LM3431EVAL National Semiconductor, LM3431EVAL Datasheet - Page 12

• Manufacturer Part Number: LM3431EVAL
• Description: BOARD EVAL FOR LM3431 LED DRIVER
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM3431EVAL

• Current - Output / Channel: 140mA
• Outputs And Type: 4, Non-Isolated
• Features: Dimmable
• Voltage - Input: 8 ~ 18V
• Utilized Ic / Part: LM3431
• Kit Contents: Board, Datasheet
• Svhc: No SVHC (15-Dec-2010)
• Kit Features: Designed To Drive 4 Parallel Strings Of 8 Series LEDs At 140mA Per String From An Input 8V To 18V
• Mcu Supported Families: LM3431
• Rohs Compliant: No
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Voltage - Output: -
• Lead Free Status / Rohs Status: Supplier Unconfirmed

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SS/SH voltage below its maximum. The AFB pin voltage at
the end of softstart is 1.85V typically, which determines the
ratio of the feedback resistors according to the following equa-
tion:
The AFB resistors also set the output over-voltage (OVP)
threshold. The OVP threshold is monitored during both LED
on and LED off states and protects against any over voltage
condition, including all LEDs open (See Open LED
section).The OVP threshold at Vout can be calculated as fol-
lows:
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 LED Protection 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. Figure 2 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.
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 volt-
age 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 cy-
cles 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 Figure 1) can be determined according
to the following empirically derived equation:
Where the f
SW
is in kHz and the R
R
T
= 35403 x f
T
SW
result is in kohm.
-1.06
12
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 rip-
ple will increase. To avoid this, set the operating frequency
below the following maximum setting:
INDUCTOR SELECTION
Figure 4 shows how the inductor current, I
switching cycle.
The important quantities in determining a proper inductance
value are I
peak to peak inductor current ripple). If Δi
I
cycle and the converter will operate in discontinuous conduc-
tion mode. If Δi
stay above zero and the converter will operate in continuous
conduction mode.
To determine the minimum L, first calculate the I
minimum and maximum input voltage:
L(AVE)
FIGURE 4. Inductor Current, SW Voltage, and V
, the inductor current will drop to zero for a portion of the
FIGURE 3. Switching Frequency vs R
L(AVE)
L
is smaller than 2 x I
(the average inductor current) and Δi
L
, the inductor current will
L
L
is larger than 2 x
30041126
, varies during a
L(AVE)
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T
at both
OUT
L
(the