LM3450 datasheet(14/28 Pages) NSC

14 / 28 page

30127426

FIGURE 5. PFC Control Circuit

POWER FACTOR CONTROLLER

The LM3450 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, RFB2) is used to

sense the output voltage, as shown in Figure 5. The divider

is connected to the inverting input (FB) of the internal error

amplifier. The LM3450 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 feed-

back short which can cause excessive energy transfer to the

output. In the same manner, the FB pin also has an OVP

mode which terminates switching when output over-voltage

is sensed.

With the FB shutdown mode, it is necessary to have a pre-

liminary 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

CMP) exceeds the minimum operational voltage (VTHM).

For an isolated topology flyback topology, where the error

amplifier is on the secondary, the LM3450 internal error am-

plifier can be bypassed engaging a 5k

Ω pull-up resistor at

COMP. COMP can then be connected directly to the optical

isolation as shown in Figure 5.

COMP and the sensed rectified AC input voltage (V

AC), pro-

vided via a resistor divider (R

AC1, RAC2), are inputs to the

multiplier. The current through the sense resistor (R

CS) pro-

duces a voltage (V

CS) that is compared to the multiplier out-

put. When V

CS 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 4:

Once Q

SW is turned off, the LM3450 waits until the inductor

(boost) or transformer (flyback) is demagnetized to turn Q

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 re-

mains relatively constant and, coupled with the variable peak

detect, creates a variable off-time.

The sinusoidal peak detection envelope creates an input cur-

rent that is sinusoidal and in phase with the input voltage

providing excellent PF. The PWM comparator 30mV input

offset voltage ensures current is also drawn at the zero-cross-

ings of the rectified AC line, reducing distortion and further

improving PF.

CURRENT SENSE

The LM3450 senses current through Q

SW via a sense resistor

CS) between the source of QSW and GND. When VCS ex-

ceeds the output of the multiplier (V

MLT), QSW is turned off.

MLT is variable over the line cycle and is a function of the

scaled rectified AC voltage (V

AC), the COMP voltage refer-

enced from its operational minimum (V

COMP-VTHM), the mul-

tiplier gain (K

M) and the PWM comparator offset (VOS):

It should be noted that the LM3450 leading edge blanking

(LEB) circuit pulls the current sense input to the PWM com-

parator 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 has a current limit threshold (V

LIM = 1.5V) at CS

to protect Q

SW along with the rest of the system from over-

current conditions. If V

CS exceeds VLIM, QSW is immediately

turned off.

LM3450 Datasheet(PDF) 14 Page - National Semiconductor (TI)

LM3450 Datasheet(HTML) 14 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

Datasheet Download

Link URL