MB3775 参数 Datasheet PDF下载

MB3775

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OPERATION DESCRIPTION

1. Reference voltage

The reference voltage circuit generates a stable, temperature-compensated 2.5 V reference from Vcc terminals

(pin 9) for use by internal circuits.

A reference voltage of temperature compensated 1/2 V

REF

can be obtained to external circuit by V

REF

terminal

(pin 16).

2. Oscillator

A triangular waveform of any frequency is obtained by connecting an external capacitor and resistor to the C

T

terminal (pin 1) and R

T

terminals (pin 2).

The amplitude of this waveform is from 1.3 V to 1.9 V. The oscillator is internally connected to the non-inverting

inputs of the PWM comparators. The oscillator waveform is available at the C

T

terminal (pin 1).

3. Error amplifiers

The error amplifier detects the output voltage of the switching regulator.

The common-mode input voltage range is

−0.2

V to 1.45 V, so the input reference voltage can be set the V

REF

terminal (pin 16) and GND terminal levels. Error amplifiers can be used as either inverting and non-inverting

amplifiers.

The voltage gain is fixed. Phase compensation is possible by connecting a capacitor to the FB terminals (pins

5 and 12) of the error amplifiers.

The error amplifier output are internally connected to the inverting inputs of the PWM comparators and also to

the short protection circuit.

4. Timer latch short protection circuit

The timer latch short protection circuit detects the output levels of the error amplifiers. If one or both error amplifier

outputs are 1.1 V or lower, 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 V

BE

( 0.65 V), the latch circuit turns the output drive transistor off

and sets the dead time to 100 %.

5. Under voltage lockout protection circuit

An ambiguous transition state at power-on or a momentary fluctuation in the supply line may result in loss of

control and may adversely affect or even destroy the system. The under voltage lockout protection circuit com-

pares the internal reference voltage level with the supply voltage level. If the supply voltage level falls below the

reference level the latch circuit is reset the output drive transistor is turned off and the dead time is set to 100%.

The protection enable terminal (pin 15) is pulled “Low”.

6. PWM comparator

Each PWM comparator has two inverting inputs and one non-inverting input. This voltage-to-pulse-width con-

verter controls the output pulse width according to the input voltage.

The PWM comparator turns the output drive transistor on when the oscillator triangular waveform is higher than

the error amplifier output and the dead time control terminal voltage.

7. Output drive transistor

The open-collector output-drive transistors provide common-emitter output of 18 V dielectric capability. The

output drive transistors can source up to 50 mA of drive current to the switching power transistor.

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