ld7578j Leadtrend Technology, ld7578j Datasheet - Page 11

• Manufacturer Part Number: ld7578j
• Description: High Voltage Green-mode Pwm Controller With Over Temperature Protection
• Manufacturer: Leadtrend Technology

Frequency Trembling
The LD7578J is implemented with a frequency trembling
function which helps the power supply designers to optimize
EMI performance with lower system cost. The Trembling
frequencies is fixed internally at ±4KHz, and incorporates
with the 65KHz switching frequencies respectively.
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 LD7578J, the
internal slope compensation circuit has been implemented
to simplify the external circuit design.
On/Off Control
The LD7578J can be turned off by pulling COMP pin to
lower than 1.2V. The gate output pin of LD7578J will be
Leadtrend Technology Corporation
LD7578J-DS-00 March 2009
Fig. 16
www.leadtrend.com.tw
11
disabled immediately under such condition. The off-mode
can be released when the pull-low signal is removed.
Dual-Oscillator Green-Mode Operation
Lots of topologies have been implemented in different chips
for the green-mode or power saving requirements such as
“burst-mode control”, “skipping-cycle mode”, “variable
off-time control “…etc. The basic operation theory of all
these approaches intends to reduce the switching cycles
under light-load or no-load condition either by skipping some
switching pulses or by reducing the switching frequency.
What the LD7578J uses to implement the power-saving
operation is Leadtrend Technology’s own IP. By using this
dual-oscillator control, the burst-mode frequency can be well
controlled to avoid the generation of audible noise.
Over Load Protection (OLP) - Auto Recovery
To protect the circuit from the damage caused by overload
condition or output short condition, a smart OLP function is
implemented in the LD7578J for it. The OLP function in
LD7578J is an auto-recovery type protection. Fig. 17 shows
the waveforms of the OLP operation.
condition, the feedback system will force the voltage loop
toward saturation and thus pull the voltage on COMP pin
(VCOMP) to high. Whenever the VCOMP trips the OLP
threshold of 5.0V and stays for over 30mS (if switching
frequency is 65KHz), the protection will be activated to turn
off the gate output and to shutdown the switching of power
circuit. The 30mS 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 under OLP behavior.
activated, the output is latched off and the divided-by-2
counter starts to count the number of UVLO(off).
latch is 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
component temperature and stress can be controlled within
a safe operating area.
LD7578J
Whenever OLP is
Under such fault
The