ld7578h Leadtrend Technology, ld7578h Datasheet - Page 11

• Manufacturer Part Number: ld7578h
• 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 to achieve voltage regulation. The LD7578H 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.
Leadtrend Technology Corporation
LD7578H-DS-00 February 2010
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
11
However, the total pulse width of the turn-on spike is
related to the output power, circuit design and PCB layout.
Nevertheless, it is strongly recommended to 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.
LD7578H is limited to 75% to avoid the transformer
saturation.
Voltage Feedback Loop
The voltage feedback signal is issued from the TL431 in
the secondary side through the photo-coupler to the
COMP pin of LD7578H. The input stage of LD7578H,
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 thus can be
eliminated on the external circuit.
V
+
(
PWM
COMPARATOR
And the maximum duty-cycle of
)
=
1
3
LD7578H
×
(
V
COMP
−
2
V
F
)