FA5626 FUJI [Fuji Electric], FA5626 Datasheet - Page 16

• Manufacturer Part Number: FA5626
• Description: Fuji Switching Power Supply Control IC
• Manufacturer: FUJI [Fuji Electric]

FA5626
(2)Oscillator
This oscillator is used to determine the switching frequency.
The switching frequency in the normal operation mode is
set to 65kHz (typ) within the IC.
To minimize the loss of power in the standby state, this IC is
equipped with a function of automatically decreasing the
switching frequency under light load.
When the FB pin voltage decreases down to 1.8V (typ) or
lower under light load, the frequency decreases almost
linearly proportional to the FB pin voltage.( See Fig.4) The
minimum frequency, Fmin, has been set to 0.45kHz (typ).
When the load further decreases and thus the FB pin
voltage decreases down to 0.4V (typ) or lower, the
switching is stopped. (See one-shot circuit.)
In addition to trigger signals for determining switching
Fig.4 Oscillation frequency
frequency, the oscillator generates pulse signals for
determining the maximum duty cycle and ramp signals for
performing slope compensation.
Frequency diffusion（Spread spectrum）
FA5626 perform frequency modulation of ± 4.5 kHz for
switching frequency 65 kHz (during the operation in which
the FB pin voltage is higher than 1.55 V.). This function
enables more noise energy of the switching to disperse
compared to the case with fixed frequency and obtains a
conduction EMI reduction effect. While the reduction effect
depends on the filter parts mounted on the power supply
board, effective use of this function allows the reduction of
the number of the filter parts and the constants.
(3) Current comparator & PWM circuit
The IC performs current mode control. Fig.5 shows a circuit
block for basic operations, and Fig.6 shows a timing chart.
Fig.5 Current mode basic operation circuit block
The polarity of the current detection voltage of the IS pin is
negative. The GND of the IC is connected between the
current detection resistor Rs and the MOSFET. (See Fig.5)
A trigger signal having the switching frequency that is
output from the oscillator is input to the PWM (F.F.) through
the one-shot circuit as a set signal. Then the output of the
PWM as well as the OUT pin voltage reaches the High
state.
On the other hand, the current comparator (IS comp.)
monitors the MOSFET current, and if the threshold voltage
is reached, a reset signal is output. When a reset signal is
input, the output of PWM (F.F.) as well as the OUT pin
voltage reaches the Low state.
The ON pulse width of the OUT pin is thus controlled with
the threshold voltage of the current comparator (IS comp.).
The output is controlled by changing the threshold voltage
of this IS comp. with feedback signals.
As shown in Fig.7, the FB pin voltage is level-shifted by a
reverse amplifier and input into the current comparator (IS
Fig.6 Current mode basic operation timing chart
comp.) as the threshold voltage. In addition, -0.5V (typ)
reference voltage is input inside the IC to regulate the
maximum input threshold voltage of the IS pin, VthIS1 (over
current control threshold).
Fuji Electric Co., Ltd.
AN-071E Rev.1.2
http://www.fujielectric.co.jp/products/semiconductor/
16
April-2011