Fujitsu MB39A104 1. DC/DC Converter Functions, Reference voltage block REF, Output block

MB39A104

■FUNCTIONS

1.DC/DC Converter Functions

(1)Reference voltage block (REF)

The reference voltage circuit generates a temperature-compensated reference voltage (5.0 V Typ) from the voltage supplied from the power supply terminal (pin 7). The voltage is used as the reference voltage for the IC’s internal circuitry.

The reference voltage can supply a load current of up to 1 mA to an external device through the VREF terminal (pin 17).

(2) Triangular-wave oscillator block (OSC)

The triangular wave oscillator incorporates a timing capacitor and a timing resistor connected respectively to the CT terminal (pin 13) and RT terminal (pin 12) to generate triangular oscillation waveform amplitude of 1.5 V to 2.5 V.

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

(3) 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 CS1 terminal (pin 11) and CS2 terminal (pin 14) which are 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.

(4) 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 remain higher than the triangular wave voltage.

(5) Output block

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

(6) Bias voltage block (VH)

This bias voltage circuit outputs VCC − 5 V(Typ) as minimum potential of the output circuit. In standby mode, this circuit outputs the potential equal to VCC.

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