LD7537 datasheet(12/16 Pages) LEADTREND

12 / 16 page

LD7537

Leadtrend Technology Corporation

www.leadtrend.com.tw

LD7537-DS-01a December 2011

Brownout Protection

The LD7537 programmable to set the brownout protection

point though BNO pin. The voltage across the BNO pin is

proportional to the bulk capacitor voltage, referred as the

line voltage. A 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 20 shows the operation.

When VBNO falls below 0.80V, 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 1.05V, 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. Figure 20 shows the circuit.

Fig. 20

Oscillator and Switching Frequency

The LD537 is implemented with Frequency Swapping

function which helps the power supply designers to both

optimize EMI performance and lower system cost. The

switching frequency substantially centers at 65KHz, and

swap between a range of ±4KHz.

Green-Mode Operation

By using the green-mode control, the switching frequency

can be reduced under the light load condition. This feature

helps to improve the efficiency in light load conditions.

The green-mode control is Leadtrend Technology’s own

property.

Fault Protection

There are several critical protections integrated in the

LD7537 to prevent from damage to the power supply.

Those damages usually come from open or short

conditions on the pins of LD7537.

In case under such conditions listed below, the gate

output will turn off immediately to protect the power circuit.

CS pin floating

COMP pin floating

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