LD7538GL datasheet(11/17 Pages) LEADTREND

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LD7538

Leadtrend Technology Corporation

LD7538-DS-00

November 2012

Adjustable

Over

Current

Compensation

(CS/OVP Pin)

In general, the power converter can deliver more current

at high input voltage than at low input voltage. To

compensate this, an offset voltage is added to the CS

signal by an internal current source (IOCP) and an external

resistor (ROCP) in series between the sense resistor (Rs)

and the CS/OVP pin, as shown in Fig. 17. By selecting a

proper value of the resistor in series with the CS pin, the

amount of compensation can be adjusted. The value of

IOCP depends on the duty cycle of OUT pin. The equation

of IOCP is decreased as:























)

125

Duty

(

240

)

625

Duty

(

)

625

Duty

125

(

480

)

Duty

625

(

IOCP

In light load conditions, the offset should be removed

since it is in same order of magnitude as the current

sense signal. Therefore the compensation current is only

fully added when the COMP voltage is higher than 2.9V.

ROCP:470~1.2k; COCP:47pF~390pF

0.85V

OCP

Comparator

V BIAS

Duty/VCOMP

OUT

LEB

ROCP

CS/OVP

IOCP

COCP

Fig. 17

Output Over Voltage Protection (CS/OVP

Pin) - Latch mode

An output overvoltage protection is implemented in the

LD7538, as shown in Fig. 18 and 19. It senses the

auxiliary voltage via the divided resistors. The auxiliary

winding voltage is reflected from secondary winding and

therefore the flat voltage on the

CS/OVP pin is

proportional to the output voltage. LD7538 can sample

this flat voltage level after a delay time to perform output

over voltage protection. This delay time is used to ignore

the voltage ringing from leakage inductance of PWM

transformer. The sampling voltage level is compared with

internal threshold voltage 0.2V. If the sampling voltage

exceeds the OVP trip level, an internal counter starts

counting subsequent OVP events. The counter has been

added to prevent incorrect OVP detection which might

occur during ESD or lightning events. However, when

typically 8 cycles of subsequent OVP events are detected,

the OVP circuit switches the power MOSFET off. As the

protection is latched, the converter only restarts after the

internal latch is reset.

ROCP

AUX

0.2V

OUT

Sample

Delay

Debouce

8 cycle

OVP

Fig. 18

Delay

Sample

Out

AUX Winding

CS/OVP

Fig. 19

Over

Load

Protection

(OLP)

Auto

Recovery

To protect the circuit from damage in over-load condition

and short or open-loop condition, the

LD7538

implemented with smart OLP function. It also features

auto recovery function; see Fig. 20 for the waveform. In

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