LD7552 查看數據表（PDF） - Unspecified
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LD7552 Datasheet PDF : 10 Pages
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Vin
Cbulk
R1
D1
C1
VCC
OUT
LD7552
Comp
CS
GND
Rs
Fig. 8
Oscillator and Switching Frequency
Connecting a resistor from RT pin to GND according to the
equation can program the normal switching frequency:
fSW
66.5 100(KHz)
RT(K )
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. The input stage of LD7552, like the
UC384X, is with 2 diodes voltage offset then feeding into the
voltage divider with 1/3 ratio, that is,
V (PWMCOMPARATOR )
1
3
( VCOMP
2VF )
VCC
OUT
LD7552
CS
GND
250ns
blanking
time
remove
Fig. 9
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.
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 internal slope compensation circuit has been
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 are many difference topologies has been
implemented in different chips for the green-mode or power
saving requirements such as “burst-mode control”,
“skipping-cycle Mode”, “variable off-time control “…etc. The
7
Leadtrend Technology Corporation
LD7552-DS-00 February, 2005
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