LM3450 Datasheet, PDF (14/40 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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POWER FACTOR CONTROLLER
The LM3450/50A uses CRM control to regulate the output voltage and provide power factor correction. In a non-
isolated boost topology, an external voltage divider (R_FB1, R_FB2) is used to sense the output voltage, as shown in
Figure 21. The divider is connected to the inverting input (FB) of the internal error amplifier. The LM3450/50A
regulates the feedback voltage (V_FB) to 2.5V in a closed loop fashion.
The FB pin has a shutdown mode to protect against a feedback short and an OVP mode which terminates
switching when output over-voltage is sensed.
With the FB shutdown mode, it is necessary to have a preliminary biasing method for the output of the error
amplifier (COMP). Otherwise, the converter would never start. COMP is pre-biased with a 415µA current until the
voltage at COMP (V_CMP) exceeds the minimum operational voltage (V_THM).
For an isolated flyback topology, where the error amplifier is on the secondary, the LM3450/50A internal error
amplifier can be bypassed using a single 5.11kΩ resistor (R_FB1) from FB to GND. This engages an internal 5kΩ
pull-up resistor at COMP. COMP can then be connected directly to the optical isolation as shown in Figure 21.
COMP and the sensed rectified AC input voltage (V_AC), provided via a resistor divider (R_AC1, R_AC2), are inputs to
the multiplier. The current through the sense resistor (R_CS) produces a voltage (V_CS) that is compared to the
multiplier output. When V_CS exceeds the multiplier output, Q_SW is turned off. The peak detect threshold and the
current slope during an on-time are proportionally changing which yields a nearly constant on-time, shown in
Figure 20:
t
ON
=
L x I_P-PK
V_{IN- PK}
(4)
Once Q_SW is turned off, the LM3450/50A waits until the inductor (boost) or transformer (flyback) is demagnetized
to turn Q_SW on again. Demagnetization, sensed at ZCD, occurs when the current through the magnetic
component falls to zero. Since the output voltage is regulated, the slope of the current remains relatively constant
and, coupled with the variable peak detect, creates a variable off-time.
The sinusoidal peak detection envelope creates an input current that is sinusoidal and in phase with the input
voltage providing excellent PF. The PWM comparator 30mV input offset voltage ensures current is drawn at the
zero-crossings of the AC line, reducing distortion and further improving PF.
CURRENT SENSE
The LM3450/50A senses current through Q_SW via a sense resistor (R_CS) between the source of Q_SW and GND.
When V_CS exceeds the output of the multiplier (V_MLT), Q_SW is turned off. V_MLT is variable over the line cycle and is
a function of the scaled rectified AC voltage (V_AC), the COMP voltage referenced from its operational minimum
(V_COMP-V_THM), the multiplier gain (K_M) and the PWM comparator offset (V_OS):
( ) V_MLT = K_M x V_AC x V_COMP - V_THM + V_OS
(5)
The LM3450/50A has a leading edge blanking (LEB) circuit that pulls the current sense input to the PWM
comparator low for 140µs at the beginning of each on-time. The LEB blanks the current spike and associated
ringing due to the turn-on transient of Q_SW, limiting the minimum achievable duty cycle.
OVER CURRENT PROTECTION
The LM3450/50A has a current limit threshold (V_LIM = 1.5V) at CS to protect the system from over-current
conditions. If V_CS exceeds V_LIM, Q_SW is immediately turned off until ZCD triggers a new on-time.
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