ld7577gs Leadtrend Technology, ld7577gs Datasheet - Page 10

• Manufacturer Part Number: ld7577gs
• Description: High Voltage Green-mode Pwm Controller With Brown-out 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. LD7577 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
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 remain a small R-C filter (as
Leadtrend Technology Corporation
LD7577-DS-01 January 2009
UVLO(on)
UVLO(off)
HV Current
Vcc
Vcc current
Startup Current
current
1mA
I
PEAK
(
MAX
Fig. 14
limit.
Operating Current
(Supply from Auxiliary Winding)
)
=
The
. 0
~ 0mA (off)
R
85
S
V
maximum
www.leadtrend.com.tw
t
t
voltage
10
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. And the maximum duty-cycle of the LD7577 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 LD7577.
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
LD7577
+
VCC
(
PWM
GND
The input stage of the LD7577, like the
OUT
COMPARATOR
CS
Fig. 15
Can be removed if the negative
spike is not over spec. (-0.3V).
)
=
1
3
×
(
V
LD7577
COMP
250ns
blanking
time
−
2
V
F
)