LM3450MT/NOPB National Semiconductor, LM3450MT/NOPB Datasheet - Page 11

• 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: -

Theory of Operation
The LM3450 is a single device with both power factor control
(PFC) and phase dimming decoder functions. This device is
designed to control isolated flyback converters and provide
active power factor correction. In addition to being a PFC, the
LM3450 can interpret a phase dimming (frequently called triac
dimming) input and provide a corresponding PWM output to
properly dim an LED load. This combination of features pro-
vides an excellent method to convert a standard AC mains
input to a dimmable LED output of 10-100W. It should be not-
ed that the LM3450 can control a boost converter in a similar
manner. However, this datasheet will focus mostly on the fly-
back topology due to the high demand for isolated LED driver
applications. Discussion of the LM3450 functionality will refer
to
The PFC control operates in critical conduction mode (CRM)
using zero crossing detection (ZCD) to terminate the off-time.
The PFC portion of this device includes an error amplifier,
multiplier, current sense circuit, zero crossing detector, and
gate driver. The internal error amplifier is used for feedback
of the output voltage in non-isolated designs. However, it can
Figure 1
component designators.
FIGURE 1. Typical Flyback Application
11
be disabled for isolated designs where the error amplifier
needs to be on the secondary side.
The phase dimmer decoder detects the dimming angle of the
rectified AC line, decodes, filters and remaps it to a 500Hz
PWM output. The PWM output can then be sent directly, or
through optical isolation, to the dimming input of a second
stage LED driver. To ensure the decoder properly interprets
the dimming angle, dynamic hold is provided which prevents
the phase dimmer from misfiring. The input current is sensed
and when the current drops below a preset minimum, the
system adds more current.
Both the dynamic hold and the decoder are sampled syn-
chronously to reduce the overall efficiency drop due to the
additional hold current. When a decoding sample period oc-
curs, the dynamic hold is activated to ensure a proper angle
is decoded. Because of this sampling method, non-sampled
cycles will potentially cause the phase dimmer to misfire but
should not affect the output LED current regulation. Finally,
the dynamic filter and variable sampling rate provide fast,
smooth dimming transitions.
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