AN557 datasheet(10/52 Pages) STMICROELECTRONICS

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AN557 APPLICATION NOTE

10/52

Figure 7. Voltage Feeforward Waveform.

Fig. 7 shows the duty-cycle varies as a result of the changes in slope of the ramp with the input voltage

Vi. The output of the error amplifier should not change to maintain the output voltage in regulation. This

function allows for the increase of speed in response to the rapid change of the supply voltage and for a

greatly reduced ouput ripple at the mains frequency.

In fact, the slope of the ramp is modulated by the ripple, generally present in the order of several volts on

the input of the regulator, particularly when the solution with a mains transformer is used.

Fig. 8 shows the simplified electrical diagram of the oscillator.

A resistor, connected between the Rosc pin and GND, sets the current that is internally reflected in the pin

Cosc, in order to charge the external capacitor to which it is connected. The voltage to the Rosc pin is not

fixed, but is tied to the instantaneous value of Vi; this is needed to achieve the feedforward voltage func-

tion, in which the slope of the ramp is directly proportional to the supply voltage. A comparator senses the

voltage at the Cosc capacitor. When the voltage reaches the value present at the inverting input of the

comparator, the output from the comparator goes high and is sent to the two transistors Q1 and Q2. Q1

is responsible for discharging the external Cosc capacitor with a current of approx. 20mA, while Q2 im-

poses at the inverted input of the comparator a voltage of 2Vbe (approx. 1.3V) that is the low-threshold of

the ramp. Some useful formulas for calculating the various parameters of the oscillator block are:

Figure 8. Oscillator Circuit.

D93IN006

Vi=30V

Vi=15V

Vi=30V

Vi=15V

Rosc

Cosc

PWM

COMP.

CLOCK

D93IN007

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