Fujitsu MB39A105 Functions, Triangular-wave oscillator block OSC, PWM comparator block PWM Comp

MB39A105

■FUNCTIONS

1.DC/DC Converter Functions

(1)Triangular-wave oscillator block (OSC)

The triangular wave oscillator incorporates a timing resistor connected to RT terminal (pin 7) to generate triangular oscillation waveform amplitude of 0.3 V to 0.7 V.

The triangular waveforms are input to the PWM comparator in the IC.

(2) Error amplifier block (Error Amp1, Error Amp2)

The error amplifier detects the DC/DC converter output voltage and outputs PWM control signals. In addition, an arbitrary loop gain can be set by connecting a feedback resistor and capacitor from the output terminal to inverted input terminal of the error amplifier, enabling stable phase compensation to the system.

Also, it is possible to prevent rush current at power supply start-up by connecting a soft-start capacitor with the CSCP terminal (pin 2) which is the non-inverted input terminal for Error Amp. The use of Error Amp for soft-start detection makes it possible for a system to operate on a fixed soft-start time that is independent of the output load on the DC/DC converter.

(3) PWM comparator block (PWM Comp.)

The PWM comparator is a voltage-to-pulse width modulator that controls the output duty depending on the input/ output voltage.

The comparator keeps output transistor on while the error amplifier output voltage and the DTC voltage remain higher than the triangular wave voltage.

(4) Output block (Drive)

The output block is in the totem pole configuration, capable of driving an external N-channel MOS FET.

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