Texas Instruments LM3423 Evaluation Board LM3423BBLSCSEV/NOPB LM3423BBLSCSEV/NOPB 데이터 시트
제품 코드
LM3423BBLSCSEV/NOPB
SNVS574E – JULY 2008 – REVISED MAY 2013
The chosen input capacitors must also have the necessary RMS current rating. Ceramic capacitors are again the
best choice due to their high ripple current rating, long lifetime, and good temperature performance. An X7R
dieletric rating is suggested.
best choice due to their high ripple current rating, long lifetime, and good temperature performance. An X7R
dieletric rating is suggested.
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 LM3421/23 device, a
10
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.
MAIN MosFET / DIMMING MosFET
The LM3421/23 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
).
When PWM dimming, the LM3421/23 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
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 chosen for better thermal performance.
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.
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 output capacitor, charging it to a higher potential than the input voltage.
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 output 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. EN < 1.3V: no discharge path (leakage only).
2. EN > 1.3V, the OVP divider resistor path, if present, and 10µA into each of the HSP & HSN pins.
In applications using the OVP divider and with EN > 1.3V, the output capacitor voltage can charge higher than
V
V
TURN-OFF
. In 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:
•
condition are:
•
Add an inrush diode from V
IN
to the output as shown in
•
Add an NTC thermistor in series with the input to prevent the inrush from overcharging the output capacitor
too high.
too high.
•
Use a current limited source supply.
•
Raise the OVP threshold.
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