ld7578j Leadtrend Technology, ld7578j Datasheet - Page 10

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

Current Sensing, Leading-edge Blanking and
the Negative Spike on CS Pin
The typical current mode PWM controller feeds back both
current signal and voltage signal to close the control loop
and achieve regulation. The LD7578J 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
current limit. The maximum voltage threshold of the current
sensing pin is set as 0.85V. Thus the MOSFET peak current
can be calculated as:
A 250nS leading-edge blanking (LEB) time is included in the
input of CS pin to prevent false-trigger caused by the current
spike. In the low power application, if the total pulse width of
the turn-on spike is less than 250nS and the negative spike
on the CS pin is not as low as -0.3V, the R-C filter (as shown
in Fig.15) 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
Leadtrend Technology Corporation
LD7578J-DS-00 March 2009
I
PEAK
(
MAX
Fig. 14
)
=
. 0
R
85
S
V
www.leadtrend.com.tw
10
Fig. 16) for higher power applications to avoid the CS pin
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. The maximum duty-cycle of LD7578J 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 LD7578J.
UC384X, is incorporated with 2 diodes voltage offset circuit
and a voltage divider with 1/3 ratio. Therefore,
A pull-high resistor is embedded internally and thus an
external one is not required.
Oscillator and Switching Frequency
The switching frequency of LD7578J is fixed at 65kHz to
provide the optimized operations by considering the EMI
performance, thermal treatment, component sizes and
transformer design.
V
+
(
PWM
COMPARATOR
The input stage of LD7578J, like the
Fig. 15
)
=
3
1
LD7578J
×
(
V
COMP
−
2
V
F
)