LM3431AMH National Semiconductor, LM3431AMH Datasheet - Page 14

• Manufacturer Part Number: LM3431AMH
• Description: LM3431/LM3431A/LM3431Q/LM3431AQ 3-Channel Constant Current LED Driver with Integrated Boost Controller; Package: TSSOP EXP PAD; No of Pins: 28; Qty per Container: 48; Container: Rail
• Manufacturer: National Semiconductor

www.national.com
4.5V. It may also be necessary to add additional V
capacitance for low V
DIODE SELECTION
The average current through D1 is the average load current
(total LED current), and the peak current through the diode is
the peak inductor current. Therefore, the diode should be rat-
ed to handle more than the peak inductor current which was
calculated earlier. The diode must also be capable of handling
the peak reverse voltage, which is equal to the output voltage
(LED Anode voltage). To improve efficiency, a low Vf Schottky
diode is recommended. Diode power loss is calculated as:
NFET SELECTION
The drive pin of the LM3431 boost switcher, LG, must be
connected to the gate of an external NFET. The NFET drain
is connected to the inductor and the source is connected to
the sense resistor. The LG pin will drive the gate at 5V typi-
cally.
The critical parameters for selection of a MOSFET are:
1. Maximum drain current rating, I
2. Maximum drain to source voltage, V
3. On-resistance, R
4. Total gate charge, Q
In the on-state, the switch current is equal to the inductor cur-
rent. Therefore, the maximum drain current, I
higher than the current limit setting. The average switch cur-
rent (
ID(AVE)
) is given in the equation below:
DS(ON)
I
P
D(AVE)
IN
DIODE
g
operation.
= I
= Vf x I
L(AVE)
D(MAX)
OUT
x D
DS(MAX)
D
, must be rated
IN
and VCC
14
The off-state voltage of the NFET is approximately equal to
the output voltage plus the diode Vf. Therefore, V
the NFET must be rated higher than the maximum output
voltage. The power losses in the NFET can be separated into
conduction losses and switching losses. The conduction loss,
Pcond, is the I
duction loss is given by:
where D
tion and R
switching losses can be roughly calculated by the following
equation:
Where t
Power is also consumed in the LM3431 in the form of gate
charge losses, P
formula:
where Q
power dissipation of the LM3431 (See TSD section).
Fast switching FETs can cause noise spikes at the SW node
which may affect performance. To reduce these spikes a drive
resistor up to 10Ω can be placed between LG and the NFET
gate.
ON
MAX
g
DS(ON)
is the NFET total gate charge. Pg adds to the total
and t
is the maximum duty cycle for the given applica-
P
2
COND
R loss across the NFET. The maximum con-
OFF
is the on resistance at high temperature. The
g
. These losses can be calculated using the
are the NFET turn-on and turn-off times.
= R
P
g
= f
DS(ON)
SW
x Q
x D
g
MAX
x V
IN
x I
L(AVE)
2
DS(MAX)
of