ld7552 Leadtrend Technology, ld7552 Datasheet - Page 7

• Manufacturer Part Number: ld7552
• Description: Green-mode Pwm Controller
• Manufacturer: Leadtrend Technology

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 LD7552 is limited
to 75% to avoid the transformer saturation.
Leadtrend Technology Corporation
LD7552-DS-00 February, 2005
LD7552
VCC
remove
GND
Vin
OUT
CS
Cbulk
Comp
LD7552
Fig. 8
Fig. 9
VCC
GND
OUT
R1
CS
D1
C1
250ns
blanking
time
Rs
7
Oscillator and Switching Frequency
Connecting a resistor from RT pin to GND according to the
equation can program the normal switching frequency:
The suggested operating frequency range of LD7552 is
within 50KHz to 130KHz.
Voltage Feedback Loop
The voltage feedback signal is provided from the TL431 in
the secondary side through the photocoupler to the COMP
pin of LD7552.
UC384X, is with 2 diodes voltage offset then feeding into the
voltage divider with 1/3 ratio, that is,
A pull-high resistor is embedded internally thus can be
eliminated on the external circuit.
Internal Slope Compensation
A fundamental issue of current mode control is the stability
problem when its duty-cycle is operated more than 50%. To
stabilize the control loop, the slope compensation is needed
in the traditional UC384X design by injecting the ramp signal
from the RT/CT pin through a coupling capacitor. In LD7552,
the
implemented to simplify the external circuit design.
On/Off Control
The LD7552 can be controlled to turn off by pulling COMP
pin to lower than 1.2V. The gate output pin of LD7552 will
be disabled immediately under such condition. The off mode
can be released when the pull-low signal is removed.
Dual-Oscillator Green-Mode Operation
There
implemented in different chips for the green-mode or power
saving
“skipping-cycle Mode”, “variable off-time control “…etc. The
internal
V
are
requirements
+
(
PWM
many
f
slope
SW
COMPARATOR
The input stage of LD7552, like the
=
difference
RT
compensation
66
such
K (
5 .
Ω
)
)
=
×
as
3
1
100
×
topologies
(
V
“burst-mode
(
LD7552
KHz
COMP
circuit
)
−
2
V
has
has
F
)
control”,
been
been