Fujitsu MB3788 Functional Description, Major Functions, Channel Control Function, Error amplifier

MB3788

■FUNCTIONAL DESCRIPTION

1. Major Functions

(1)Reference voltage power circuit

The reference voltage power supply produces a reference voltage (≈ 2.50 V) which is temperature-compensated by the voltage supplied from the power pin (pin 11); it is used as the IC internal circuit operating power supply. The reference voltage can also be output externally at 1 mA from VREF pin (pin12).

(2)Error amplifier

The error amplifier detects the switching regulator output voltage and outputs a PWM control signal. It has a wide in-phase input voltage range of -0.2 V to VCC - 1.8 V to make setting from an external power supply easy.

Connecting the output pin and inversion input pin of the error amplifier through a feedback resistor and capacitor allows setting of any loop gain to provide stable phase compensation.

(3)PWM comparator

The PWM comparator controls the output pulse ON time according to the input voltage.

The voltage input to the -IN pin (PWM) turns the output transistor on when it is lower than the output voltage of the error amplifier.

(4)Output circuit

The output circuit is configured in a push-pull form and uses a PNP transistor drive system to drive a transistor of up to 30 mA. (See How to Set Output Current.)

2. Channel Control Function

Channels can be set ON/OFF by combining the voltage levels at pin CTL1 (pin 13) and pin CTL2 (pin 14).

Channel ON/OFF Setting Conditions

Voltage level at CTL pin			Channel ON/OFF status
CTL1	CTL2	Power circuit		Channel 1	Channel
L	⋅			Stand by state*
H	H		ON		ON
	L				OFF

*: The power current in the standby state is 10 ∝A Max.

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