LD5537GL datasheet(12/17 Pages) LEADTREND

12 / 17 page

LD5537

09/07/2015

Leadtrend Technology Corporation

www.leadtrend.com.tw

LD5537-DS-01 September 2015

Voltage Feedback Loop

The voltage feedback signal is provided from the TL431 at

the secondary side through the photo-coupler to the

COMP pin of the LD5537.

Similar to UC3842, the

LD5537 would carry 2 diode voltage offset at the stage to

feed the voltage divider at the ratio of RA and RB, that is,

)

(

COMP

)

PWM

(

COMPARATOR











A pull-high resistor is embedded internally and therefore

no external one is required.

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 the ramp signal of the

RT/CT pin injected through a coupling capacitor. It

therefore requires no extra design for the LD5537 since it

has integrated it already.

On/Off Control

To pull COMP below 1.6V can disable the gate output pin

of the LD5537. The off-mode can be released when the

pull-low signal is removed.

Over

Power

Protection

(OPP)

Auto

Recovery

To protect the circuit from damage in over-power short or

open-loop condition, the LD5537 is implemented with

smart OPP function. It also features auto recovery

function, see Fig. 19 for the waveform. In case of fault

condition, the feedback system will force the voltage loop

enter toward saturation and then pull the voltage high on

COMP pin (VCOMP).

When the VCOMP ramps up to the

OPP threshold of 4.7V and continues over OPP 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)

COMP

OPP

4.7V

OUT

TOPP

Switching

Non-Switching

OPP Trip Level

OPP Reset

Fig. 19

Over

Current

Protection (OCP)

Auto

Recovery

When the switching current is higher than the OCP

threshold, the internal counter counts up. When the total

accumulated counting time is more than 110ms, the

controller triggers the OCP. This protection is auto

recovery function.

OSCP (Output Short Circuit Protection) -

Auto Recovery

Even when the output shorts to GND, there

’s no way to

turn off the signal unless the following four conditions are

met.

1. The CS is higher than limit voltage.

2. The COMP voltage is higher than 4.7V

3. This duration is greater than 8 cycles.

4. Turn on time is lower than 1us.

The out signal could not be charged either, if it fails to

meet the four conditions.

Once the protection is triggered, switching is terminated

and the MOSFET remains off.

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