Functional description

AN1518

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again. For this reason the switching frequency is also reduced, thus keeping the inductor

current under its maximum threshold. The Frequency Shifter (Figure 6) functions based on

the feedback voltage. As the feedback voltage decreases (due to the reduced duty cycle),

the switching frequency decreases also.

Figure 7.

Current limitation circuitry

3.5

Error amplifier

The voltage error amplifier is the core of the loop regulation. It is a transconductance

operational amplifier whose non inverting input is connected to the internal voltage

reference (1.235 V), while the inverting input (FB) is connected to the external divider or

directly to the output voltage. The output (COMP) is connected to the external compensation

network. The uncompensated error amplifier has the following characteristics:

The error amplifier output is compared to the oscillator sawtooth to perform PWM control.

3.6

PWM comparator and power stage

This block compares the oscillator sawtooth and the error amplifier output signals to

generate the PWM signal for the driving stage.

The power stage is a highly critical block, as it functions to guarantee a correct turn ON and

turn OFF of the PDMOS. The turn ON of the power element, or more accurately, the rise

time of the current at turn ON, is a very critical parameter. At a first approach, it appears that

the faster the rise time, the lower the turn ON losses.

Table 2.

Uncompensated error amplifier characteristics

Description

Values

Transconductance

2300 µS

Low frequency gain

65dB

Minimum sink/source voltage

1500 µA/300 µA

Output voltage swing

0.4 V/3.65 V

Input bias current

2.5 µA