LD7537

11

Leadtrend Technology Corporation

www.leadtrend.com.tw

LD7537-DS-01a December 2011

Internal Slope Compensation

In the conventional applications, the problem of the

stability is a critical issue for current mode controlling,

when it operates over 50% duty-cycle. As UC384X, It

takes slope compensation from injecting the ramp signal

of the RT/CT pin through a coupling capacitor. It therefore

requires no extra design for the LD7537 since it has

integrated it already.

On/Off Control

The LD7537 can be turned off by pulling COMP pin lower

than 1.5V. The gate output pin of the LD7537 will be

disabled immediately under such condition. The off-mode

can be released when the pull-low signal is removed.

Over

Load

Protection

(OLP)

-

Auto

Recovery

To protect the circuit from damage due to over-load

condition and short or open-loop condition, the LD7537 is

implemented with smart OLP function. It also features

auto recovery function; see Fig. 18 for the waveform. In

case of fault condition, the feedback system will force the

voltage loop toward the saturation and then pull the

voltage high on COMP pin (VCOMP). When the VCOMP

ramps up to the OLP threshold of 4.5V and continues over

OLP delay time, the protection will be activated and then

turn off the gate output to stop the switching of power

circuit.

With the protection mechanism, the average input power

will be minimized to remain the component temperature

and stress within the safe operating area.

VCC

UVLO(on)

UVLO(off)

t

t

COMP

OLP

4.5V

t

OUT

OLP delay time

Switching

Switching

Non-Switching

OLP trip Level

UVLO(off)

OLP Reset

Fig. 18

OVP (Over Voltage Protection) on Vcc -

Latch mode

The Vcc OVP functions of LD7537 are latch mode.

Whenever the voltage on the Vcc pin is higher than the

OVP threshold, the output gate drive circuit will be

shutdown simultaneous to latch off the switching of the

power MOSFET. As soon as the voltage on Vcc pin drops

below OVP threshold and starts AC-recycling again, it will

recover to normal operation. Figure 19 shows its

operation.

Fig. 19

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