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LD7750E

High Voltage Green-Mode PWM Controller with Over Temperature Protection

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LD7750E

Oscillator and Switching Frequency

The LD7750E is implemented with Frequency Swapping

function which helps the power supply designers to both

optimize EMI performance and minimize the system

cost. The switching frequency substantially centers at

65KHz, and swaps between a range of ±4KHz.

Output Stage and Maximum Duty-Cycle

An output stage of a CMOS buffer with driving capability

of 500mA typically is incorporated to drive a power

MOSFET directly. And the maximum duty-cycle of

LD7750E is limited to 75% to avoid the transformer

saturation.

Voltage Feedback Loop

The voltage feedback signal is provided from the TL431 in

the secondary side through the photo-coupler to the

COMP pin of LD7750E. The input stage of LD7750E,

like the UC384X, is incorporated with 2 diodes voltage

offset circuit and a voltage divider with 1/3 ratio.

Therefore,

V+ (PWM COMPARATOR

)

=

1

3

× (VCOMP

−

2VF )

A pulled-high resistor is embedded internally and thus no

external one is required.

Internal Slope Compensation

Stability is crucial for current mode control when it

operates at more than 50% of duty-cycle. To stabilize the

control loop, the slope compensation is required in the

traditional UC384X design by injecting the ramp signal

from the RT/CT pin through a coupling capacitor. In the

LD7750E, the internal slope compensation circuit has

been implemented to simplify the external circuit design.

On/Off Control

The LD7750E can be turned off by pulling COMP pin to

below 1.8V. The gate output pin of LD7750E will be

disabled immediately under such condition. The off-mode

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

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

IP.

Over Load Protection (OLP) - Auto

Recovery

To protect the circuit from damage due to overload

condition or output short condition, a smart OLP function

is implemented in the LD7750E for it. The OLP function is

an auto-recovery type protection. Fig. 17 shows the

waveforms of the OLP operation. Under such fault

condition, the feedback system will force the voltage loop

toward saturation and thus pull up the voltage of COMP

pin (VCOMP). If the VCOMP trips the OLP threshold of

5.0V and stays for over 64mS, the protection will be

activated to turn off the gate output and to shutdown the

switching of power circuit. The 64mS delay time is to

prevent the false-trigger during the power-on and turn-off

transient.

A divided-by-2 counter is implemented to reduce the

average power consumption under OLP behavior.

Whenever OLP is activated, the output is latched off and

the divided-by-2 counter starts to count the number of

UVLO(off). The latch will be released if the 2nd UVLO(off)

point is counted, and then the output recovers switching

again.

By using such protection mechanism, the average input

power can be reduced to a very low level so that the

Leadtrend Technology Corporation

LD7750E-DS-00 February 2012

11

www.leadtrend.com.tw

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