LM3450 Datasheet, PDF (16/40 Pages) National Semiconductor (TI) – LED Driver with Active Power Factor Correction and Phase Dimming Decoder

LM3450
SNVS681D – NOVEMBER 2010 – REVISED MAY 2013
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If the COMP pin voltage (V_CMP) falls below 1.4V at any time, the device enters burst mode where the GATE is off
for 340µs then is turned on. If V_CMP is still below 1.4V at the end of the on-time then another 340µs off-time
occurs. However, if V_CMP has risen above 1.4V, the converter continues switching until it falls below the threshold
again. This feature is necessary to prevent the output of the converter from rising arbitrarily high because the
minimum on-time of the device prevents less energy transfer.
The LM3450/50A also implements both feedback short circuit protection and output over-voltage protection
(OVP) functions at the FB pin. If V_FB exceeds 3V, then OVP is engaged and the part stops switching until V_FB
falls below 3V. In the same manner, if V_FB falls below 168mV, then shutdown is engaged and switching stops
until V_FB exceeds 188mV.
The flyback topology is frequently used to provide isolation from input to output. Since, the current transfer ratio
(CTR) of standard optical isolation varies over temperature, proper regulation using primary error amplifiers is
difficult. An error amplifier is usually placed in the secondary to regulate the output voltage accurately. To
accommodate isolated designs, the LM3450/50A internal error amplifier can be bypassed by placing a 5.11kΩ
resistor from FB to GND. This engages a 5kΩ pull-up resistor from COMP to an internal 5V rail.
SECONDARY ERROR AMPLIFIER
For isolated designs, the error amplifier on the secondary should take the form of a proportional integral (PI)
compensator. The amplifier is frequently implemented with an LMV431. The output voltage resistor divider (R_FB1,
R_FB2) provides the sensed output voltage to the LMV431 inverting input. The PI compensation is achieved by
connecting R_SC and C_SC in between the LMV431 input and output, shown in Figure 23. In addition, C_CMP is
placed from COMP to GND on the primary for higher frequency noise attenuation.
In addition to the basic error amplifier, a soft-start circuit can be implemented using a capacitor, two diodes and a
Zener diode as shown in Figure 23. This secondary softstart circuit has no restart mechanism, therefore a
primary side softstart is recommended as described in the SOFTSTART section of this document.
LM3450/50A
5V
5k
FB COMP
V_SB
R_SBS
V_OUT
SOFT
START
R_FB
C_CMP
OPTO
R_SC
R_FB2
C_SC
LMV431
R_FB1
Figure 23. Secondary Error Amplifier
PRECISION VOLTAGE REFERENCE
The LM3450/50A provides a 3V voltage reference (V_REF) for biasing the V_ADJ pin as well as any external circuitry.
V_REF is regulated once V_CC exceeds 3V. There is a 2mA current limit for the reference. A 10nF ceramic bypass
capacitor should be placed from V_REF to GND.
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