LD5537

09/07/2015

14

Leadtrend Technology Corporation

www.leadtrend.com.tw

LD5537-DS-01 September 2015

the compensation current will be fully added once the

COMP voltage is above 3.05V, as shown in Fig. 23.

ROCP: 470~1.4k; COCP:82pF~390pF

0.75V

Limit

Comparator

VBIAS

Duty/VCOMP

OUT

LEB

ROPP

RS

CS

IOPP

Fig. 22

VCOMP

IOPP

Duty

4.2V

3.9V

Duty

≤ 0.1

Duty= 0.2

Duty= 0.3

Fig. 23

Output Over Voltage Protection

– Auto

Recovery

An output overvoltage protection is implemented in the

LD5537 to sense the auxiliary voltage via the divided

resistors as shown in Fig. 24. The auxiliary winding

voltage is reflected to the secondary winding and

therefore the flat voltage on the CS pin is in proportion to

the output voltage. LD5537 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 0.30V. If the sampling voltage exceeds the OVP

trip

level,

an internal counter

starts

counting

the

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 auto

recovery, the converter restarts after the VCC is lower than

UVLO OFF level and then recharge to UVLO ON.

ROPP

RS

CS

AUX

0.30V

OUT

Sample

Delay

Debouce

8 cycle

OVP

Fig. 24

Delay

Sample

Out

AUX Winding

Fig. 25

Oscillator and Switching Frequency

The LD5537 is implemented with Frequency Swapping

function which helps the power supply designers optimize

EMI performance and reduce system cost. The switching

frequency substantially centers at 130kHz, and swap

between a range of ±6%.

Green-Mode Operation

By using the green-mode control, the switching frequency

can be reduced in light load condition. This feature helps

to improve the efficiency in light load conditions. The

green-mode control is Leadtrend Technology

’s own

property.