LM3450MT/NOPB National Semiconductor, LM3450MT/NOPB Datasheet - Page 18

• Manufacturer Part Number: LM3450MT/NOPB
• Description: IC LED DRIVER DIMMING 16-TSSOP
• Manufacturer: National Semiconductor

Specifications of LM3450MT/NOPB

• Featured Product: LM3450 LED Driver
• Internal Driver: No
• Type - Primary: General Purpose
• Type - Secondary: High Brightness LED (HBLED)
• Mounting Type: Surface Mount
• Topology: PWM
• Number Of Outputs: 1
• Voltage - Supply: 8.5 V ~ 20 V
• Voltage - Output: 11.5V
• Package / Case: 16-TSSOP (0.173", 4.40mm Width)
• Operating Temperature: -40°C ~ 125°C
• Led Driver Application: General Lighting
• No. Of Outputs: 1
• Output Current: 200mA
• Output Voltage: 11.5V
• Input Voltage: 8.5V To 20V
• Dimming Control Type: PWM / Analog
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Frequency: -

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This PWM signal at the DIM pin can be used as the dim input
to a secondary LED driver. DIM is an open drain output de-
signed for isolated solutions. Optical isolation is used to trans-
mit signals across the isolation boundary. With most opto-
isolators, the edge rate is dependent on the amount of drive
current through the photodiode. The open-drain configuration
allows the primary bias supply (V
shown in
V
enables the user to trade-off PWM accuracy with system ef-
ficiency.
The open drain configuration also ensures that the secondary
has a resistor from the phototransistor’s emitter to secondary
ground (not from collector to secondary bias). During system
turn-off, this prevents an undesired LED blink because the
secondary stage LED driver is forced off.
A variable sample rate and dynamic filter ensure fast, smooth
dimming transitions (movement of the dimmer) while main-
taining robust flicker-free behavior when the dimmer is static.
The sample rate depends on past and present angle infor-
mation. The dynamic filter is a dual mode filter. During stand-
by mode, when a transition has not been made and the
dimmer is static, a 500kΩ series resistor is connected be-
tween the buffered output and FLT1 as shown in
The 500kΩ resistor is shorted when the LM3450 senses a
large transition of the dimmer. This increases the filter speed
while the dimmer is transitioning between levels to improve
response time.
The FLT1 and FLT2 poles created by each RC pair (R
C
•
CC
F1
C
, R
and the photodiode anode will set the drive current. This
F1
F2
and C
and C
Figure
F2
F2
can be 1µF ceramic capacitors for all designs.
) should be set as follows:
10. The choice of resistor (R
CC
) to provide the current as
PB
Figure
) between
F1
10.
and
18
•
2 Hz poles provide a “smooth fade” while 10Hz poles create
a “snappy” response.
These component values ensure that the static filter condition
in standby mode has 1 pole approximately a decade lower
than the nominal in order to provide good noise immunity to
the system.
Since the buffered decoder output has amplitude equal to
V
voltage at FLT2, the V
mapping (extend the usable range of some dimmers). The
maximum LED current (DIM = 0) when V
to decoded angles of 70% or greater. Some dimmers have a
maximum angle greater than this. If V
the maximum LED current will correspond to an angle of 80%
and at V
of 95%.
The V
log adjust function. If the demodulated phase angle at V
above 85%, then the fast filter is always enabled (500kΩ
shorted) and the V
pin duty cycle. When V
ters low power shutdown so the maximum attainable contrast
ratio using V
Both FLT1 and FLT2 have pull-down MosFETs that are
turned on when V
provides a quick discharge path for the capacitors and elimi-
nates the possibility of an undesired light level at the next
startup.
ADJ
R
75kΩ (~2Hz).
F1
and the resulting PWM signal is filtered into an analog
ADJ
and R
ADJ
pin can also be used to implement a standard ana-
= 2V the maximum will occur at a decoded angle
ADJ
F2
should be set between 15kΩ (~10Hz) and
only is approximately 40:1.
CC
ADJ
UVLO falling threshold is triggered. This
pin can solely be used to scale the DIM
ADJ
ADJ
is pulled below 75mV the part en-
pin can be used to change the
ADJ
ADJ
is reduced to 2.5V,
= 3V corresponds
AC
is