FA3686 Datasheet, PDF (16/18 Pages) List of Unclassifed Manufacturers – For Switching Power Supply Control

FA3686V
s Design advice
1. Setting the oscillation frequency
As described in item 1, “Description of each circuit,” a desired
oscillation frequency can be determined by the value of the
resistor connected to the RT pin. When designing an oscillation
frequency, you can set any frequency between 300kHz and
1.5MHz. You can obtain the oscillation frequency from the
characteristic curve “Oscillation frequency (fosc) vs. timing
resistor resistance (RT)” or the value can be approximately
calculated by the following expression.
fOSC = 4050 ϫ RT ^–0.86
( ) ^1.16
RT = ⁴⁰⁵⁰
fOSC
fOSC: Oscillation frequency [kHz]
RT: Timing resistor [kΩ]
This expression, however, can be used for rough calculation,
the obtain value is not guaranteed. The operation frequency
varies due to the conditions such as tolerance of the
characteristics of the ICs, influence of noises, or external
discrete components. When determining the values, examine
the effectiveness of the values in an actual circuit. The timing
resistor RT should be wired to the GND pin as shortly as
possible because the RT pin is a high impedance pin and is
easy affected by noises.
2. Determining soft start period
The period from the start of charging the capacitor CCS to
widening n% of output duty cycle can be roughly calculated by
the following expression: (see Fig. 5 for symbols)
t [s] = ^{VCS2n Ï« CCS1} For CS1 pin
ICS1
t [s] = ^{(VREG – VCS2n) ϫ CCS1} For CS2 pin
ICS2
CCS1, CCS2: Capacitance connected to the CS1 or CS2 pin [µF]
ICS1, ICS2: CS charge current [µA] (2µA typ.)
VCS1n and VCS2n are the voltage of the CS1 and CS2 pins in
n% of output duty cycle, and vary in accordance with operating
frequency. The value can be obtained from the characteristic
curve “Output duty cycle vs. CS voltage”
The charging of the CCS1 and CCS2 starts after the UVLO is
unlocked. Therefore, the period from power-on of VCC to
widening n% of output duty cycle is the sum of t0 and t.
To reset the soft start function, the supply voltage VCC is
lowered below the UVLO voltage (2.1V typ.) and then the
internal switch discharges the CS capacitor. The characteristics
of the internal switch for discharge are shown in following the
characteristics curves of “Characteristics of CS1 internal
discharge switch current vs. voltage” and “Characteristics of
CS2 internal discharge switch current vs. voltage”. Therefore,
when determining the period of soft start at restarting the power
supply, consider the characteristics carefully.
Threshold
voltage
VCS1n
t0
VCC pin voltage
CS1 pin voltage
t
Fig. 9
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