Texas Instruments LM3431 Evaluation Board LM3431EVAL/NOPB LM3431EVAL/NOPB Datenbogen
Produktcode
LM3431EVAL/NOPB
P
SW
=
f
SW
x IL
(AVE)
x V
OUT
x (t
ON
+ t
OFF
)
2
SNVS547G – NOVEMBER 2007 – REVISED MAY 2013
VCC
The VCC pin is the output of the internal voltage regulator. It must be bypassed to PGND with a minimum 4.7 µF
ceramic capacitor. Although VCC is capable of supplying up to 72 mA, external loads will increase the power
dissipation and temperature rise within the LM3431. See the
ceramic capacitor. Although VCC is capable of supplying up to 72 mA, external loads will increase the power
dissipation and temperature rise within the LM3431. See the
section for more detail. Above 72 mA, the VCC
voltage will drop due to current limit. Since the UVLO threshold is monitored at this pin, UVLO may be enabled
by a VCC over current event.
by a VCC over current event.
For input voltages between 4.5V and 5.5V, connect VCC to V
IN
through a 4.7
Ω
resistor. This will hold VCC
above the UVLO threshold and allow operation at input voltages as low as 4.5V. It may also be necessary to add
additional V
additional V
IN
and VCC capacitance for low V
IN
operation.
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 rated 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:
diode is the peak inductor current. Therefore, the diode should be rated 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:
P
DIODE
= Vf x I
OUT
(15)
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 typically.
drain is connected to the inductor and the source is connected to the sense resistor. The LG pin will drive the
gate at 5V typically.
The critical parameters for selection of a MOSFET are:
1. Maximum drain current rating, I
D(MAX)
2. Maximum drain to source voltage, V
DS(MAX)
3. On-resistance, R
DS(ON)
4. Total gate charge, Q
g
In the on-state, the switch current is equal to the inductor current. Therefore, the maximum drain current, I
D
, must
be rated higher than the current limit setting. The average switch current (
ID(AVE)
) is given in the equation below:
I
D(AVE)
= I
L(AVE)
x D
(16)
The off-state voltage of the NFET is approximately equal to the output voltage plus the diode Vf. Therefore,
V
V
DS(MAX)
of 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
2
R loss across the
NFET. The maximum conduction loss is given by:
P
COND
= R
DS(ON)
x D
MAX
x I
L(AVE)
2
where
•
D
MAX
is the maximum duty cycle for the given application
•
R
DS(ON)
is the on resistance at high temperature
(17)
wThe switching losses can be roughly calculated by the following equation:
Where
•
t
ON
and t
OFF
are the NFET turn-on and turn-off times.
(18)
Power is also consumed in the LM3431 in the form of gate charge losses, P
g
. These losses can be calculated
using the formula:
P
g
= f
SW
x Q
g
x V
IN
where
•
Q
g
is the NFET total gate charge
(19)
Pg adds to the total power dissipation of the LM3431 (See
section).
Copyright © 2007–2013, Texas Instruments Incorporated
15
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