L7986TA datasheet(12/43 Pages) STMICROELECTRONICS

12 / 43 page

Functional description

L7986TA

12/43

Doc ID 022098 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, therefore, with high DC gain and low output impedance.

The uncompensated error amplifier characteristics are the following:

In continuous 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. If 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 must be used (see Section 6.4 for details of the compensation

network selection).

Anyway, the methodology to compensate the loop is to introduce zeroes to obtain a safe

phase margin.

Table 5.

Uncompensated error amplifier characteristics

Parameter

Value

Low frequency gain

100 dB

GBWP

4.5 MHz

Slew rate

7 V/

μs

Output voltage swing

0 to 3.3 V

Maximum source/sink current

17 mA/25 mA

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