Texas Instruments LM3431 Evaluation Board LM3431EVAL/NOPB LM3431EVAL/NOPB Scheda Tecnica
Codici prodotto
LM3431EVAL/NOPB
f
SW(MAX)
=
D
t
ON(MIN)
0
200
400
600
800
1000
1200
0
20
40
60
80
100 120 140 160
R
T
(k:)
F
R
EQ
U
EN
C
Y
(kH
z
)
SNVS547G – NOVEMBER 2007 – REVISED MAY 2013
Because OVP has a fixed 2.0V threshold sensed at AFB, a larger value for R19 will increase the OVP threshold
of the output voltage. During an open LED fault, the output voltage will increase by 2.6V typically (see
of the output voltage. During an open LED fault, the output voltage will increase by 2.6V typically (see
section). Therefore, at least this much headroom above the nominal output voltage is required to avoid
a false OVP error. Note that because of the high output voltage setting at the end of softstart, a brief open LED
error may occur during the short time it takes for the cathode voltage to drop to its nominal level.
error may occur during the short time it takes for the cathode voltage to drop to its nominal level.
shows a typical startup waveform, where both Vout and the SS/SH voltage reach their peak before the LED
current turns on. Once LED current starts, SS/SH and Vout drop to the nominal operating point.
current turns on. Once LED current starts, SS/SH and Vout drop to the nominal operating point.
While the LEDs are on, the AFB voltage is sampled to the SS/SH pin. During LED-off time, this SS/SH voltage is
used as the reference voltage to regulate the output. This allows the output voltage to remain stable between on
and off dimming cycles, even though there may be wide variation in the LED string forward voltage. The SS/SH
pin has a maximum voltage of 1.9V. Therefore, the AFB voltage when the LEDs are on must be below this limit
for proper regulation. This will be ensured by setting the AFB resistors as described above. During LED-off
cycles, there is minimal loading on the output, which forces the boost controller into pulse skipping mode. In this
mode, switching is stopped completely, or for multiple cycles until the AFB feedback voltage falls below the
SS/SH reference level.
used as the reference voltage to regulate the output. This allows the output voltage to remain stable between on
and off dimming cycles, even though there may be wide variation in the LED string forward voltage. The SS/SH
pin has a maximum voltage of 1.9V. Therefore, the AFB voltage when the LEDs are on must be below this limit
for proper regulation. This will be ensured by setting the AFB resistors as described above. During LED-off
cycles, there is minimal loading on the output, which forces the boost controller into pulse skipping mode. In this
mode, switching is stopped completely, or for multiple cycles until the AFB feedback voltage falls below the
SS/SH reference level.
Switching Frequency
The switching frequency can be set between 200 kHz and 1 MHz with a resistor from the RT pin to ground. The
frequency setting resistor (R6 in
frequency setting resistor (R6 in
) can be determined according to the following empirically derived
equation:
R
T
= 35403 x f
SW
-1.06
Where
•
f
SW
is in kHz
•
the R
T
result is in kohm.
(6)
Figure 16. Switching Frequency vs R
T
For a given application, the maximum switching frequency is limited by the minimum on time. When the LM3431
reaches its minimum on-time, pulse skipping will occur and output ripple will increase. To avoid this, set the
operating frequency below the following maximum setting:
reaches its minimum on-time, pulse skipping will occur and output ripple will increase. To avoid this, set the
operating frequency below the following maximum setting:
(7)
Inductor Selection
shows how the inductor current, I
L
, varies during a switching cycle.
12
Copyright © 2007–2013, Texas Instruments Incorporated
Product Folder Links: