SD4844 Silan Microelectronics, SD4844 Datasheet - Page 6

• Manufacturer Part Number: SD4844
• Description: (SD4840 - SD4844) Current Mode PWM Controller
• Manufacturer: Silan Microelectronics

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FUNCTION DESCRIPTION
gate driver and current mode PWM controller which includes frequency oscillator and various protections such as
undervoltage lockout, overvoltage protection, overcurrent protection and overtemperature protection. Frequency
jitter generated from oscillator is used to lower EMI and built-in soft start is used for reducing transformer stress
when the circuit is powered on. Burst mode is adopted during light load to lower standby power dissipation, and
function of lead edge blanking eliminates the MOSFET error shutdown caused by interference through
minimizing MOSFET turning on time. Few peripheral components are needed for higher efficiency and higher
reliability and it is suitable for flyback converter and forward converter.
1.
At the beginning, the capacitor connected to pin V
circuit start to work if voltage at Vcc is 12V. The output is shutdown if there is any protection during normal
operation and Vcc is decreased because of powering of auxiliary winding. The whole control circuit is shutdown
if voltage at Vcc is 8V below to lower current dissipation and the capacitor is recharged for restarting.
2.
In order to decrease transformer stress and to prevent its saturation during power on, it is recommended to
increase peak current value of primary winding slowly by increasing feedback voltage slowly. After about 15ms,
the soft start is completed and it has no effect on normal operation.
HANGZHOU SILAN MICROELECTRONICS CO.,LTD
Http: //www.silan.com.cn
SD4840/4841/4842/4843/4844 is designed for off-line SMPS, consisting of high voltage MOSFET, optimized
Under Voltage Lockout and Self-Start
Built-In Soft Start Circuit
CC
is charged via start resistor by high voltage AC and the
SD4840/4841/4842/4843/4844
REV:1.2
Page 6 of 11
2008.12.17