A5974D datasheet(12/46 Pages) STMICROELECTRONICS

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Functional description

A5974D

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DocID018774 Rev 2

the switching frequency is also reduced, thus keeping the inductor current under its

maximum threshold. The frequency shifter (Figure 5) 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

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.

DRIVER

NOT

PWM

VCC

OUT

A1/A2=95

RSENSE

AM00008v1

IOFF

RTH

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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