LM3450MTSLASHNOPB datasheet(14/40 Pages) TI1

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THM

COMP

MLT

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 (RFB1, RFB2) 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 (VFB) 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 (VCMP) exceeds the minimum operational voltage (VTHM).

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 5.11k

Ω resistor (RFB1) 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 (VAC), provided via a resistor divider (RAC1, RAC2), are inputs to

the multiplier. The current through the sense resistor (RCS) produces a voltage (VCS) that is compared to the

multiplier output. When VCS exceeds the multiplier output, QSW 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:

(4)

Once QSW is turned off, the LM3450/50A waits until the inductor (boost) or transformer (flyback) is demagnetized

to turn QSW 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 QSW via a sense resistor (RCS) between the source of QSW and GND.

When VCS exceeds the output of the multiplier (VMLT), QSW is turned off. VMLT is variable over the line cycle and is

a function of the scaled rectified AC voltage (VAC), the COMP voltage referenced from its operational minimum

(VCOMP-VTHM), the multiplier gain (KM) and the PWM comparator offset (VOS):

(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 QSW, limiting the minimum achievable duty cycle.

OVER CURRENT PROTECTION

The LM3450/50A has a current limit threshold (VLIM = 1.5V) at CS to protect the system from over-current

conditions. If VCS exceeds VLIM, QSW is immediately turned off until ZCD triggers a new on-time.

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