 para-
sitic 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 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.
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