MB3873 Datasheet, PDF (12/22 Pages) Fujitsu Component Limited. – Multi-Resonance AC/DC Converter IC

MB3873
(3) Overvoltage detection comparator 2 (OVP Comp.2)
When the input voltage at the +IN terminal (pin 15) is greater than the input voltage at the -IN terminal (pin 16),
the CS terminal is set to “L” level causing the frequency to increase. When the +IN input voltage falls below the
-IN input voltage, soft start processing is performed to restart operation. Overvoltage detection comparator 2
does not provide the same latch operation as OVP Comp.1.
Note that if OVP Comp.2 is not used, the +IN terminal (pin 15) should be shorted to GND, and the -IN terminal
(pin 16) should be connected to the VREF terminal (pin 10) by the shortest path (see “PROCESSING WHEN
OVERVOLTAGE DETECTION COMPARATOR 2 IS NOT USED”).
(4) Overload detection comparator circuit (OCP Comp.)
When an overload occurs, the OCP Comp. circuit detects the overload signal output by the oscillator frequency
control circuit, and after a given interval sets the error detection latch and sets the VREF terminal (pin 10) and
OUT terminal (pin 8) to “L” level. The time interval from overload detection to setting of the error latch is determined
by the delay interval setting capacitor connected to the CD terminal (pin 4).
Note that if the overload detection function is not used, the CD terminal (pin 4) should be shorted to GND by
the shortest path (see “PROCESSING WHEN THE CD PIN IS NOT USED”).
(5) Overtemperature detection comparator (OTP Comp.)
The over temperature detection comparator detects the input voltage at the OTP terminal (pin 13) and if greater
than the threshold voltage ( =: 0.98V) sets the error detection latch, and sets the VREF terminal (pin 10) and
OUT terminal (pin 8) to “L” level.
Note that if the overtemperature detection function is not used, the OTP terminal (pin 13) should be shorted to
GND by the shortest path (see “PROCESSING WHEN OTP PIN IS NOT USED”).
3. Soft Start Function
Soft Start Circuit (CS)
The MB3873 oscillator frequency control circuit includes an on-chip soft start circuit. Soft starting can be provided
by connecting a capacitor to the CS terminal (pin 6). At start up, this causes the PFM control signal to be input
to the triangular wave oscillator, thereby controlling the control frequency and preventing current rush.
Note that if the soft start function is not used, the CS terminal (pin 6) should be left open. (See “PROCESSING
WHEN CS PIN IS NOT USED.”)
s SETTING THE OSCILLATOR FREQUENCY
The oscillator frequency is set by the timing capacitor CT and timing resistor RT connected to the CT pin and RT
pin respectively.
Oscillator frequency fOSC (when frequency control is not exerted by the FB, CS pins)
fOSC [kHz]=:
7.6 × 10⁵
CT [pF] × RT [kΩ]
s SETTING THE DEAD TIME
The dead time is set by the dead time resistor RD connected to the RD pin.
Dead time (output pin square wave pulse width)
tDEAD [ns] =: 4.8 × RD [kΩ] – 44
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