FA5558 Fuji Electric holdings CO.,Ltd, FA5558 Datasheet - Page 17

• Manufacturer Part Number: FA5558
• Description: Green Mode Pwm Ic
• Manufacturer: Fuji Electric holdings CO.,Ltd

(2) Oscillator
This oscillator is used to determine the switching frequency.
The switching frequency in the normal operation mode is
set to 60kHz (typ) or 100kHz (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 terminal voltage decreases down to 1.15V
(typ) or lower under light load, the frequency decreases
almost
voltage.( See Fig.4) The minimum frequency, Fmin, has
been set to 0.34kHz (typ).
When the load further decreases and thus the FB terminal
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
frequency, the oscillator generates pulse signals for
determining the maximum duty cycle and ramp signals for
performing slope compensation.
(3) Current comparator and PWM latch circuit
The IC performs current mode control. Fig.5 shows a
circuit block for basic operations, and Fig.6 shows a timing
chart.
The polarity of the current detection voltage of the IS
terminal 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 latch (F.F.)
through the one-shot circuit as a set signal. Then the
output of the PWM latch as well as the OUT terminal
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 latch (F.F.) as well as the OUT
terminal voltage reaches the Low state.
The ON pulse width of the OUT terminal 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 terminal voltage is level-shifted
by a reverse amplifier and input into the current comparator
(IS comp.) as the threshold voltage. In addition, -0.95V
(typ) or -0.45V (typ) reference voltage is input inside the IC
to regulate the maximum input threshold voltage of the IS
terminal, VthIS1 (overcurrent control threshold).
The reverse amplifier output or the maximum IS terminal
input threshold voltage, VthIS1, whichever is higher, is
given precedence as the IS terminal threshold voltage.
(Example: When the output of the reverse amplifier is -0.5V
in a product whose maximum threshold voltage of the IS
terminal, VthIS1, is -0.95V, the output of the reverse
amplifier is given precedence and thus the current
comparator is reversed when the IS terminal voltage
reaches -0.5V.)
AN-025E Rev.0.1
December-2008
linearly
proportional
to
the
FB
terminal
FA5558/87/88/89
FA5581/82/83/84
17
Fig. 6. Current mode basic operating timing chart
Fig. 5 Current mode basic operation circuit block
Fig. 4 Oscillation frequency
Fuji Electric Device Technology