MB3800

Document Number: 002-08716 Rev. *A

Page 12 of 21

10. Functional Description

10.1 Switching Regulator Function

10.1.1 Reference voltage circuit

The reference voltage circuit generates a temperature-compensated reference voltage (

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

SW

Mode

OFF

Stand-by mode

ON

Operating mode

RB

SW

4

BR/CTL

MB3800