LM3431MH National Semiconductor, LM3431MH Datasheet - Page 11

• Manufacturer Part Number: LM3431MH
• Description: 3-Channel Constant Current LED Driver with Integrated Boost Controller
• Manufacturer: National Semiconductor

SOFT-START
The SS/SH pin is a dual function softstart and sample/hold
pin. The SH function is described in sections below. When the
EN pin is pulled above the programmable UVLO threshold
and VCC rises above the internal UVLO threshold, the SS/SH
pin begins sourcing current. This charges the SS cap (C6)
and the SS pin voltage in turn controls the output voltage
ramp-up, sensed via the AFB pin. The softstart capacitor is
calculated as shown below, where 20 µA is the typical soft-
start source current:
The LED current regulators are held off until softstart is com-
pleted. During softstart, current limit is active and the CFB pin
is monitored for a cathode short fault (See LED protection
section). When the SS/SH voltage reaches 1.85V, the current
regulators are activated, LED current begins flowing, and out-
put voltage control is transferred to the CFB pin. Typical
startup is shown below in Figure 2.
OUTPUT VOLTAGE, OVP, and SH
The LM3431 boost controls the LED cathode voltage in order
to drive the LED strings with sufficient headroom at optimum
efficiency. When the LED strings are on, voltage is regulated
to 1.7V (typical) at the CFB pin, which is one diode Vf above
the LED cathode voltage. Therefore, when the LED strings
are on, the output voltage (LED anodes, shown as VA in Fig-
ure 1) will vary according to the Vf of the LED string, while the
LED cathode voltage will be regulated via the CFB pin.
The AFB pin is used to regulate the output voltage when the
LED strings are off, which is during startup and dimming off
cycles. During LED-off times, the cathode voltage is not reg-
ulated.
AFB should set the initial output voltage to at least 1.0V (CFB
voltage minus one diode drop) above the maximum LED
string forward voltage. This ensures that there is enough
headroom to drive the LED strings at startup and keeps the
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:
FIGURE 2. Typical Startup Waveforms
(From power-on, DIM = high)
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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
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