LM3431MHX National Semiconductor, LM3431MHX Datasheet - Page 17

• Manufacturer Part Number: LM3431MHX
• Description: 3-Channel Constant Current LED Driver with Integrated Boost Controller
• Manufacturer: National Semiconductor

on-times. To ensure good linearity, select NPN regulators
with short and similar rise and fall times.
SETTING FF
To minimize voltage transients during LED dimming, the out-
put voltage is regulated via the AFB pin during LED off times.
However, because the control loop has a limited response
time, voltage transients can never be completely eliminated.
If these transients are large enough, LED current will be af-
fected and ceramic output capacitors may generate audible
noise. The FF pin speeds up the loop response time, and thus
minimizes output voltage transients during dimming.
A resistor connected from FF to ground, Rff, sets the FF cur-
rent which is injected into the control loop at the rising and
falling edge of the dimming signal. In this way, the FF pin cre-
ates a correction signal before the control loop can respond.
A smaller FF resistor will generate a larger correction signal.
The minimum recommended Rff value is 10k.
Since the amount of FF correction required for a given appli-
cation depends on many factors, it is best to determine a FF
resistor value through bench testing. Use the following pro-
cedure to determine an optimal Rff value:
An Rff value of approximately 20k is a good starting point. A
20 kΩ potentiometer in series with a 10 kΩ resistor works well
for bench testing.
The dimming frequency must be selected before setting Rff.
Confirm that boost switching operation is stable at 100% dim-
ming duty cycle.
Adjust Rff until the COMP pin voltage is between 0.8V and
0.9V. Next, monitor the cathode voltage response at a low
dimming duty cycle while adjusting Rff until the overshoot and
undershoot is minimal or there is a slight overshoot.
Check the cathode voltage response at the lowest input volt-
age and lowest dimming duty cycle and adjust Rff if neces-
sary. This is typically the worst case condition.
The curves in Figure 10 below show the variation in cathode
voltage with different Rff settings. Notice that at the ideal set-
ting, both the cathode voltage and COMP voltage are flat. For
clarity, the 3 cathode voltage curves in this figure have been
offset; all FF settings will result in the cathode voltage settling
at 1.2V typically.
Once an Rff value has been set, check the cathode voltage
over the input voltage range and dimming duty range. Some
further adjustment may be necessary.
FIGURE 10. FF Setting Example
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In practice the FF pin also has a small effect on the control
loop response. As a final step, switching stability at 100%
dimming duty should be re-verified once the Rff value has
been selected. At the optimal Rff setting, output voltage tran-
sients will be minimized and the cathode voltage will be stable
across the range of input voltage and dimming duty cycle.
The ideal cathode response illustrated in Figure 10 may not
be achievable over the entire input voltage range. However,
LED current will not be affected as long as the cathode voltage
remains above the regulator saturation voltage and below the
open LED fault threshold (See Open LED section).
A wide input voltage range will cause a wider variation in the
feedforward effect, thus making duty cycles less than 1%
more difficult to achieve. For any given application there is a
minimum achievable dimming duty cycle. Below this duty cy-
cle, the cathode voltage will begin to drift higher, eventually
appearing as an open LED fault (See LED Protection section).
During an LED open fault condition, cathode voltage over-
shoot will tend to increase. If Rff is not set appropriately, high
overshoots may be detected as an LED short fault and lead
to shutdown.
LED PROTECTION
Fault Modes and Fault Delay
The LM3431 provides 3 types of protection against several
types of potential faults. The table below summarizes the fault
protections and groups the fault responses into three types
(the auto-restart option is described in the next section).
When Type 1 or Type 2 faults occur, the DLY pin begins
sourcing current (57 µA typical). A capacitor connected from
DLY to ground (C7) sets the DLY voltage ramp and shutdown
delay time. For a Type 1 fault, the LM3431 will continue to
LED short
All LEDs
Cathode
multiple
Multiple
VIN<6V
Current
voltage
Output
1 LED
1 LED
UVLO
short,
over-
open
short
open
short
THM
fault
TSD
LED
limit
AFB < 0.85V
mechanism
temperature
THM < 1.2V
AFB > 2.0V
AFB > 2.0V
CFB low at
VCC or EN
SC > 3.1V
SC > 3.1V
SC > 6.0V
COMP at
IC over
startup
max
low
Fault Mode Summary
charges
charges
charges
charges
charges
charges
charges
charges
No DLY
No DLY
No DLY
action
DLY
DLY
DLY
DLY
DLY
DLY
DLY
DLY
flag
flag
flag
Shutdown or
Shutdown or
Shutdown or
Shutdown or
Shutdown or
Shutdown or
auto-restart
auto-restart
auto-restart
auto-restart
auto-restart
auto-restart
continue to
continue to
response
stand by
stand by
stand by
regulate
regulate
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type
1
2
3