MB3782 Datasheet, PDF (5/28 Pages) Fujitsu Component Limited. – Switching Regulator Controller

MB3782
s FUNCTIONAL DESCRIPTIONS
1. Reference Voltage Source
The reference voltage source uses the voltage provided at the power supply pin (pin 20) to generate a
temperature-compensated reference voltage (≅ 2.50 V), which is used as the operating power supply for the
internal circuits of the IC. The reference voltage source can be output through the VREF pin (pin 1).
2. Triangular Wave Oscillator
By connecting a timing capacitor and resistor respectively to the CT pin (pin 2) and RT pin (pin 3), the oscillator
can provide a triangular waveform at any desired frequency.
The waveform has an amplitude of 1.3 V to 1.9 V, and can be connected to the non-inverting input of the on-
chip PWM comparator and also output through the CT pin.
3. Error Amps
The error amps are amplifiers that detect the output voltage of the switching regulator and send the PWM control
signal. The common-mode input voltage range is 1.05 V to 1.45 V, so that the voltage applied to the non-inverting
input pin as a reference voltage should be either the voltage obtained by dividing the IC reference voltage output
(recommended value: VREF/2) or the voltage obtained from the RT pin (1.25 V). The non-inverting input for the
error amps 1 and 2 is internally connected to VREF/2 voltage.
Also, a feedback transistor and capacitor can be connected between the error amp output pin and inverting
input pin to provide any desired level of loop gain, enabling stable phase compensation.
4. Timer Latch (S-R Latch) Type Short Prevention Circuit
The timer-latch type short prevention circuit detects the output levels from each of the error amps. Whenever
one or more error amps produces an output level of 2.1 V or higher, the timer circuit is activated starting the
charging of the external protection enabler capacitor.
If the error amp output voltage does not return to normal range before the voltage in this capacitor reaches the
transistor’s base-emitter junction voltage (VBE (≅ 0.65 V)), the latch circuit will operate to turn the output transistor
off and at the same time set the dead time to 100%.
Once the prevention circuit is activated, the power must be switched on again to resume normal operation.
5. Low Input Voltage Fault Prevention Circuit (Under Voltage Lock-Out (UVLO) function)
When power is switched on, excess power or momentary drops in power line current can cause operating faults
in the controller IC, which can in turn lead to damage or deterioration in systems.
The low input voltage fault prevention circuit detects the internal reference voltage level with respect to the
power supply voltage level and acts to reset the latch circuit, thereby turning the output transistor off and at the
same time setting the dead time to 100% and holding the SCP pin (pin 14) at “low.” Operation returns to normal
when the power supply voltage reaches or exceeds the UVLO threshold voltage level.
6. PWM Comparator
The PWM comparator is a voltage comparator with one inverting and two non-inverting inputs, which acts as a
voltage to pulse width converter controlling the on-time of the output pulse according to the input voltage level.
When the triangular waveform produced by the oscillator is lower than either the error amp output or the DTC
pin voltage, the output transistor is switched on.
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