MB3775 Datasheet, PDF (3/25 Pages) Fujitsu Component Limited. – SWITCHING REGULATOR CONTROLLER

MB3775
s OPERATION DESCRIPTION
1. Reference voltage
The reference voltage circuit generates a stable, temperature-compensated 2.5 V reference from Vcc (pin 9) for use by internal
circuits.
A reference voltage of temperature compensated 1/2 Vref can be obtained to external circuit by Vref terminal (pin 16).
2. Oscillator
A triangular waveform of any frequency is obtained by connecting an external capacitor and resistor to the CT (pin 1) and RT
terminals (pin 2).
The amplitude of this waveform is from 1.3 V to 1.9 V. The oscillator is internally connected to the non-inverting inputs of the
PWM comparators. The oscillator waveform is available at the CT terminal.
3. Error amplifiers
The error amplifier detects the output voltage of the switching regulator.
The common-mode input voltage range is -0.2 V to 1.45 V, so the input reference voltage can be set the VREF and GND levels.
Error amplifiers can be used as either inverting and non-inverting amplifiers.
The voltage gain is fixed. Phase compensation is possible by connecting a capacitor to the FB terminals (pins 5 and 12) of the
error amplifiers.
The error amplifier output are internally connected to the inverting inputs of the PWM comparators and also to the short protection
circuit.
4. Timer latch short protection circuit
The timer latch short protection circuit detects the output levels of the error amplifiers. If one or both error amplifier outputs are
1.1 V or lower, the timer circuit begins charging the externally connected protection enable capacitor.
If the output level of the error amplifier does not drop below the normal voltage range before the capacitor voltage reaches the
transistor base-emitter voltage VBE ( 0.65 V), the latch circuit turns the output drive transistor off and sets the dead time to 100 %.
5. Under voltage lockout protection circuit
An ambiguous transition state at power-on or a momentary fluctuation in the supply line may result in loss of control and may
adversely affect or even destroy the system. The under voltage lockout protection circuit compares the internal reference voltage
level with the supply voltage level. If the supply voltage level falls below the reference level the latch circuit is reset the output
drive transistor is turned off and the dead time is set to 100 %. The protection enable terminal (pin 15) is pulled “Low”.
6. PWM comparator
Each PWM comparator has two inverting inputs and one non-inverting input. This voltage-to-pulse-width converter controls the
output pulse width according to the input voltage.
The PWM comparator turns the output drive transistor on when the oscillator triangular waveform is higher than the error amplifier
output and the dead time control terminal voltage.
7. Output drive transistor
The open-collector output-drive transistors provide common-emitter output of 18 V dielectric capability. The output drive tran-
sistors can source up to 50 mA of drive current to the switching power transistor.
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