MB3800 Fujitsu Microelectronics, Inc., MB3800 Datasheet - Page 11

• Manufacturer Part Number: MB3800
• Description: Switching Regulator Controller (low Voltage)
• Manufacturer: Fujitsu Microelectronics, Inc.

1. Switching Regulator Function
(1) Reference voltage circuit
The reference voltage circuit generates a temperature-compensated reference voltage ( 1.25V) from voltage
supplied from the power supply pin (pin 3). In addition to providing the reference voltage for the switching regulator,
the circuit also sets the idle period.
(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).
(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.
(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.
(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).
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
the pin open circuit.
FUNCTIONAL DESCRIPTION
OFF
SW
ON
Stand-by mode
Operating mode
Mode
MB3800
RB
4
SW
BR/CTL
MB3800
CC
or by making
11