ld7576jps Leadtrend Technology, ld7576jps Datasheet - Page 11

• Manufacturer Part Number: ld7576jps
• Description: Green-mode Pwm Controller With High-voltage Start-up Circuit And Adjustable Olp Delay Time
• Manufacturer: Leadtrend Technology

and achieve regulation. The LD7576X series detects the
primary MOSFET current from the CS pin, which is not only
for the peak current mode control but also for the
pulse-by-pulse
threshold of the current sensing pin is set at 0.85V. Thus the
MOSFET peak current can be calculated as:
A 230nS leading-edge blanking (LEB) time is provided in the
input of CS pin to prevent false-triggering from the current
spike. In the low power applications, if the total pulse width
of the turn-on spikes is less than 230nS and the negative
spike on the CS pin does not exceed -0.3V, the R-C filter (as
shown in figure15) can be eliminated.
However, the total pulse width of the turn-on spike is related
to the output power, circuit design and PCB layout. It is
strongly recommended to add a small R-C filter (as shown in
figure 16) for higher power application to avoid the CS pin
from being damaged by the negative turn-on spike.
Output Stage and Maximum Duty-Cycle
An output stage of a CMOS buffer, with typical 500mA
driving capability, is incorporated to drive a power MOSFET
directly. And the maximum duty-cycle of LD7576X series
are limited to 75% to avoid the transformer saturation.
Voltage Feedback Loop
The voltage feedback signal is provided from the TL431 on
the secondary side through the photo-coupler to the COMP
pin of LD7576X series. The input stage of LD7576X series,
like the UC384X, is with 2 diodes voltage offset to feed the
voltage divider with 1/3 ratio, that is,
A pull-high resistor is embedded internally to optimize the
external circuit. Generally, an external capacitor in parallel
to photo-coupler is required in application.
Leadtrend Technology Corporation
LD7576-DS-03 December 2007
V
+
(
PWM
COMPARATOR
current
I
PEAK
(
MAX
limit.
)
)
=
=
1
3
The
. 0
×
R
85
(
S
V
V
COMP
maximum
www.leadtrend.com.tw
−
2
V
F
)
voltage
11
Oscillator and Switching Frequency
The switching frequency of LD7576X series are fixed at
65KHz and 100KHz internally to provide the optimized
LD7576X
VCC
GND
LD7576/76H/76J/76K
OUT
CS
Fig. 15
Fig. 16
Can be removed if the negative
spike is not over spec. (-0.3V).
230ns
blanking
time