MB3789 Просмотр технического описания (PDF) - Fujitsu

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MB3789

Switching Regulator Controller

Fujitsu 

MB3789

s FUNCTIONAL DESCRIPTION

1. Switching Regulator Functions

(1) Reference voltage generator

The reference voltage generator uses the voltage supplied from the power supply pin (pin 1) to generate a

temperature-compensated, reference voltage (about 2.50 V) as the reference supply voltage for the IC’s internal

circuitry.

The reference voltage can be output, up to 50 µA, to an external device through the VREF pin (pin 2).

This regulated reference voltage can be used as the reference voltage for the switching regulator and also

used for setting the dead time.

(2) Sawtooth waveform oscillator

With a timing capacitor and a timing resistor connected to the CT pin (pin 3), the sawtooth waveform oscillator

generates a sawtooth wave which remains stable even with supply voltage variations or temperature changes.

The sawtooth wave is input to the PWM comparator. The amplitude of oscillating waveform is 0.3 V to 0.9 V.

In addition, the oscillator can be used for external synchronization, where it generates a sawtooth waveform

synchronous to the input signal from the SYNC pin (pin 4).

(3) Error amplifiers

The error amplifiers detect the output voltage from the switching regulator and outputs the PWM control signal.

Since they support a wide range of in-phase input voltages from –0.2 V to “VCC – 1.8 V”, they can be set easily

from an external power supply.

An arbitrary loop gain can be set by connecting a feedback resistor and capacitor from the error amplifier output

pin to the inverting input pin, enabling stable phase compensation to the system.

The MB3789 can make a current-regulated DC/DC converter using the two internal error amplifiers respectively

for voltage control and current control.

(4) PWM comparator

The PWM comparator is a voltage comparator with one inverting input and three noninverting inputs, serving

as a voltage-pulse width converter for controlling the output duty depending on the input voltage.

The PWM comparator turns on the output transistor during the interval in which the sawtooth wave voltage

level is lower than the voltage levels at all of the error amplifier output pins, the SCP pin (pin 5), and at the

DTC pin (pin 12).

(5) Output circuit

The output circuit is a power MOSFET driven, output circuit in a totem-pole configuration. It can drive the gate

voltage up to near the supply voltage with a bootstrap capacitor connected between the OUT pin (pin 15) and

CB pin (pin 13). (See “s SETTING THE BOOTSTRAP CAPACITOR (CBS).”)

2. Protection Functions

(1) Timer-latch short-circuit protection circuit

SCP comparator 1 detects the output voltage levels of error amplifiers 1 and 2. When the output voltage level

of either (or both) of the two error amplifiers reaches 1.25 V, the timer circuit is actuated to start charging the

external protection-enable capacitor connected to the SCP pin (pin 5).

If the error amplifier output is not restored to the normal voltage level before the capacitor voltage reaches

1.8 V, the latch circuit is actuated to turn off the output transistor while making the dead time 100%.

To reset the actuated protection circuit, turn the power supply on back. (See “s SETTING THE SOFT START/

SHORT-CIRCUIT DETECTION TIME.”)

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