LM3421MH/NOPB National Semiconductor, LM3421MH/NOPB Datasheet - Page 20

• Manufacturer Part Number: LM3421MH/NOPB
• Description: IC LED DRVR HP CONS CURR 16TSSOP
• Manufacturer: National Semiconductor
• Series: PowerWise®r
• Type: High Power, Constant Currentr

Specifications of LM3421MH/NOPB

• Constant Current: Yes
• Topology: PWM, SEPIC, Step-Down (Buck), Step-Up (Boost)
• Number Of Outputs: 1
• Internal Driver: No
• Type - Primary: Automotive
• Type - Secondary: High Brightness LED (HBLED)
• Frequency: 2MHz
• Voltage - Supply: 4.5 V ~ 75 V
• Voltage - Output: 3 V ~ 72 V
• Mounting Type: Surface Mount
• Package / Case: 16-TSSOP Exposed Pad, 16-eTSSOP, 16-HTSSOP
• Operating Temperature: -40°C ~ 125°C
• Current - Output / Channel: 1A
• Internal Switch(s): No
• Efficiency: 95%
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: *LM3421MH/NOPB; LM3421MH

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MAIN MosFET / DIMMING MosFET
The LM3421/23 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
When PWM dimming, the LM3421/23 requires another Mos-
FET (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
10% higher than the nominal LED current (I
rating is simply V
ming duty cycle (continuous operation) will occur.
In general, the NFETs should be chosen to minimize total gate
charge (Q
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
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.
BOOST INRUSH CURRENT
When configured as a boost converter, there is a “phantom”
power path comprised of the inductor, the output diode, and
the output capacitor. This path will cause two things to happen
when power is applied. First, there will be a very large inrush
of current to charge the output capacitor. Second, the energy
stored in the inductor during this inrush will end up in the out-
put capacitor, charging it to a higher potential than the input
voltage.
Depending on the state of the EN pin, the output capacitor
would be discharged by:
1.
2.
In applications using the OVP divider and with EN > 1.3V, the
output capacitor voltage can charge higher than V
EN < 1.3V: no discharge path (leakage only).
EN > 1.3V, the OVP divider resistor path, if present, and
10µA into each of the HSP & HSN pins.
g
) when f
OFF
O
SW
. The most efficient choice for D1 is a
multiplied by I
is high and minimize R
O
) and a current rating at least
DS-ON
LED
).
, assuming 100% dim-
LED
DS-ON
) . The power
TURN-OFF
otherwise.
. In
20
this situation, the FLT pin (LM3423 only) is open and the PWM
dimming MosFET is turned off. This condition (the system
appearing disabled) can persist for an undesirably long time.
Possible solutions to this condition are:
•
•
•
•
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 LM3421/23 buck
regulator contains the input capacitor (C
diode (D1), the N-channel MosFET (Q1), and the sense re-
sistor (R
tinuous current flows through the output capacitor (C
Q1, and R
continuous and should be carefully layed out. These loops
should be kept as small as possible and the connections be-
tween all the components should be short and thick to mini-
mize 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, 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 LM3421/23, or on a sepa-
rate 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.
Add an inrush diode from V
Figure
Add an NTC thermistor in series with the input to prevent
the inrush from overcharging the output capacitor too high.
Use a current limited source supply.
Raise the OVP threshold.
FIGURE 21. Boost Topology with Inrush Diode
LIM
21.
LIM
). In the LM3421/23 boost regulator, the discon-
. In the buck-boost regulator both loops are dis-
IN
to the output as shown in
IN
), the recirculating
300673i9
O
), D1,