ld7577j Leadtrend Technology, ld7577j Datasheet - Page 10

• Manufacturer Part Number: ld7577j
• Description: High Voltage Green-mode Pwm Controller With Brown-out Protection
• Manufacturer: Leadtrend Technology

auxiliary winding of the transformer.
power losses on the startup circuit beyond the startup
period can be eliminated and the power saving can be
easily achieved. In general application, a 39KΩ resistor is
still recommended to be placed in high voltage path to
limit the current if there is a negative voltage applying in
any case.
An UVLO comparator is included to detect the voltage on
the V
power on the LD7577J PWM controller and in addition to
drive the power MOSFET.
Hysteresis is provided to prevent the shutdown caused by
the voltage dip during startup. The turn-on and turn-off
threshold levels are set at 16V and 10.0V, respectively.
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. LD7577J detects the
Leadtrend Technology Corporation
LD7577J-DS-00b July 2009
UVLO(on)
UVLO(off)
CC
pin to ensure the supply voltage is high enough to
HV Current
Vcc
Vcc current
Startup Current
1mA
Fig. 15
Operating Current
(Supply from Auxiliary Winding)
As shown in Fig. 15, a
~ 0mA (off)
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Therefore, the
t
t
10
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.16) can be
eliminated. Nevertheless, it is strongly recommended to
remain a small R-C filter (as shown in Fig. 17) 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. And the maximum duty-cycle of the
LD7577J 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 LD7577J. The input stage of the LD7577J, 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.
V
+
(
PWM
COMPARATOR
I
PEAK
(
MAX
)
)
=
=
3
1
. 0
LD7577J
×
R
85
(
S
V
V
COMP
−
2
V
F
)