SD4844 Silan Microelectronics, SD4844 Datasheet - Page 7

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

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3.
The oscillation frequency is kept changed for low EMI and decreasing radiation on one frequency. The oscillation
frequency changes within a very small range to simplify EMI design. The rule of frequency changing: change
from 65KHz to 69KHz.
4.
350mV<FB <500mV, there are two different conditions: when FB changes from low to high, there is no action for
switch and it is the same with condition of FB lower than 350mV; the other is that FB changes form high to low,
comparison value is increased for increasing turning on time to decrease switch loss.
by load) because of the high comparison value to decrease FB until it is 350mV below; when FB <350mV, there
is no action for switch and output voltage decrease (decreasing speed is also decided by load) to increase FB.
This is repeated to decrease action of switch for lower power dissipation.
5.
For this current-controlled circuit, there is pulse peak current during the transient of switch turning on and there is
an error operation if the current is sampled during this time. And leading edge blanking is adopted to eliminate
this error operation. The output of PWM comparator is used for controlling shutdown after the leading edge
blanking if there is any output drive.
HANGZHOU SILAN MICROELECTRONICS CO.,LTD
Http: //www.silan.com.cn
Working in this mode to reduce power dissipation. It works normally when FB is 500mV above and during
For this mode, during FB changes form high to low, the output voltage increases (increasing speed is decided
Frequency Jitter
Light Load Mode
Leading Edge Blanking
SD4840/4841/4842/4843/4844
REV:1.2
Page 7 of 11
2008.12.17