Сергей Борщ (11.12.2009 02:43, просмотров: 102) ответил Сергей Борщ на Конкретно у той микросхемы в цепи коррекции стояла емкость порядка 3.9пФ. Паразитные емкости разводки примерно такого же порядка. Вам ваш САПР выдает паразитные емкости получившейся разводки? Или вы можете прямо на плате их измерить? Проще подобрать.
Ой, соврал - то была не LT, LT тоже рассматривал, но остановился на LM2623 CHOOSING THE CORRECT C3 CAPACITOR
The C3 capacitor allows for the duty cycle of the internal
oscillator to be programmable. Choosing the correct C3
capacitor to get the appropriate duty cycle for a particular
application circuit is a trial and error process. The non-linear
effect that C3 produces is dependent on the input voltage
and output voltage values. The correct C3 capacitor for
particular input and output voltage values cannot be calcu-
lated. Choosing the correct C3 capacitance is best done by
trial and error, in conjunction with the checking of the induc-
tor peak current to make sure your not too close to the
current limit of the device. As the C3 capacitor value in-
creases, so does the duty cycle. And conversely as the C3
capacitor value decreases, the duty cycle decreases. An
incorrect choice of the C3 capacitor can result in the part
prematurely tripping the current limit and/or double pulsing,
which could lead to the output voltage not being stable.
Выделено мной.