LD7523GS datasheet(12/18 Pages) LEADTREND

12 / 18 page

LD7523

Leadtrend Technology Corporation

www.leadtrend.com.tw

LD7523-DS-00

June 2009

Brownout Protection & Line Compensation

The LD7523 employs BNO pin to set the brownout

protection point and the line voltage OVP point though

BNO pin.

The voltage across the BNO pin is proportional to the bulk

capacitor voltage, referred as the line voltage.

brownout comparator is implemented to detect the

abnormal line condition. As soon as the condition is

detected, it will shut down the controller to prevent the

damage.

Figure 22 shows the operation.

When VBNO

falls below 1.25V, the gate output will be kept off even Vcc

has already achieved UVLO(ON). It therefore makes Vcc

hiccup between UVLO(ON) and UVLO(OFF). Unless the

line voltage is large enough to pull VBNO larger than 1.25V,

the gate output will not start switching even when the next

UVLO(ON) is tripped. A hysteresis is implemented to

prevent the false trigger during turn-on and turn-off.

Meanwhile, LD7523 detects the voltage across BNO pin

to feed the line compensation signal to the current sense

circuit. Figure 23 shows the circuit. The OCP level of

high-line and low-line can be set to a very close point.

The

voltage

gain

from

the

BNO

voltage

line

compensation is 0.04 (V/V).

The relationship between

BNO pin voltage and the line compensation is illustrated

in figure 24.

1.25V

Vcc

OUT

Switching

Non-Switching

VBNO

Line Voltage

1.10V

AC OK area

UVLO(on)

UVLO(off)

Fig. 22

∑

⋅

Fig. 23

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