FA3686 Datasheet, PDF (13/18 Pages) List of Unclassifed Manufacturers – For Switching Power Supply Control

5. PWM comparator
The PWM output generates from the oscillator output, the error
amplifier output (FB1, FB2) and CS voltage (CS1, CS2) (Fig. 4).
The oscillator output is compared with the preferred lower
voltage between FB1 and CS1 for ch1. While the preferred
voltage is lower than oscillator output, the PWM output is low.
While the preferred voltage is higher than oscillator output, the
PWM output is high. Since the phase of Ch2 is the opposite
phase of Ch1, higher voltage between FB2 and CS2 is
preferred and while the preferred voltage is lower than the
oscillator output, the PWM output 2 is high. (Cannot be
observed externally) The output polarity of OUT1, OUT2
changes according to the condition of SEL pin. (See Fig. 6)
The maximum duty cycle (DMAX1, DMAX2) is internally set
approximately 85%. Note that the maximum duty cycle depends
on operation frequencies. (See the characteristics curve: Output
duty cycle vs. oscillation frequency)
6. Soft start function
This IC has a soft start function to protect DC-to-DC converter
circuits from damage when starting operation. CS1 pin (Pin10)
and CS2 pin (Pin7) are used for soft start function of ch1 and
ch2 respectively. (Fig. 5) When the supply voltage is applied to
the VCC pin and UVLO is cancelled, the capacitor CCS1 and
CCS2 is charged by the internal constant current sources (2µA,
typ.). Then, the CS1 voltage gradually increases, and the CS2
voltage gradually decreases. Since the CS1, and CS2 are
connected to the PWM comparator, the pulses gradually widen
and then the soft start function operates. (Fig. 6)
FA3686V
PWM
FB1
Comp.1
CS1
Oscillator
output
DMAX1
UVLO
DMAX2
CS2
FB2
PWM
Comp.2
Fig. 4
PWM
output1
Nch.
drive
OUT1
9
PWM output2
Pch.
drive
OUT2
8
10
CS1
CCS1
VREG
13
CCS2
7
CS2
VREG
13
Fig. 5
Oscillator output
Er. amp.1 output
CS1 pin voltage
PWM output 1
OUT1
Nch.drive
CS2 pin voltage
Oscillator output ^{Er. amp.2 output}
PWM output2
OUT2
Pch. drive
Fig. 6
13