L7981 datasheet(12/44 Pages) STMICROELECTRONICS

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Functional description

L7981

12/44

Doc ID 15182 Rev 3

5.3

Error amplifier and compensation

The error amplifier (E/A) provides the error signal to be compared with the sawtooth to

perform the pulse width modulation. Its non-inverting input is internally connected to a 0.6 V

voltage reference, while its inverting input (FB) and output (COMP) are externally available

for feedback and frequency compensation. In this device the error amplifier is a voltage

mode operational amplifier so with high DC gain and low output impedance.

The uncompensated error amplifier characteristics are the following:

In continuos conduction mode (CCM), the transfer function of the power section has two

poles due to the LC filter and one zero due to the ESR of the output capacitor. Different

kinds of compensation networks can be used depending on the ESR value of the output

capacitor. In case the zero introduced by the output capacitor helps to compensate the

double pole of the LC filter a type II compensation network can be used. Otherwise, a type

III compensation network has to be used (see Chapter 6.4 for details about the

compensation network selection).

Anyway the methodology to compensate the loop is to introduce zeros to obtain a safe

phase margin.

Table 5.

Uncompensated error amplifier characteristics

Low frequency gain

100dB

GBWP

4.5MHz

Slew rate

7V/

μs

Output voltage swing

0 to 3.3V

Maximum source/sink current

17mA/25mA

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