A5974AD datasheet(15/49 Pages) STMICROELECTRONICS

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A5974AD

Functional description

Doc ID 018762 Rev 1

15/49

Figure 7.

Current limitation circuitry

5.5

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.

5.6

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 the first approach, it appears that the

faster the rise time, the lower the turn-on losses.

Table 5.

Uncompensated error amplifier characteristics

Description

Values

Transconductance

2300 µS

Low frequency gain

65 dB

Minimum sink/source voltage

1500 µA/300 µA

Output voltage swing

0.4 V/3.65 V

Input bias current

2.5 µA

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