MB3800 Datasheet, PDF (12/21 Pages) Fujitsu Component Limited. – Switching Regulator Controller (Low Voltage)

MB3800
10. Functional Description
10.1 Switching Regulator Function
10.1.1 Reference voltage circuit
The reference voltage circuit generates a temperature-compensated reference voltage (1.25V) from voltage supplied from the V_CC
pin (pin 3). In addition to providing the reference voltage for the switching regulator, the circuit also sets the idle period.
10.1.2 Sawtooth wave oscillator
The sawtooth oscillator generates a sawtooth wave (up to 1 MHz) that is stable with respect to the supply voltage and temperature.
The capacitor and resistor that set the oscillation frequency are connected to the OSC pin (pin 7).
10.1.3 Error amplifier (Error Amp)
The error amplifier detects the output voltage of the switching regulator and outputs the PWM control signal. The voltage gain is fixed,
and connecting a phase compensation capacitor to the FB pin (pin 8) provides stable phase compensation for the system.
10.1.4 PWM comparator (PWM Comp.)
The voltage comparator has one inverting and three non-inverting inputs. The comparator is a voltage/pulse width converter that
controls the ON time of the output pulse depending on the input voltage. The output level is high (H) when the sawtooth wave is lower
than the error amplifier output voltage, soft start setting voltage, and idle period setting voltage.
10.1.5 Output circuit
The output circuit has a totem pole type configuration and can drive an external NPN transistor directly. The value of the ON/OFF
current can be set by a resistor connected to the BR/CTL pin (pin 4).
10.2 Power Supply Control Function
Stand-by mode (supply current 1 A or less) can be set by connecting the BR/CTL pin (pin 4) to V_CC or by making the pin open circuit.
SW
Mode
OFF
Stand-by mode
ON
Operating mode
MB3800
4
BR/CTL
RB
SW
Document Number: 002-08716 Rev. *A
Page 12 of 21