MB3778 Datasheet, PDF (5/29 Pages) Fujitsu Component Limited. – Switching Regulator Controller

MB3778
s OPERATION DESCRIPTION
1. Reference voltage circuit
The reference voltage circuit generates a temperature-compensated reference voltage (=: 2.46 V) from VCC (pin
9) . The reference voltage is used as an operation power supply for internal circuit.
The reference is obtained from the VREF terminal (pin 16).
2. Triangular wave oscillator
Triangular waveforms can be generated at any frequency by connecting a timing capacitor and resistor to the
CT terminal (pin 1) and to the RT terminal (pin 2) .
The amplitude of this waveform is from 1.3 V to 1.9 V. These waveforms are connected to the non-inverting
inputs of the PWM comparator and can be output through the CT terminal.
3. Error amplifiers (Error Amp.)
The error amplifier detects the output voltage of the switching regulator and outputs PWM control signals.The
in-phase input voltage range is from 1.05 V to 1.45 V.The reference voltage obtained by dividing the reference
voltage output (recommended value : VREF/2) or the RT terminal voltage (1.23 V) is supplied to the non-inverting
input. The VREF/2 voltage is internally connected to non-inverting input of the other error amplifier.
Any loop gain can be chosen by connecting the feedback resistor and capacitor to the inverting input terminal
from the output terminal of the error amplifier.Stable phase compensation is possible.
4. Timer latch short circuit protection circuit
This circuit detects the output levels of each error amplifier. If the output level of one or both of the error amplifiers
is 2.1 V or higher, 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 idle period to 100%.
5. Under voltage lock-out circuit
The transition state at power-on or a momentary drops in supply voltage may cause the control IC to malfunction,
which may adversely affect or even destroy the system. The under voltage lockout circuit monitors VCC with
reference to the internal reference voltage and resets the latch circuit to turn the output drive transistor off. The
idle period is set to 100%. It also pulls the SCP terminal (pin 15) “Low”.
6. PWM comparator unit
Each PWM comparator has one inverting input and two non-inverting inputs. This voltage-to-pulse-width con-
verter controls the turning on time of the output pulse according to the input voltage.
The PWM comparator turns the output drive transistor on while triangular waveforms from the oscillator are
lower than the error amplifier output and the DTC terminal voltage.
7. Output drive transistor
The output drive transistors have open collector outputs with common source supply and common grounds
independent of VCC and signal ground. The output drive transistors for switching can sink or source up to 50 mA.
8. Power control unit
The power control terminal (pin 14) controls power on/off modes(the power supply current in stand-by mode is
10 µA or lower).
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