ld7578 Leadtrend Technology, ld7578 Datasheet - Page 10

• Manufacturer Part Number: ld7578
• Description: Green-mode Pwm Controller With Hv Start-up Circuit And Otp 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 to achieve voltage regulation. LD7578 detects the
primary MOSFET current from the CS pin, which is
applied 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 incorporated
in the input of CS pin to prevent the false-trigger caused
by any current spike. For low power applications, if the
total pulse width of each 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. Nevertheless, it is strongly recommended to
Leadtrend Technology Corporation
LD7578-DS-00 October 2008
UVLO(on)
UVLO(off)
HV Current
Vcc
Vcc current
Startup Current
1mA
I
PEAK
(
MAX
Fig. 14
)
Operating Current
(Supply from Auxiliary Winding)
=
. 0
R
85
~ 0mA (off)
S
V
www.leadtrend.com.tw
t
t
10
remain a small R-C filter (as shown in Fig. 16) for higher
power applications to avoid the CS pin being damaged by
negative turn-on spikes.
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.
LD7578 is limited to 75% in order to avoid the transformer
flux saturation.
Voltage Feedback Loop
The voltage feedback signal is issued from the TL431 in
the secondary side through the photo-coupler to COMP
pin of the LD7578. The input stage of the LD7578, like
the 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, eliminating
external corresponding components on a board.
LD7578
V
VCC
+
(
PWM
GND
OUT
COMPARATOR
CS
The maximum duty-cycle of the
Fig. 15
Can be removed if the negative
spike is not over spec. (-0.3V).
)
=
3
1
×
(
V
COMP
LD7578
250ns
blanking
time
−
2
V
F
)